"user manual"
Contents
Practical Industrial Data Communications
Best Practice Techniques
i
Contents
i
Other titles in the series
Practical Cleanrooms: Technologies and Facilities (David Conway)
Practical Data Acquisition for Instrumentation and Control Systems (John Park, Steve Mackay)
Practical Data Communications for Instrumentation and Control (John Park, Steve Mackay,
Edwin Wright)
Practical Digital Signal Processing for Engineers and Technicians (Edmund Lai)
Practical Electrical Network Automation and Communication Systems (Cobus Strauss)
Practical Embedded Controllers (John Park)
Practical Fiber Optics (David Bailey, Edwin Wright)
Practical Industrial Data Networks: Design, Installation and Troubleshooting (Steve Mackay,
Edwin Wright, John Park, Deon Reynders)
Practical Industrial Safety, Risk Assessment and Shutdown Systems for Instrumentation and Control
(Dave Macdonald)
Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems (Gordon Clarke, Deon
Reynders)
Practical Radio Engineering and Telemetry for Industry (David Bailey)
Practical SCADA for Industry (David Bailey, Edwin Wright)
Practical TCP/IP and Ethernet Networking (Deon Reynders, Edwin Wright)
Practical Variable Speed Drives and Power Electronics (Malcolm Barnes)
Practical Centrifugal Pumps (Paresh Girdhar and Octo Moniz)
Practical Electrical Equipment and Installations in Hazardous Areas (Geoffrey Bottrill and
G. Vijayaraghavan)
Practical E-Manufacturing and Supply Chain Management (Gerhard Greef and Ranjan Ghoshal)
Practical Grounding, Bonding, Shielding and Surge Protection (G. Vijayaraghavan, Mark Brown and
Malcolm Barnes)
Practical Hazops, Trips and Alarms (David Macdonald)
Practical Machinery Safety (David Macdonald)
Practical Machinery Vibration Analysis and Predictive Maintenance (Cornelius Scheffer and
Paresh Girdhar)
Practical Power Distribution for Industry (Jan de Kock and Cobus Strauss)
Practical Process Control for Engineers and Technicians (Wolfgang Altmann)
Practical Power Systems Protection (Les Hewitson, Mark Brown and Ben. Ramesh)
Practical Telecommunications and Wireless Communications (Edwin Wright and Deon Reynders)
Practical Troubleshooting of Electrical Equipment and Control Circuits (Mark Brown, Jawahar Rawtani
and Dinesh Patil)
Practical Hydraulics (Ravi Doddannavar, Andries Barnard)
Practical Batch Process Management (Mike Barker, Jawahar Rawtani)
Contents
Practical Industrial Data
Communications
Best Practice Techniques
Deon Reynders Pr.Eng, BSc (ElecEng) (Hons), MBA,
Senior Staff Engineer, IDC Technologies, Perth, Australia
Steve Mackay FIE (Aust), CPEng, BSc (ElecEng), BSc (Hons), MBA,
Gov.Cert.Comp., Technical Director – IDC Technologies
Edwin Wright MIPENZ, BSc (Hons), BSc (ElecEng),
Senior Staff Engineer for IDC Technologies, Perth, Australia
Series editor: Steve Mackay FIE (Aust), CPEng, BSc (ElecEng), BSc (Hons), MBA,
Gov.Cert.Comp., Technical Director – IDC Technologies
AMSTERDAM • BOSTON • HEIDELBERG • LONDON
NEW YORK • OXFORD • PARIS • SAN DIEGO
SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
Newnes is an imprint of Elsevier
i
Contents
Newnes
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
30 Corporate Drive, Burlington, MA 01803
First published 2005
Copyright © 2005, IDC Technologies. All rights reserved
No part of this publication may be reproduced in any material form (including
photocopying or storing in any medium by electronic means and whether
or not transiently or incidentally to some other use of this publication) without
the written permission of the copyright holder except in accordance with the
provisions of the Copyright, Designs and Patents Act 1988 or under the terms of
a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road,
London, England W1T 4LP. Applications for the copyright holder’s written
permission to reproduce any part of this publication should be addressed
to the publishers
British Library Cataloguing in Publication Data
Reynders, D.
Practical industrial data communications : best practice
techniques. – (Practical professional)
1.
2. Computer network resources
3. I. Title II. Mackay, S. III. Wright, E.
4. 621.3'981
Library of Congress Cataloguing in Publication Data
A catalogue record for this book is available from the Library of Congress
ISBN
0 7506 6395 2
For information on all Newnes Publications
visit our website at www.newnespress.com
Typeset and edited by Integra Software Services Pvt. Ltd, Pondicherry, India
www.integra-india.com
Printed and bound in The Netherlands
Working together to grow
libraries in developing countries
www.elsevier.com | www.bookaid.org | www.sabre.org
i
Contents
Preface..................................................................................................................................xi
1
General topics ............................................................................................................ 1
1.1
Overview ...................................................................................................... 1
1.2
OSI reference model .................................................................................... 2
1.3
Systems engineering approach .................................................................... 8
1.4
State transition structure ............................................................................ 10
1.5
Detailed design .......................................................................................... 11
1.6
Media ........................................................................................................ 11
1.7
Physical connections ................................................................................. 12
1.8
Protocols ................................................................................................... 12
1.9
Noise ......................................................................................................... 15
1.10
Cable spacing ............................................................................................ 21
1.11
Ingress protection ...................................................................................... 24
2
Copper cable ........................................................................................................... 26
2.1
Cable characteristics ................................................................................. 26
2.2
Cable selection .......................................................................................... 29
2.3
Coaxial cables ........................................................................................... 31
2.4
Twisted-pair cable ..................................................................................... 34
2.5
Distribution/installation standards .............................................................. 39
2.6
Connector standards ................................................................................. 41
2.7
Earthing/grounding .................................................................................... 43
2.8
Termination ............................................................................................... 45
2.9
Transient protection ................................................................................... 47
3
Fiber optics .............................................................................................................. 50
3.1
Introduction ............................................................................................... 50
3.2
Fiber-optic cable components .................................................................... 51
3.3
Fiber-optic cable parameters ..................................................................... 53
3.4
Types of optical fiber ................................................................................. 54
3.5
Basic cable types ...................................................................................... 55
3.6
Connecting fibers ...................................................................................... 57
3.7
Splicing trays/organizers and termination cabinets .................................... 59
3.8
Troubleshooting ......................................................................................... 62
3.9
Fiber installation rules................................................................................. 63
3.10
Cleaning optical connectors ....................................................................... 65
3.11
Locating broken fibers ............................................................................... 65
vi
Contents
4a
RS-232 overview ..................................................................................................... 71
Objectives .................................................................................................. 71
4a.1
RS-232 interface standard (CCITT V.24 interface standard) ...................... 71
4a.2
Half-duplex operation of the RS-232 interface ........................................... 78
4a.3
Summary of EIA/TIA-232 revisions ............................................................ 80
4a.4
Limitations ................................................................................................. 81
4b
RS-232 troubleshooting ........................................................................................... 82
4b.1
Introduction ................................................................................................ 82
4b.2
Typical approach ....................................................................................... 82
4b.3
Test equipment .......................................................................................... 83
4b.4
Typical RS-232 problems ........................................................................... 86
4b.5
Summary of troubleshooting ...................................................................... 90
5a
RS-485 overview ..................................................................................................... 92
Objectives .................................................................................................. 92
5a.1
The RS-485 interface standard .................................................................. 92
5b
RS-485 troubleshooting ........................................................................................... 97
5b.1
Introduction ................................................................................................ 97
5b.2
RS-485 vs RS-422 ..................................................................................... 98
5b.3
RS-485 installation ..................................................................................... 98
5b.4
Noise problems .......................................................................................... 99
5b.5
Test equipment ........................................................................................ 103
5b.6
Summary ................................................................................................. 106
6a
Current loop and RS-485 converters overview ....................................................... 107
Objectives ................................................................................................ 107
6a.1
The 20 mA current loop ........................................................................... 107
6a.2
Serial interface converters ....................................................................... 108
6b
Current loop and RS-485 converters troubleshooting ............................................. 110
6b.1
Troubleshooting converters ..................................................................... 110
7a
TCP/IP overview .................................................................................................... 112
7a.1
Introduction .............................................................................................. 112
7a.2
Internet layer protocols (packet transport) ................................................ 114
7a.3
Host-to-host layer: end to end reliability ................................................... 124
7b
TCP/IP troubleshooting .......................................................................................... 127
7b.1
Introduction .............................................................................................. 127
7b.2
Common problems .................................................................................. 127
7b.3
Tools of the trade ..................................................................................... 127
7b.4
Typical network layer problems ................................................................ 128
7b.5
Transport layer problems ......................................................................... 130
Contents
vii
8a
Modbus overview ....................................................................................................132
Objectives ................................................................................................132
8a.1
General overview .....................................................................................132
8a.2
Modbus protocol structure ........................................................................134
8a.3
Function codes .........................................................................................135
8b
Modbus troubleshooting .........................................................................................142
8b.1
Common problems and faults ...................................................................142
8b.2
Description of tools used ..........................................................................142
8b.3
Detailed troubleshooting ...........................................................................143
8b.4
Conclusion ...............................................................................................147
9
Fundamentals of DNP3 ..........................................................................................149
9.1
Fundamental concepts ..............................................................................149
9.2
Understanding the DNP3 message structure ............................................153
9.3
Physical layer ...........................................................................................155
9.4
Data-link layer ..........................................................................................157
9.5
Transport layer (pseudo-transport) ...........................................................170
9.6
Application layer message handling .........................................................171
9.7
Application layer message functions .........................................................181
10
Fundamentals of IEC 60870-5 ................................................................................195
10.1
The IEC 60870-5 standard .......................................................................195
10.2
Protocol architecture .................................................................................199
10.3
Physical layer ...........................................................................................201
10.4
Data-link layer ..........................................................................................203
10.5
Application layer .......................................................................................217
10.6
Information elements ................................................................................228
11a
Industrial Ethernet overview ...................................................................................231
Objectives ................................................................................................231
11a.1 Introduction ..............................................................................................231
11a.2 10 Mbps Ethernet .....................................................................................232
11a.3 100 Mbps Ethernet ...................................................................................244
11a.4 Gigabit Ethernet .......................................................................................247
11a.5 Industrial Ethernet ....................................................................................250
11b
Industrial Ethernet troubleshooting .........................................................................255
11b.1 Introduction ..............................................................................................255
11b.2 Common problems and faults ...................................................................255
11b.3 Tools of the trade .....................................................................................255
11b.4 Problems and solutions ............................................................................257
11b.5 Troubleshooting switched networks ..........................................................269
11b.6 Troubleshooting fast Ethernet ...................................................................270
11b.7 Troubleshooting Gigabit Ethernet .............................................................270
viii
Contents
12a
AS-interface (AS-i) overview .................................................................................. 271
Objectives ................................................................................................ 271
12a.1 Introduction .............................................................................................. 271
12a.2 Layer 1: the physical layer ....................................................................... 272
12a.3 Layer 2: the data-link layer ....................................................................... 274
12a.4 Operating characteristics ......................................................................... 276
12b
AS-i troubleshooting .............................................................................................. 277
12b.1 Introduction .............................................................................................. 277
12b.2 Tools of the trade ..................................................................................... 277
13a
DeviceNet overview ............................................................................................... 279
Objectives ................................................................................................ 279
13a.1 Introduction .............................................................................................. 279
13a.2 Physical layer .......................................................................................... 280
13a.3 Connectors .............................................................................................. 281
13a.4 Cable budgets ......................................................................................... 283
13a.5 Device taps .............................................................................................. 284
13a.6 Cable description ..................................................................................... 287
13a.7 Network power ......................................................................................... 288
13a.8 System grounding .................................................................................... 291
13a.9 Signaling .................................................................................................. 292
13a.10 Data-link layer .......................................................................................... 292
13a.11 The application layer ................................................................................ 295
13b
DeviceNet troubleshooting ..................................................................................... 296
13b.1 Introduction .............................................................................................. 296
13b.2 Tools of the trade ..................................................................................... 296
13b.3 Fault finding procedures .......................................................................... 298
14a
Profibus PA/DP/FMS overview .............................................................................. 303
Objectives ................................................................................................ 303
14a.1 Introduction .............................................................................................. 303
14a.2 Profibus protocol stack ............................................................................ 305
14a.3 The Profibus communication model ......................................................... 311
14a.4 Relationship between application process and communication ................ 311
14a.5 Communication objects ........................................................................... 312
14a.6 Performance ............................................................................................ 313
14a.7 System operation ..................................................................................... 313
14b
Profibus troubleshooting ........................................................................................ 317
14b.1 Introduction .............................................................................................. 317
14b.2 Troubleshooting tools .............................................................................. 317
14b.3 Tips ......................................................................................................... 320
15a
Foundation Fieldbus overview ................................................................................ 322
Objectives ................................................................................................ 322
15a.1 Introduction to Foundation Fieldbus ......................................................... 322
15a.2 The physical layer and wiring rules .......................................................... 323
Contents
15a.3
15a.4
15a.5
15a.6
15a.7
15a.8
ix
The data-link layer ....................................................................................325
The application layer ................................................................................326
The user layer ..........................................................................................327
Error detection and diagnostics ................................................................328
High-speed Ethernet (HSE) ......................................................................328
Good wiring and installation practice ........................................................329
15b
Foundation Fieldbus troubleshooting ......................................................................331
15b.1 Introduction ..............................................................................................331
15b.2 Power problems .......................................................................................332
15b.3 Communication problems .........................................................................333
15b.4 Foundation Fieldbus test equipment .........................................................335
16a
Modbus Plus protocol overview ..............................................................................336
Objectives ................................................................................................336
16a.1 General overview .....................................................................................336
16b
Modbus Plus troubleshooting .................................................................................340
16b.1 Common problems and faults ...................................................................340
16b.2 Description of tools used ..........................................................................341
16b.3 Detailed troubleshooting ...........................................................................341
17a
Data Highway Plus/DH485 overview ......................................................................347
Objectives ................................................................................................347
17a.1 Allen Bradley Data Highway (Plus) protocol .............................................347
17b
Data Highway Plus/DH485 troubleshooting ............................................................355
17b.1 Introduction ..............................................................................................355
17b.2 Data Highway Plus wiring troubleshooting ................................................355
17b.3 Data Highway Plus network diagnostics ...................................................356
18a
HART overview ......................................................................................................358
Objectives ................................................................................................358
18a.1 Introduction to HART and smart instrumentation ......................................358
18a.2 HART protocol ..........................................................................................358
18a.3 Physical layer ...........................................................................................359
18a.4 Data-link layer ..........................................................................................362
18a.5 Application layer .......................................................................................362
18b
HART troubleshooting ............................................................................................365
18b.1 Troubleshooting HART systems ...............................................................365
19
Wireless technologies .............................................................................................366
19.1
Satellite systems ......................................................................................366
19.2
Wireless LANs (WLANs) ..........................................................................372
19.3
Radio and wireless communications .........................................................378
x Contents
20
System design methodology .................................................................................. 396
20.1
Introduction .............................................................................................. 396
20.2
Point-to-point links ................................................................................... 397
20.3
Networked systems ................................................................................. 401
21
Installation, commissioning, troubleshooting .......................................................... 405
21.1
Introduction .............................................................................................. 405
21.2
Methodology ............................................................................................ 405
21.3
Common problems .................................................................................. 406
Index ................................................................................................................................. 408
Preface
The objective of this book is to outline the best practice in designing, installing, commissioning and
troubleshooting industrial data communications systems. In any given plant, factory or installation
there are a myriad of different industrial communications standards used and the key to successful
implementation is the degree to which the entire system integrates and works together. With so
many different standards on the market today, the debate is not about what is the best – be it
Foundation Fieldbus, Profibus, Devicenet or Industrial Ethernet – but rather about selecting the most
appropriate technologies and standards for a given application and then ensuring that best practice is
followed in designing, installing and commissioning the data communications links to ensure they
run fault-free.
The industrial data communications systems in your plant underpin your entire operation. It is
critical that you apply best practice in designing, installing and fixing any problems that may occur.
This book distills all the tips and tricks learnt with the benefit of many years of experience and gives
the best proven practices to follow.
The main steps in using today's communications technologies involve selecting the correct
technology and standards for your plant based on your requirements; doing the design of the overall
system; installing the cabling and then commissioning the system. Fiber Optic cabling is generally
accepted as the best approach for physical communications but there are obviously areas where you
will be forced to use copper wiring and, indeed, wireless communications. This book outlines the
critical rules followed in installing the data communications physical transport media and then
ensuring that the installation will be trouble-free for years to come.
The important point to make is that with today’s wide range of protocols available, you only need to
know how to select, install and maintain them in the most cost-effective manner for your plant or
factory – knowledge of the minute details of the protocols is not necessary.
This book will be useful to anyone working with or required to follow best practice in the
installation of industrial data communications systems ranging from RS-232 to Fieldbus and Ethernet
systems, including:
•
•
•
•
•
•
•
•
•
•
•
•
•
Instrumentation and Control Engineers/Technicians
Process Control Engineers
Network Planners
Electrical Engineers
Test Engineers
System Integrators
Designers
Electronic Technicians
Consulting Engineers
Design Engineers
Plant Managers
Systems Engineers
Shift Electricians
xii
Preface
We would hope you would gain the following knowledge from reading this book:
• Best practice in industrial data communications design, installation and commissioning
• Practical hands-on experience in jointing, splicing and testing of copper and fiber based
cabling
• How to design and install your own fully operational industrial data communications
systems
• How to integrate different industrial communications protocols and standards into a
complete working system
You should have a modicum of electrical knowledge and some exposure to industrial automation
systems to derive maximum benefit from this book.
General topics
1.1
Overview
This book can be divided into several distinct sections.
1.1.1
Introduction
This introductory chapter deals with general topics such as the OSI model, systems
engineering concepts, physical (layer 1) connections, protocols, and noise and ingress
protection.
1.1.2
Media
Chapters 2 and 3 deal with media – specifically conductive media viz. copper (coax,
UTP, STP) and fiber.
1.1.3
Physical layer standards
Chapters 4–6 (inclusive) cover RS-232, RS-485, and 4–20 mA.
Note: Throughout this book, we will refer to RS-232, RS-422, and RS-485. One is often
criticized for using these terms of reference, since in reality they are obsolete. However, if
we briefly examine the history of the organization that defined these standards, it is not
difficult to see why they are still in use today and will probably continue as such.
The Electronics Industry Association (EIA) of America defined the common serial
interface RS-232. ‘RS’ stands for ‘recommended standard’, and the number (suffix-232)
refers to the interface specification of the physical device. The EIA has since established
many standards and amassed a library of white papers on various implementations of
them. So to keep track of them all, it made sense to change the prefix to EIA. (It is
interesting to note that most of the white papers are NOT free.)
The Telecommunications Industry Association (TIA) was formed in 1988, by
merging the telecommunications arm of the EIA and the United States Telecommunications Suppliers Association. The prefix changed again to EIA/TIA-232 (along with
all the other serial implementations, of course). So now we have TIA-232, TIA485, etc.
It should also be noted that the TIA is a member of the Electronics Industries Alliance
(EIA). This alliance is made up of several trade organizations (including the CEA, ECA,
GEIA . . . ) that represent the interests of manufacturers of electronics-related products.
2
Practical Industrial Data Communications
Now when someone refers to ‘EIA’, they are talking about the Alliance, not the
Association!
If we still use the terms RS-232, RS-422, etc., then they are just as equally obsolete as the
‘RS’ equivalents. However, when they are referred to as TIA standards some people might
give you a quizzical look and ask you to explain yourself . . . So to cut a long story short,
one says ‘RS-xxx’ – and the penny drops. ‘RS’ has become more or less a de facto
approach, as a search on the Internet will testify.
Copies of the relevant standards are available from Global Engineering documents, the
official suppliers of EIA documents. A brief perusal of their website (http://global.ihs.com)
will reveal the name changes over time, since names were not changed retroactively. The
latest ‘232’ revision refers to TIA-232, but earlier revisions and other related documents
still refer to TIA/EIA-232, EIA-232, and RS-232.
1.1.4
Industrial protocols
Chapters 7–10 (inclusive) deal with a few well-known industrial protocols such as
TCP/IP, Modbus, DNP3, and IEC 60870.
1.1.5
Industrial networks
Chapters 11–18 (inclusive) deal with some popular industrial networks (both old and
new) such as Industrial Ethernet, AS-i, DeviceNet, Profibus, Foundation Fieldbus,
Modbus Plus, Data Highway Plus, and HART. Although the topic of troubleshooting
only follows towards the end of the workshop, each chapter has been divided into
an A and a B section, where the A section deals with the operation of the system and
B deals with troubleshooting the same system. This has been done for ease of
reference.
1.1.6
Other technologies
Chapter 19 deals with several wireless technologies, including VSAT, IEEE 802.11, and
wireless point-to-point.
1.1.7
Selection methodology
Chapter 20 covers the appropriate steps to be taken in choosing the components for an
industrial data communications system.
1.1.8
Installation, commissioning, troubleshooting
Chapter 21 covers recommended practice in installing, commissioning, and troubleshooting industrial data communications systems.
1.2
OSI reference model
Faced with the proliferation of closed network systems, the International Organization
for Standardization (ISO) defined a ‘Reference Model for Communication between Open
Systems’ in 1978. This has become known as the Open Systems Interconnection
Reference model or simply as the OSI model (ISO7498). The OSI model is essentially a
data communications management structure, which breaks data communications down
into a manageable hierarchy of seven layers.
General topics 3
Each layer has a defined purpose and interfaces with the layers above and below it. By
laying down standards for each layer, some flexibility is allowed so that the system
designers can develop protocols for each layer independent of each other. By conforming
to the OSI standards, a system is able to communicate with any other compliant system,
anywhere in the world.
At the outset, it should be realized that the OSI reference model is not a protocol or set
of rules for how a protocol should be written, but rather an overall framework in which to
define protocols. The OSI model framework specifically and clearly defines the functions
or services that have to be provided at each of the seven layers (or levels).
Since there must be at least two sites to communicate, each layer also appears to
converse with its peer layer at the other end of the communication channel in a virtual
(‘logical’) communication. These concepts of isolation of the process of each layer,
together with standardized interfaces and peer-to-peer virtual communication, are
fundamental to the concepts developed in a layered model such as the OSI model. The
OSI layering concept is shown in Figure 1.1.
Figure 1.1
OSI layering concept
The actual functions within each layer are provided by entities that are abstract devices,
such as programs, functions, or protocols that implement the services for a particular layer
on a single machine. A layer may have more than one entity – for example a protocol
entity and a management entity. Entities in adjacent layers interact through the common
upper and lower boundaries by passing physical information through service access points
(SAPs). A SAP could be compared to a predefined ‘post-box’ where one layer would
collect data from the previous layer. The relationship between layers, entities, functions,
and SAPs are shown in Figure 1.2.
In the OSI model, the entity in the next higher layer is referred to as the N + 1 entity and
the entity in the next lower layer as N – 1. The services available to the higher layers are
the result of the services provided by all the lower layers.
The functions and capabilities expected at each layer are specified in the model.
However, the model does not prescribe how this functionality should be implemented.
The focus in the model is on the ‘interconnection’ and on the information that can be
passed over this connection. The OSI model does not concern itself with the internal
operations of the systems involved.
4
Practical Industrial Data Communications
Figure 1.2
Relationship between layers, entities, functions, and SAPs
When the OSI model was being developed, a number of principles were used to
determine exactly how many layers this communication model should encompass. These
principles are:
• A layer should be created where a different level of abstraction is required.
• Each layer should perform a well-defined function.
• The function of each layer should be chosen with thought given to defining
internationally standardized protocols.
• The layer boundaries should be chosen to minimize the information flow across
the boundaries.
• The number of layers should be large enough that distinct functions need not be
thrown together in the same layer out of necessity and small enough that the
architecture does not become unwieldy.
The use of these principles led to seven layers being defined, each of which has been
given a name in accordance with its process purpose. Figure 1.3 shows the seven layers of
the OSI model.
Figure 1.3
The OSI reference model
At the transmitter, the user invokes the system by passing data and control information
(physically) to the highest layer of the protocol stack. The system then passes the data
physically down through the seven layers, adding headers (and possibly trailers), and
invoking functions in accordance with the rules of the protocol. At each level, this
General topics 5
combined data and header ‘packet’ is termed as protocol data unit or PDU. At the
receiving site, the opposite occurs with the headers being stripped from the data as it is
passed up through the layers. These header and control messages invoke services and a
peer-to-peer logical interaction of entities across the sites.
At this stage, it should be quite clear that there is no connection or direct communication
between the peer layers of the network. Rather, all communication is across the physical
layer, or the lowest layer of the stack. Communication is down through the protocol stack
on the transmitting stack and up through the stack on the receiving stack. Figure 1.4 shows
the full architecture of the OSI model, whilst Figure 1.5 shows the effects of the addition of
headers (protocol control information) to the respective PDUs at each layer. The net effect
of this extra information is to reduce the overall bandwidth of the communications channel,
since some of the available bandwidth is used to pass control information.
Figure 1.4
Full architecture of OSI model
1.2.1
OSI layer services
Briefly, the services provided at each layer of the stack are:
• Application (layer 7): The provision of network services to the user’s
application programs (clients, servers, etc.). Note: the user’s actual application
programs do not reside here.
• Presentation (layer 6): Maps the data representations into an external data
format that will enable correct interpretation of the information on receipt. The
mapping can also possibly include encryption and/or compression of data.
• Session (layer 5): Control of the communications between the users. This
includes the grouping together of messages and the coordination of data
transfer between grouped layers. It also affects checkpoints for (transparent)
recovery of aborted sessions.
6
Practical Industrial Data Communications
Figure 1.5
OSI message passing
• Transport (layer 4): The management of the communications between the
two end systems.
• Network (layer 3): Responsible for the control of the communications
network. Functions include routing of data, network addressing, fragmentation
of large packets, congestion, and flow control.
• Data link (layer 2): Responsible for sending a frame of data from one system
to another. Attempts to ensure that errors in the received bit stream are not
passed up into the rest of the protocol stack. Error correction and detection
techniques are used here.
• Physical (layer 1): Defines the electrical and mechanical connections at the
physical level or the communication channel itself. Functional responsibilities
include modulation, multiplexing, and signal generation. Note that the physical
layer defines, but does not include the medium. This is located below the
physical layer and is sometimes referred to as layer 0.
A more specific discussion of each layer is now presented.
Application layer
The application layer is the topmost layer in the OSI model. This layer is responsible for
giving applications access to the network. Examples of application layer tasks include file
transfer, electronic mail services, and network management. Application layer services
are more varied than the services in lower layers, because the entire range of application
and task possibilities is available here. To accomplish its tasks, the application layer
passes program requests and data to the presentation layer, which is responsible for
encoding the application layer’s data in the appropriate form.
Presentation layer
The presentation layer is responsible for presenting information in a manner suitable for
the applications or users dealing with the information. Functions, such as data conversion
General topics 7
from EBCDIC to ASCII (or vice versa), use of special graphics or character sets, data
compression or expansion, and data encryption or decryption are carried out at this layer.
The presentation layer provides services for the application layer above it and uses the
session layer below it. In practice, the presentation layer rarely appears in pure form and
is the least well-defined of the OSI layers. Application or session layer programs will
often encompass some or all of the presentation layer functions.
Session layer
The session layer is responsible for synchronizing and sequencing the dialogue and
packets in a network connection. This layer is also responsible for making sure that the
connection is maintained until the transmission is complete and ensuring that
appropriate security measures are taken during a ‘session’ (i.e. a connection). The
session layer is used by the presentation layer above it, and uses the transport layer
below it.
Transport layer
In the OSI model, the transport layer is responsible for providing data transfer at an
agreed upon level of quality, such as at specified transmission speeds and error rates. To
ensure delivery, outgoing packets are assigned numbers in sequence. The numbers are
included in the packets that are transmitted by lower layers. The transport layer at the
receiving end checks the packet numbers to make sure all have been delivered and to put
the packet contents into the proper sequence for the recipient. The transport layer
provides services for the session layer above it and uses the network layer below it to find
a route between source and destination. In many ways, the transport layer is crucial
because it sits between the upper layers (which are strongly application dependent) and
the lower ones (which are network based).
The layers below the transport layer are collectively known as the subnet layers.
Depending on how well (or not) they perform their function, the transport layer has to
interfere less (or more) in order to maintain a reliable connection.
Network layer
The network layer is the third lowest layer or the uppermost subnet layer. It is responsible
for the following tasks:
• Determining addresses or translating from hardware to network addresses.
These addresses may be on a local network or they may refer to networks
located elsewhere on an internetwork. One of the functions of the network layer
is, in fact, to provide capabilities needed to communicate on an internetwork.
• Finding a route between a source and a destination node or between two
intermediate devices.
• Establishing and maintaining a logical connection between these two nodes and
to establish either a connectionless or a connection-oriented communication.
The data is processed and transmitted using the data-link layer below the
network layer. Responsibility for guaranteeing proper delivery of the packets
lies with the transport layer, which uses network layer services.
• Fragmentation of large packets of data into frames which are small enough to
be transmitted by the underlying data-link layer. The corresponding network
layer at the receiving node undertakes re-assembly of the packet.
8
Practical Industrial Data Communications
Data-link layer
The data-link layer is responsible for creating, transmitting, and receiving data packets. It
provides services for the various protocols at the network layer, and uses the physical
layer to transmit or receive material. The data-link layer creates packets appropriate for
the network architecture being used. Requests and data from the network layer are part of
the data in these packets (or frames, as they are often called at this layer). These packets
are passed down to the physical layer and from there the data is transmitted to the
physical layer on the destination machine. Network architectures (such as Ethernet,
ARCnet, token ring, and FDDI) encompass the data-link and physical layers, which is
why these architectures support services at the data-link level. These architectures also
represent the most common protocols used at the data-link level.
The IEEE 802.x networking working groups have refined the data-link layer into two
sub-layers: the logical link control (LLC) sub-layer at the top and the media access
control (MAC) sub-layer at the bottom. The LLC sub-layer must provide an interface for
the network layer protocols and control the logical communication with its peer at the
receiving side. The MAC sub-layer must provide access to a particular physical encoding
and transport scheme.
Physical layer
The physical layer is the lowest layer in the OSI reference model. This layer gets data
packets from the data-link layer above it and converts the contents of these packets into a
series of electrical signals that represent 0 and 1 values in a digital transmission. These
signals are sent across a transmission medium to the physical layer at the receiving end.
At the destination, the physical layer converts the electrical signals into a series of bit
values. These values are grouped into packets and passed up to the data-link layer.
The mechanical and electrical properties of the transmission medium are defined at this
level. These include the following:
• The type of cable and connectors used. A cable may be coaxial, twisted pair, or
fiber optic. The types of connectors depend on the type of cable.
• The pin assignments for the cable and connectors. Pin assignments depend on
the type of cable and also on the network architecture being used.
• The format for the electrical signals. The encoding scheme used to signal
0 and 1 values in a digital transmission or particular values in an analog
transmission depends on the network architecture being used. Most networks
use digital signaling and some form of Manchester encoding for the signal.
1.3
Systems engineering approach
1.3.1
System specifications
Systems engineering, especially in a military context, is a fully fledged subject and proper
treatment thereof will warrant a two-day workshop on its own. However, the basic
principles of systems engineering can be applied very advantageously throughout the life
cycle of any project, and hence, we will briefly look at the concepts. The project, in the
context of this workshop, would involve the planning, installation, commissioning, and
ongoing maintenance of some sort of industrial data communications system.
The question is: what is a system, where does it start and where does it end? The
answer is a bit ambiguous – it depends where the designer draws the boundaries. For
example, the engine of a motor vehicle, excluding gearbox, radiator, battery, and engine
General topics 9
mounts, but including fuel injection system, could be seen as a system in its own right.
On the other hand, the car in its entirety could be seen as a system, and the engine, one of
its sub-systems. Other sub-systems could include the gearbox, drive train, electrical
system, etc. In similar fashion, a SCADA system integrator could view the entire product
as the ‘system’ with, for example, the RTUs as sub-systems, whereas for a hardware
developer the RTU could be viewed as a ‘system’ in its own right.
The point of departure should be the physical, mechanical, and electrical environment
in which the system operates. For a car engine, this could include the dimensions of the
engine compartment, minimum and maximum ambient temperatures, and levels of
humidity. An engine operating in Alaska in mid-winter faces different problems than its
counterpart operating in Saudi Arabia.
In similar fashion, an RTU developer or someone contemplating an RTU installation
should consider:
•
•
•
•
•
•
Minimum and maximum temperatures
Vibration
Humidity
Mounting constraints
IP rating requirements
Power supply requirements (voltage levels, tolerances, current consumption,
power backup and redundancy, etc.).
These should all be included in the specifications. Let us return to the engine. There are
five attributes necessary to fully describe it, but we will initially look at the first three
attributes namely inputs, outputs, and functions.
Inputs
What goes ‘into’ the system? Inputs would include fuel from the fuel pump, air input
from the air filter, cold water input from the radiator, and electrical power from the
battery. For each input, the mechanical, electrical, and other details, as required, must be
stated. For example, for the electrical inputs of the engine, the mechanical details of
the +12 V and ground terminals must be given as well as the voltage and current limits.
For an RTU the inputs could include:
•
•
•
•
Digital inputs (e.g. contact closures)
Analog inputs (e.g. 4–20 mA)
Communication input (RS-232)
Power (e.g. 12 V DC at 100 mA).
Specifications should include all relevant electrical and mechanical considerations
including connector types, pin allocations, minimum and maximum currents, minimum
and maximum voltage levels, maximum operating speeds, and any transient protection.
Stated in general, in the mathematical equation y = f (x), where x would be the input.
Outputs
What comes ‘out of ’ the system? Engine outputs would include torque output to the
gearbox, hot water to the radiator, and exhaust gases to the exhaust system. For each output,
the exact detail (including flange dimensions, bolt sizes, etc.) has to be stated. The reason
for this is simple. Each output of the engine has to mate exactly with the corresponding
input of the associated auxiliary sub-system. Unless the two mating entities are absolutely
complementary, dimensionally and otherwise, there will be a problem.
10
Practical Industrial Data Communications
For an RTU the outputs could include:
• Relay outputs
• Open collector transistor outputs.
Specifications should include maximum voltages and currents as well as maximum
operating speeds, relay contact lifetime, and transient protection.
Stated in general, in the mathematical equation y = f (x), y (the output) occurs as a result
of x, the input.
Functions
What does the system (viewed as a ‘black box’) do? The functions are built into the
system black box. They convert the input(s) to the output(s) according to some built-in
transfer function(s). The system can be seen as having a singular function with several
sub-functions, or as simply having several separate functions. The overall function of the
engine would be to convert fuel plus air into energy. Its main sub-function would be
to convert the fuel plus air into torque to drive the car, another sub-function could be to
provide electrical energy to the battery. In the mathematical equation above, this refers to
the f ( ) part, in other words it takes ‘x’ and does something to it in order to produce ‘y’.
The three items mentioned so far describes the behavior of the system in terms of
‘what’ it has to do, but not ‘how’. It has, in other words, not described a specific
implementation, but just a functional specification. Once this has been documented,
reviewed (several times!), and ratified, the solution can be designed.
The full (detailed) specification has to include the ‘how’. For this, two additional
descriptions are necessary. They are the structure of elements and couplings, and the state
transition diagram.
Structure of elements and couplings
It is also referred to as the EC diagram. This refers to all the ‘building blocks’ of the
system and their inter-relationship, but does not elucidate the way they operate. In a car
engine, this would show the engine block, pistons, connecting rods, crankshaft, etc., and
the way they are attached to each other.
For an RTU, this would include a full electronic circuit diagram as well as a component
placement diagram.
1.4
State transition structure
This is also referred to as the ST diagram. This is the ‘timing diagram’ of the system. It
explains, preferably in diagram form (e.g. flowchart), how all the building blocks interact.
For the engine, it would show the combustion cycle of the engine plus the firing sequence
of the spark plugs, etc.
For an RTU, this would be an explanation of the system operation by means of a flowchart.
Flowcharts could be drawn for the initial setup, normal system operation (from an operator
point of view) and program flow (from a software programmer’s point of view), etc.
1.4.1
System life cycle
Our discussion this far has focused on the specification of the system, but not on the
implementation thereof. Here is a possible approach. Each phase mentioned here should
be terminated with a proper design review. The further a system implementation
progresses, the more costly it becomes to rectify mistakes.
General topics 11
1.4.2
Conceptual phase
In this phase, the functional specification is developed. Once it has been agreed upon, one
or more possible solutions can be put together and evaluated on paper.
1.4.3
Validation phase
If there are any untested assumptions in the design concept, now is the time to validate it.
This could involve setting up a small pilot system or a test network, in order to confirm
that the design objectives can be achieved.
1.5
Detailed design
Once the validation has been completed, it is time to do the full, detailed design of the
system.
1.5.1
Implementation
This phase involves the procurement of the equipment, the installation, and subsequent
commissioning of the system.
1.5.2
Maintenance/troubleshooting
Once the system is operational, these actions will be performed for the duration of its
service life. At the end of its useful life, the system will be replaced, overhauled, or
scrapped. In fact, often overlooked is the monetary cost of maintaining a system over its
useful life, including the cost of parts, maintenance, and service infrastructure that could
exceed the initial purchase cost, be a factor of five or more.
1.6
Media
For any communication to take place between two entities, there must be some form
of medium between them. The OSI model does not include the actual medium
(although it may specify it). The medium is sometimes referred to as ‘layer 0’ (being
below layer 1) although, in fact, there is no such thing. In the context of data
communications, we can distinguish between two basic groupings, namely conductive
media and radiated media.
In the case of conductive media, there is a physical cable between the two devices. This
cable could be either a copper cable or an optic-fiber cable.
In copper cable, the signal is conducted as electrical impulses. This type of cable can be
in the form of:
• Coaxial cable, e.g. RG-58
• Twisted-pair cable (single or multi-pair), e.g. EIA/TIA-568 Cat 5, or
• Untwisted (parallel) cable, e.g. the flat cables for DeviceNet or AS-i.
Twisted-pair cable can be unshielded or shielded with foil, woven braid, or a
combination thereof.
In the case of optic fiber, the signal is conducted as impulses of light. There are two
main approaches possible with fiber-optic cables, namely:
1. Single mode (monomode) cabling and
2. Multimode cabling.
12
Practical Industrial Data Communications
This is widely used throughout industrial communications systems because of
immunity to electrical noise and optical isolation from surges and transients. As a result,
fiber is tending to dominate in all new installations that require reasonable levels of traffic
(Figure 1.6).
Figure 1.6
Single mode and multimode optic fibers
An alternative to conductive media is radiated media. Here, the medium is actually free
space, and various techniques are used to transmit the signal. These include infrared
transmission as well as VHF transmission (30–300 MHz) and UHF transmission
(300 MHz–3 GHz). A very popular band is the unlicensed 2.4 GHz ISM (industrial,
scientific, and medical) band as used in IEEE 802.15 Bluetooth and most wireless LANs
e.g. IEEE 802.11.
In microwave transmission, a differentiation is often made in terms of terrestrial
systems (i.e. transmission takes place in a predominantly horizontal plane) and satellite
transmission, where transmission takes place in a predominantly vertical plane.
1.7
Physical connections
This refers to layer 1 of the OSI model and deals with the mechanism of placing an actual
signal on the conductor for the purpose of transmitting 1s and 0s. Many network
standards such as Ethernet and AS-i have their own unique way of doing this. Many
others, such as Data Highway Plus and Profibus, use the RS-485 standard.
Here follows a brief summary of RS-485, although it is covered in detail elsewhere.
RS-485 is a balanced (differential) system with up to 32 ‘standard’ transmitters and
receivers per line, speeds up to 10 Mbps and distances up to 1200 m.
The RS-485 standard is very useful for instrumentation and control systems, where
several instruments or controllers may be connected together on the same multi-point
network. A diagram of a typical RS-485 system is shown in Figure 1.7.
1.8
Protocols
It has been shown that there are protocols operating at layers 2–7 of the OSI model.
Layer 1 is implemented by physical standards such as RS-232 and RS-485, which are
mechanisms for ‘putting the signal on the wire’ and are therefore not protocols. Protocols
are the sets of rules by which communication takes place and are implemented in
software.
General topics 13
Figure 1.7
Typical two-wire multidrop network for RS-485
Protocols vary from the very simple such as ASCII-based protocols to the very
sophisticated such as TCP and IP, which operate at high speeds transferring megabits of
data per second. There is no right or wrong protocol, the choice depends on a particular
application.
Examples of protocols include:
•
•
•
•
Layer 2: SDLC, HDLC
Layer 3: IP, IPX
Layer 4: TCP, UDP, SPX
Layers 5 + 6 + 7: CIP, HTTP, FTP, POP3, NetBIOS.
Depending on their functionality and the layer at which they operate, protocols perform
one or more of the following functions:
• Segmentation (fragmentation) and re-assembly: Each protocol has to deal
with the limitations of the PDU (protocol data unit) or packet size associated
with the protocol below it. For example, the Internet protocol (IP) (layer 3)
can only handle 65 536 bytes of data, hence the transmission control protocol
(TCP) (layer 4) has to segment the data received from layer 5 into pieces no
bigger than that. IP (layer 3), on the other hand, has to be aware that Ethernet
(layer 2) cannot accept more than 1500 bytes of data at a time and has to
fragment the data accordingly. The term ‘fragmentation’ is normally
associated with layer 3, whereas the term ‘segmentation’ is normally
associated with layer 4. The end result of both is the same but the
mechanisms differ. Obviously, the data stream fragmented by a protocol on
the transmitting side has to be re-assembled by its corresponding peer on the
14
Practical Industrial Data Communications
•
•
•
•
•
•
receiving side, so each protocol involved in the process of fragmentation has
to add appropriate parameters in the form of sequence numbers, offsets, and
flags to facilitate this.
Encapsulation: Each protocol has to handle the information received from
the layer above it ‘without prejudice’; i.e. it carries/forwards it without
regard for its content. For example, the information passed on to IP (layer
3) could contain a TCP header (layer 4) plus an FTP header (layers 5, 6, 7)
plus data from an FTP client (e.g. Cute FTP). IP simply regards this as a
package of information to be forwarded, adds its own header with the
necessary control information, and passes it down to the next layer (e.g.
Ethernet).
Connection control: Some layer 4 protocols such as TCP create logical
connections with their peers on the other side. For example, when browsing the
Internet, TCP on the client (user) side has to establish a connection with TCP
on the server side before a web site can be accessed. Obviously, there are
mechanisms for terminating the connection as well.
Ordered delivery: Large messages have to be cut into smaller fragments, but
on a packet switching network, the different fragments can theoretically travel
via different paths to their destination. This results in fragments arriving at their
destination out of sequence, which creates problems in rebuilding the original
message. This issue is normally addressed at layer 3 and sometimes at layer 4
(anywhere that fragmentation and segmentation takes place) and different
protocols use different mechanisms, including sequence numbers and fragment
offsets.
Flow control: The protocol on the receiving side must be able to liaise with
its counterpart on the sending side in order not to be overrun by data. In simple
protocols, this is accomplished by a lock-step mechanism (i.e. each packet sent
needs to be acknowledged before the next one can be sent) or XON/XOFF
mechanisms where the receiver sends an XOFF message to the sender to pause
transmission, then sends an XON message to resume transmission.
More sophisticated protocols use ‘sliding windows’. Here, the sliding
window is a number that represents the amount of unacknowledged data that
can still be sent. The receiver does not have to acknowledge every message, but
can from time to time issue blanket acknowledgments for all data received up
to a point. As the sender sends data, the window shrinks and as the receiver
acknowledges, the window expands accordingly. When the window becomes
zero, the transmitter stops until some acknowledgment is received and the
window opens up again.
Error control: The sender needs some mechanism by which it can ascertain if
the data received is the same as the data sent. This is accomplished by
performing some form of checksum on the data to be transmitted, including the
checksum in the header or in a trailer after the data. Types of checksum include
vertical and longitudinal parity, block check count (BCC) and cyclic
redundancy checking (CRC).
Addressing: Protocols at various levels need to identify the physical or
logical recipient on the other side. This is done by various means. Layer 4
protocols such as TCP and UDP use port numbers. Layer 3 protocols use
a protocol address (such as the IP address for the Internet protocol) and layer
2 protocols use a hardware (or ‘media’) address such as a station number or
MAC address.
General topics 15
• Routing: In an internetwork, i.e. a larger network consisting of two or more
smaller networks interconnected by routers, the routers have to communicate
with each other in order to know the best path to a given destination on the
network. This is achieved by routing protocols (RIP, OSPF, etc.) residing on
the routers.
• Multiplexing: Some higher-protocols such as TCP can create several ‘logical’
channels on one physical channel. The opposite can be done some lower-level
protocols such as PPP where one logical stream of data can be sent over several
physical (e.g. dial-up) connections. This mechanism is called multiplexing.
1.9
Noise
1.9.1
Sources of electrical noise
Typical sources of noise are devices that produce quick changes (or spikes) in voltage or
current, such as:
•
•
•
•
•
Large electrical motors being switched on
Fluorescent lighting tubes
Lightning strikes
High-voltage surging due to electrical faults
Welding equipment.
From a general point of view, there must be three contributing factors for the existence of
an electrical noise problem. They are:
1. A source of electrical noise
2. A mechanism coupling the source to the affected circuit
3. A circuit conveying the sensitive communication signals.
1.9.2
Electrical coupling of noise
There are four forms of coupling of electrical noise into the sensitive data communications
circuits. They are:
1.
2.
3.
4.
Impedance coupling (sometimes referred to as conductance coupling)
Electrostatic coupling
Magnetic or inductive coupling
Radio frequency radiation (a combination of electrostatic and magnetic).
Each of these noise forms will be discussed in some detail in the following sections.
1.9.3
Impedance coupling (or common impedance coupling)
For situations where two or more electrical circuits share common conductors, there can
be some coupling between the different circuits with harmful effects on the connected
circuits. Essentially, this means that the signal current from the one circuit proceeds back
along the common conductor resulting in an error voltage along the return bus that affects
all the other signals. The error voltage is due to the impedance of the return wire. This
situation is shown in the Figure 1.8.
Obviously, the quickest way to reduce the effects of impedance coupling is to minimize
the impedance of the return wire. The best solution is to use a separate return for each
individual signal (Figure 1.9).
16
Practical Industrial Data Communications
Figure 1.8
Impedance coupling
Figure 1.9
Impedance coupling eliminated with separate ground returns
1.9.4
Electrostatic or capacitive coupling
This form of coupling is proportional to the capacitance between the noise source and the
signal wires. The magnitude of the interference depends on the rate of change of the noise
voltage and the capacitance between the noise circuit and the signal circuit.
In Figure 1.10, the noise voltage is coupled into the communication signal wires
through two capacitors, C1 and C2, and a noise voltage is produced across the resistance in
the circuit. The size of the noise (or error) voltage in the signal wires is proportional
to the:
• Inverse of the distance of noise voltage from each of the signal wires
• Length (and hence impedance) of the signal wires into which the noise is
induced
General topics 17
• Amplitude (or strength) of the noise voltage
• Frequency of the noise voltage
• There are four methods for reducing the noise induced by electrostatic
coupling. They are:
–
–
–
–
Shielding of the signal wires
Separating from the source of the noise
Reducing the amplitude of the noise voltage (and possibly the frequency)
Twisting of the signal wires.
Figure 1.10
Electrostatic coupling
The problem can be addressed by installing an electrostatic shield around the signal
wires. The currents generated by the noise voltages prefer to flow down the lower
impedance path of the shield rather than the signal wires. If one of the signal wires and
the shield are tied to the ground at one point, which ensures that the shield and the signal
wires are at an identical potential, then reduced signal current flows between the signal
wires and the shield (Figure 1.11).
Figure 1.11
Shield to minimize electrostatic coupling
18
Practical Industrial Data Communications
The shield must be of a low resistance material such as aluminum or copper. For
a loosely braided copper shield (85% braid coverage), the screening factor is about
100 times or 20 dB. For a low resistance multi-layered screen, this screening factor can be
35 dB or 3000 times.
Twisting of the signal wires provides a slight improvement in reducing the induced
noise voltage by ensuring that C1 and C2 are closer together in value; thus ensuring that
any noise voltages induced in the signal wires tend to cancel each other out.
Provision of a shield by a cable manufacturer ensures that the capacitance between
the shield and each wire is equal in value, thus eliminating any noise voltages by
cancellation.
1.9.5
Magnetic or inductive coupling
This depends on the rate of change of the noise current and the mutual inductance
between the noise system and the signal wires. Expressed slightly differently, the degree
of noise induced by magnetic coupling will depend on the:
• Magnitude of the noise current
• Frequency of the noise current
• Area enclosed by the signal wires (through which the noise current magnetic
flux cuts)
• Inverse of the distance from the disturbing noise source to the signal wires.
The effect of magnetic coupling is shown in Figure 1.12.
Figure 1.12
Magnetic coupling
The easiest way of reducing the noise voltage caused by magnetic coupling is to
twist the signal conductors. This results in lower noise due to the smaller area for
each loop. This means less magnetic flux to cut through the loop and consequently, a
lower induced noise voltage. In addition, the noise voltage that is induced in each
loop tends to cancel out the noise voltages from the next sequential loop. It is
assumed that the noise voltage is induced in equal magnitudes in each signal wire due
to the twisting of the wires giving a similar separation distance from the noise voltage
(Figure 1.13).
The second approach is to use a magnetic shield around the signal wires. The magnetic
flux generated from the noise currents induces small eddy currents in the magnetic shield.
These eddy currents then create an opposing magnetic flux φ1 to the original flux φ2. This
means a lesser flux (φ2 − φ1) reaches our circuit (Figure 1.14).
General topics 19
Figure 1.13
Twisting of wires to reduce magnetic coupling
Figure 1.14
Use of magnetic shield to reduce magnetic coupling
Note: The magnetic shield does not require grounding. It works merely by being
present. High-permeability steel makes best magnetic shields for special applications.
However, galvanized steel conduit makes quite an effective shield.
1.9.6
Radio frequency radiation
The noise voltages induced by electrostatic and inductive coupling (discussed above) are
manifestations of the near field effect, which is electromagnetic radiation close to the
source of the noise. This sort of interference is often difficult to eliminate. It requires
close attention to grounding of the adjacent electrical circuit, and the ground connection
is only effective for circuits in close proximity to the electromagnetic radiation. The
effects of electromagnetic radiation can be neglected unless the field strength exceeds
1 V/m. This can be calculated by the formula:
Field strength =
Where
Field strength: V/m
Power: kilowatt
Distance: km.
2(Power)
Distance
20
Practical Industrial Data Communications
The two most commonly used mechanisms to minimize electromagnetic radiation are:
1. Proper shielding (iron)
2. Capacitors to shunt the noise voltages to ground.
Any incompletely shielded conductors will perform as a receiving aerial for the radio
signal, and hence, care should be taken to ensure good shielding of any exposed wiring.
1.9.7
Shielding
It is important that electrostatic shielding is only grounded at one point. More than one
ground point will cause circulating currents. The shield should be insulated to prevent
inadvertent contact with multiple ground points, which could result in circulating
currents. The shield should never be left floating because that would tend to allow
capacitive coupling, rendering the shield useless.
Two useful techniques for isolating one circuit from the other are by the use of optoisolation as shown in the Figure 1.15, and transformer coupling as shown in Figure 1.16.
Figure 1.15
Opto-isolation of two circuits
Figure 1.16
Transformer coupling
Although opto-isolation does isolate one circuit from the other, it does not prevent noise
or interference being transmitted from one circuit to another.
Transformer coupling can be preferable to optical isolation when there are high-speed
transients in one circuit. There is some capacitive coupling between the LED and the base
of the transistor, which is in the opto-coupler, can allow these types of transients to
penetrate one circuit from another. This is not the case with transformer coupling.
1.9.8
Good shielding performance ratios
The use of some form of low resistance material covering the signal conductors is
considered good shielding practice for reducing electrostatic coupling. When comparing
shielding with no protection, this reduction can vary from copper braid (85% coverage),
which returns a noise reduction ratio of 100 : 1 to aluminum Mylar tape with drain wire,
with a ratio of 6000 : 1.
General topics 21
Twisting the wires to reduce inductive coupling reduces the noise (in comparison to no
twisting) by ratios varying from 14 : 1 (for 4 in. lay) to 141 : 1 (for 1 in. lay). In
comparison, putting parallel (untwisted) wires into steel conduit only gives a noise
reduction of 22 : 1.
On very sensitive circuits with high levels of magnetic and electrostatic coupling, the
approach is to use coaxial cables. Double-shielded cable can give good results for very
sensitive circuits.
Note: With double shielding, the outer shield could be grounded at multiple points to
minimize radio frequency circulating loops. This distance should be set at intervals of less
than one-eighth of the wavelength of the radio frequency noise.
1.9.9
Cable ducting or raceways
These are useful in providing a level of attenuation of electric and magnetic fields. These
figures are done at 60 Hz for magnetic fields and 100 kHz for electric fields.
Typical screening factors are:
• 5 cm (2 in.) aluminum conduit with 0.154 in. thickness: magnetic fields
(at 60 Hz) 1.5 : 1, electric fields (at 100 kHz) 8000 : 1
• Galvanized steel conduit 5 cm (2 in.), wall thickness 0.154 in. width: magnetic
fields (at 60 Hz) 40 : 1, electric fields (at 100 kHz) 2000 : 1.
1.10
Cable spacing
In situations where there are a large number of cables varying in voltage and current
levels, the IEEE 518 – 1982 standard has developed a useful set of tables indicating
separation distances for various classes of cables. There are four classification levels of
susceptibility for cables. Susceptibility, in this context, is understood to be an indication
of how well the signal circuit can differentiate between the undesirable noise and required
signal. It follows a data communication physical standard such as RS-232E that would
have a high susceptibility and a 1000 V, 200 A AC cable that has a low susceptibility.
The four susceptibility levels defined by the IEEE 518 standard are briefly:
• Level 1 (high): This is defined as analog signals less than 50 V and digital signals
less than 15 V. This would include digital logic buses and telephone circuits.
Data communication cables fall into this category.
• Level 2 (medium): This category includes analog signals greater than 50 V
and switching circuits.
• Level 3 (low): This includes switching signals greater than 50 V and analog
signals greater than 50 V. Currents less than 20 A are also included in this
category.
• Level 4 (power): This includes voltages in the range 0–1000 V and currents in the
range 20–800 A. This applies to both AC and DC circuits.
IEEE 518 also provides for three different situations when calculating the separation
distance required between the various levels of susceptibilities.
In considering the specific case where one cable is a high-susceptibility cable and the other
cable has a varying susceptibility, the required separation distance would vary as follows:
• Both cables contained in a separate tray
– Level 1 to Level 2–30 mm
– Level 1 to Level 3–160 mm
– Level 1 to Level 4–670 mm
22
Practical Industrial Data Communications
• One cable contained in a tray and the other in conduit
– Level 1 to Level 2–30 mm
– Level 1 to Level 3–110 mm
– Level 1 to Level 4–460 mm
• Both cables contained in separate conduit
– Level 1 to Level 2–30 mm
– Level 1 to Level 3–80 mm
– Level 1 to Level 4–310 mm.
Figures are approximate as the original standard is quoted in inches. A few words need
to be said about the construction of the trays and conduits. The trays are to be
manufactured from metal and firmly grounded with complete continuity throughout the
length of the tray. The trays should also be fully covered preventing the possibility of any
area being without shielding.
1.10.1
Grounding requirements
This is a contentious issue and a detailed discussion laying out all the theory and practice
is possibly the only way to minimize the areas of disagreement. The picture is further
complicated by different national codes, which whilst not actively disagreeing with the
basic precepts of other countries, tend to lay down different practical techniques in the
implementation of a good grounding system.
A typical design should be based around three separate ground systems. They are:
1. The equipment (or instrument) ground
2. The chassis (or safety) ground
3. The earth ground.
The aims of these systems are:
• To minimize the electrical noise in the system
• To reduce the effects of fault or ground loop currents on the instrumentation
system
• To minimize the hazardous voltages on equipment due to electrical faults.
Ground is defined as a common reference point for all signals in equipment situated at
zero potential. Below 10 MHz, a single point grounding system is the optimum solution.
Two key concepts to be considered when setting up an effective grounding system are:
1. To minimize the effects of impedance coupling between different circuits (i.e.
when three different currents, for example, flow through a common impedance)
2. To ensure that ground loops are not created (for example, by mistakenly tying
the screen of a cable at two points to ground).
There are three types of grounding system possible as shown in Figure 1.17. The series
single point is perhaps more common; while the parallel single point is the preferred
approach with a separate ground system for different groups of signals.
1.10.2
Suppression techniques
It is often appropriate to approach the problem of electrical noise proactively by limiting
the noise at the source. This requires knowledge of the electrical apparatus that is causing
the noise and then attempting to reduce the noise caused here. The two main approaches
are shown here.
General topics 23
Figure 1.17
Various grounding configurations
Figure 1.18
Suppression networks (snubbers)
In Figure 1.18, the inductance will generate a back emf across the contacts when the
voltage source applied to it is switched off. This RC network then takes this back emf and
thus reduces damage to the contacts.
The voltage can be limited by various combinations of devices, depending on whether
the circuit is AC or DC.
Circuit designers should be aware that the response time of the coil could be reduced
significantly. For example, the dropout time of a coil can be increased by a factor of ten.
Hence, this should be approached with caution, where quick response is required from
regular switched circuits (apart from the obvious negative impact on safety due to
slowness of operation).
24
Practical Industrial Data Communications
Silicon-controlled rectifiers (SCRs) and triacs generate considerable electrical noise due
to the switching of large currents. A possible solution is to place a correctly sized
inductor in series with the switching device.
1.10.3
Filtering
Filtering should be done as close to the source of noise as possible. Table 1.1 summarizes
some typical sources of noise and possible filtering means.
Typical Sources of
Noise
Filtering Remedy
Comments
AC voltage varies
Improved ferroresonant
transformer
Conventional ferroresonant transformer fails
Notching of AC
waveform form
Improved ferroresonant
transformer
Conventional ferroresonant transformer fails
Missing half cycle in
AC waveform
Improved ferroresonant
transformer
Conventional ferroresonant transformer fails
Notching in DC line
Storage capacitor
For extreme cases active power line filters are
required
Random excessively
high-voltage
spikes or transients
Non-linear filters
Also called limiters
High-frequency
components
Filter capacitors across
the line
Called low-pass filtering – great care should
be taken with high frequency vs
performance of ‘capacitors’ at this
frequency
Ringing of filters
Use T filters
From switching transients or high level of
harmonics
60 Hz or 50 Hz
interference
Twin-T RC notch filter
networks
Sometimes low-pass filters can be suitable
Common mode
voltages
Avoid filtering (isolation
transformers or
common-mode filters)
Opto-isolation is preferred eliminates ground
loop
Excessive noise
Auto or cross correlation
techniques
Extracts the signal spectrum from the closely
overlapping noise spectrum
Table 1.1
Typical noise sources and some possible means of filtering
1.11
Ingress protection
The ingress protection (IP) rating system is recognized in most countries and is described
by several standards, including IEC 60529. It describes the degree of protection offered
by an enclosure. This enclosure can be of any description, including a cable, cable
assembly, connector body, the casing of a network hub, or a large cabinet used to enclose
electronic equipment.
General topics 25
Enclosures are rated in the format ‘IP xy’ or ‘IP xyz’.
• The first digit of the IP designation (x) describes the degree of protection
against access to hazardous parts and ingress of solid objects.
• The second digit (y) designates the degree of protection against water. Refer to
the appropriate sections of IEC 60529 for complete information regarding
applications, features, and design tests.
• The third digit (z) describes the degree of protection against mechanical
impacts and is often omitted. It does, e.g. apply to metal enclosures but not to
cables or cable assemblies.
Here follows a list of meanings attributed to the digits of the IP rating.
Protection Against
Foreign Objects
1st
2nd
Protection Against
Moisture
3rd
Protection
Against
Mechanical
Impacts
0
Not protected
0
Not protected
0
Not protected
1
Protected against objects
greater than 50 mm
diameter (e.g. hand
contact)
1
Protected against dripping
water (falling vertically,
e.g. condensation)
1
Impact 0.225 J
2
Protected against objects
greater than 12 mm
(e.g. fingers)
2
Protected against dripping
water when tilted 15° to
either side
2
Impact 0.375 J
3
Protected against objects
greater than 2.5 mm
(e.g. tolls, wires)
3
Protected against rain up
to 60° from vertical
3
Impact 0.60 J
4
Protected against objects
greater than 1.0 mm
(e.g. small tools, small
wires)
4
Protected against
splashing water, any
direction
4
N/a
5
Dust protected – limited
ingress permitted
(no harmful deposits)
5
Protected against water
jets (with nozzles)
5
Impact 2.00 J
6
Dust tight – totally protected
against dust (no deposits
at all)
6
Protected against heavy
seas
6
N/a
7
N/a
7
Protection against effects
of immersion
7
Impact 6.00 J
8
N/a
8
Protection against
submersion
8
N/a
9
N/a
9
N/a
9
Impact 20.00 J
For example, a marking of IP 68 would indicate a dust tight (first digit = 6) piece of
equipment that is protected against submersion in water (second digit = 8).
Copper cable
2.1
Cable characteristics
Two main types of copper cable are used in industrial communications. They are:
1. Coaxial cable, also referred to as coax
2. Twisted-pair cable, which can be shielded (STP/ScTP/FTP) or unshielded (UTP).
2.1.1
Cable structure
All copper cable types have the following components in common:
• One or more conductors to provide a medium for the signal
• Insulation of some sort around the conductors to help keep the signal in and
interference out
• An outer sheath, or jacket, to encase the cable elements. The sheath keeps the
cable components together, and may also help protect the cable components
from water, pressure, or other types of damage.
Conductor
For copper cables, the conductor is known as the signal (carrier) wire and may consist of
either solid or stranded wire. Solid wire is a single thick strand of conductive material,
usually copper. A stranded wire consists of many thin strands of conductive material
wound tightly together.
The signal wire is described in terms of the following:
• The type of conductive material
• Whether the wire is stranded or solid
• The carrier wire diameter, expressed directly (in inches, centimeters, or
millimeters) or in terms of the wire gauge as specified in the AWG (American
wire gage)
• The diameter of the strands, which determines some of the wire’s electrical
properties such as resistance and impedance. These properties, in turn, help
determine the wire’s performance.
Insulation
The insulating layer keeps the signal from escaping and helps to protect the signal from
outside interference. The insulation is usually made of a dielectric such as polyethylene.
Copper cable
27
Some types of coaxial cable have multiple protective layers around the signal wire. The
size of the insulating layer determines the spacing between the conductors in a cable and
therefore its capacitance and impedance.
Cable sheath
The outer casing (sheath) of the cable provides a shell that keeps the cable elements
together. The sheath differs for indoor and outdoor exposure. Outdoor cable sheaths tend
to be black, with appropriate resistance to UV light, and have enhanced water resistance.
Two main classes of sheath for indoor use are plenum and non-plenum.
For certain environments, local laws require plenum (a plenum is the space between a
ceiling and a roof) cable. It must be used when the cable is being run ‘naked’ (without being
put in a conduit) inside walls and above ceilings, and should probably be used whenever
possible. Plenum sheaths are made of non-flammable fluoropolymers such as Teflon or
Kynar. They are fire resistant and do not give off toxic fumes when burning. They are also
considerably more expensive than cables with non-plenum sheaths. Plenum cable specified
for networks installed in the USA should generally meet the NEC (national electrical code)
CMP (communications plenum cable) or CL2P (class 2 plenum cable) specifications.
Networks installed in other countries may have to meet equivalent safety standards, and
these should be determined before installation. The cable should also be underwriters
laboratories (UL) listed for UL-910, which subjects plenum cable to a flammability test.
Non-plenum cable uses less expensive material for the sheath, so it is consequently less
expensive, but it can often be used only under restricted conditions. Non-plenum cable
sheaths are made of polyethylene (PE) or polyvinyl chloride (PVC), which will burn
and give off toxic fumes. PVC cable used for networks should meet the NEC CMR
(communications riser cable) or CL2R (class 2 riser cable) specifications. The cable
should also be UL-listed for UL-1666, which subjects riser cable to a flammability test.
2.1.2
Cable packaging
Cables can be packaged in different ways, depending on what it is being used for and
where it is located.
The following types of cable packaging are available:
• Simplex cable: One cable within one sheath, which is the default configuration.
The term is used mainly for fiber-optic cable to indicate that the sheath contains
only a single fiber.
• Duplex cable: Two cables or fibers within a single sheath. In fiber-optic cable,
this is a common arrangement. One fiber is used to transmit in each direction.
• Multi-fiber cable: Multiple cables or fibers within a single sheath. For fiberoptic cable, a single sheath may contain thousands of fibers. For electrical cable,
the sheath will contain at the most a few dozen cables.
2.1.3
Factors affecting cable performance
Copper cables are good for signal transfer, but they are not perfect. Ideally, the signal at the
end of a length of cable should be the same as at the beginning. Unfortunately, this will not
be true in practice. All signals degrade when transmitted over a distance through any
medium. This is because its amplitude decreases as the medium resists the flow of energy,
and signals become distorted because the higher frequencies are attenuated more than the
lower ones. Any transmission also consists of signal and noise components. Signal quality
degrades for several reasons, including attenuation, crosstalk, and impedance mismatches.
28
Practical Industrial Data Communications
Attenuation
Attenuation is the decrease in signal strength, measured in decibels (dB) per unit length.
Attenuation occurs more quickly at higher frequencies and when the cable resistance is
higher.
In networking environments, repeaters are responsible for regenerating a signal before
passing it on. Many devices such as hubs are, in fact, repeaters without explicitly saying
so. Since attenuation is sensitive to frequency, some situations require the use of
equalizers.
Characteristic impedance
The impedance of a cable is defined as the resistance offered to the flow of electrical
current at a particular frequency. The characteristic impedance is the impedance of an
infinitely long cable, where the signal never reaches the end of the cable and hence
cannot bounce back. The same situation is replicated when a cable is terminated. A short
cable terminated in its characteristic impedance appears electrically to be infinitely long
and has no signal reflected from the remote end. If one cable is connected to another of
differing characteristic impedance, then there is some reflection at the junction. These
reflections cause interference with the data signals and must be avoided (Figure 2.1).
Figure 2.1
Characteristic impedance ZO = ZL
Crosstalk
Crosstalk is electrical interference in the form of signals picked up from a neighboring
cable or circuits. For example, signals on different wires in a multistranded twisted-pair
cable may interfere with each other.
The following forms of crosstalk measurement are important for twisted-pair cables:
• Near-end crosstalk (NEXT): NEXT measurements (in dB) indicate the degree
to which unwanted signals are coupled onto adjacent wire pairs. This unwanted
‘bleeding over’ of a signal from one wire pair to another can disturb the desired
signal. As the name implies, NEXT is measured at the ‘near end’ or the end
closest to the transmitted signal. NEXT is a ‘pair-to-pair’ reading, where each
wire pair is tested for crosstalk relative to another pair. NEXT increases as the
frequency of transmission increases (see Figure 2.2).
Copper cable
29
Figure 2.2
Near end crosstalk (NEXT)
• Far-end crosstalk (FEXT): This is similar in nature to NEXT, but crosstalk is
measured at the opposite end from the transmitted signal. FEXT tests are
affected by signal attenuation to a much greater degree than NEXT, since it is
measured at the far end of the cabling link where signal attenuation is greatest.
Therefore, FEXT measurements are a more significant indicator of cable
performance if attenuation is accounted for.
• Equal-level far-end crosstalk (ELFEXT): The comparative measurement of
FEXT and attenuation is called equal-level far-end crosstalk or ELFEXT.
ELFEXT is the arithmetic difference between FEXT and attenuation.
Characterizing ELFEXT is important for cabling links intended to support fourpair full-duplex network transmissions.
• Attenuation-to-crosstalk ratio (ACR): ACR is not specifically a new test, but
rather a relative comparison between NEXT and attenuation performance.
Expressed in decibels (dB), the ratio is the arithmetic difference between NEXT
and attenuation. ACR is significant because it is more telling of cable
performance than NEXT or attenuation alone. ACR is a measure of the strength
of a signal compared to the crosstalk noise.
• Power sum NEXT: Power sum NEXT (in dB) is calculated from the six
measured pair-to-pair crosstalk results. Power sum NEXT differs from pairto-pair NEXT by determining the crosstalk induced on a given wire pair from
three disturbing pairs. This methodology is critical for supporting transmissions
that utilize all four pairs in the cable such as Gigabit Ethernet (Figure 2.3).
2.2
Cable selection
Cables are used to meet all sorts of power and signaling requirements. The demands
made on a cable depend on the location in which the cable is used and the function for
which the cable is intended. These demands, in turn, determine the features a cable
should have.
30
Practical Industrial Data Communications
Figure 2.3
PSNEXT and PSFEXT
2.2.1
Function and location
Here are a few examples of considerations involving the cable’s function and location:
• Cable designed to run over long distances, such as between floors or buildings,
should be robust against environmental factors (moisture, temperature changes,
etc.). This may require extra sheaths or sheaths made with a special material.
• Cable that must run around corners should bend easily, and the cable’s properties
and performance should not be affected by the bending. For several reasons,
twisted-pair cable is probably the best cable for such a situation (assuming it
makes sense within the rest of the wiring scheme). Of course, another way to get
around a corner is by using a connector; however, connectors may introduce
signal-loss problems.
• Cable that must run through areas in which heavy current motors are operating
(or worse, being turned on and off at random intervals) must be able to withstand
magnetic interference. Large currents produce strong magnetic fields, which can
interfere with and disrupt nearby signals. Because it is not affected by such
electrical or magnetic fluctuations, fiber-optic cable is the best choice in
machinery-intensive environments.
• If you need to run many cables through a limited area, cable weight can become
a factor, particularly if all that cable will be running in the ceiling above you. In
general, fiber-optic and twisted-pair cables tend to be lightest.
• Cables installed in barely accessible locations must be particularly reliable. It
is worth considering installing a backup cable during the initial installation.
Because the installation costs in such locations are generally much more than the
cable material cost, installation costs for the second cable add only marginally to
the total cost.
• Cables that need to interface with other worlds (e.g. with a mainframe network or
a different electrical or optical system) may need special properties or adapters.
The kinds of cable required will depend on the details of the environments and
the transition between them.
Copper cable
2.2.2
31
Main cable selection factors
Along with the function and location considerations, cable selection is determined by a
combination of factors, including the following:
• The type of installation being planned (e.g. Ethernet or RS-485). While it is
possible to use just about any type of cable in any type of network, certain cable
types have been more closely associated with particular network types. In many
cases, the exact type of cable is prescribed in the standard or the equipment vendor.
Examples are:
–
–
–
–
EIA/TIA -568 Cat5 cable for 100 Mbps Ethernet
Belden 9841-9844 or similar for RS-485
Furon 1T52-8820-234 or Belden Databus 3076F for Foundation Fieldbus
Belden 3082A, 3083A, 3084A, and 3085A for DeviceNet.
• The amount of money available for the network. Keep in mind that cable
installation can be an expensive part of the network costs.
• Whatever cabling resources are already available (and useable). There may be
available wiring that could conceivably be used. It is almost certain, however,
that at least some of that wire is defective or is not up to the requirements for
the new application.
• Building or other safety codes and regulations.
2.3
Coaxial cables
In coaxial cable, two or more separate materials share a common central axis. Coax is
used for radio frequency and data transmission. The cable is remarkably stable in terms of
its electrical properties at frequencies below 4 GHz, and this makes the cable popular for
cable television transmissions as well as for creating local area networks (LANs).
2.3.1
Coaxial cable construction
A coaxial cable consists of the following layers (moving outward from the center) as
shown in Figure 2.4.
Carrier wire
A conductor wire or signal wire is in the center. This wire is usually made of copper and
may be solid or stranded. There are restrictions regarding the wire composition for certain
network configurations. The diameter of the signal wire is one factor in determining the
attenuation of the signal over distance. The number of strands in a multistrand conductor
also affects the attenuation.
Insulation
An insulation layer consists of a dielectric around the carrier wire. This dielectric is
usually made of some form of polyethylene or Teflon.
Foil shield
This thin foil shield around the dielectric usually consists of aluminum bonded to both
sides of a tape. Not all coaxial cables have foil shielding. Some have two foil shield
layers, interspersed with copper braid shield layers.
32
Practical Industrial Data Communications
Figure 2.4
Cross section of coaxial cable
Braid shield
A braid (or mesh) conductor made of copper or aluminum that surrounds the insulation
and foil shield. This conductor can serve as the ground for the carrier wire. Together with
the insulation and any foil shield, the braid shield protects the carrier wire from
electromagnetic interference (EMI) and radio frequency interference (RFI). The braid
and foil shields provide good protection against electrostatic interference when earthed
correctly, but little protection against electromagnetic interference.
Sheath
This is the outer cover that can be either plenum or non-plenum, depending on its
composition. The signal and shield wires are concentric (coaxial) and hence the name.
2.3.2
Coaxial cable performance
The main features that affect the performance of coaxial cable are its composition,
diameter, and impedance. The carrier wire’s composition determines how good a
conductor the cable will be. Cable diameter helps determine the electrical demands that
can be made on the cable. In general, thick coaxial cable can support a much higher level
of electrical activity than thin coaxial cable.
Impedance is a measure of opposition to the flow of alternating current. The properties
of the dielectric between the carrier wire and the braid help determine the cable’s
characteristic impedance, which in turn imposes limits on where the cable can be used.
For example, Ethernet and ARCnet architectures can both use thin coaxial cable, but they
have different characteristic impedances and so Ethernet and ARCnet cables are not
compatible. Most LAN cables have an RG (recommended gage) rating and cable with the
same RG rating from different manufacturers can be safely mixed (Table 2.1).
Copper cable
Recommended
Gage
Application
Characteristic
Impedance (Ω)
RG-8
10BASE-5
50
RG-58
RG-59
10BASE-2
CATV
50
75
RG-2
ARCnet
93
33
Table 2.1
Common network coaxial cable impedances
In LANs, the characteristic cable impedances range from 50 Ω (RG-8/u or RG-58/u for
Ethernet) to 93 Ω (RG-59/u or RG-62/u for ARCnet). The impedance of the coaxial cable
is given by the formula:
Z=
138
D
log
d
k
Where k is the dielectric constant of the insulation.
Advantages of coaxial cable
Coaxial cable has the following general advantages over other types of cable that might be
used for a network. These advantages may change or disappear over time, as technology
advances and products improve:
• Broadband coaxial can be used to transmit voice, data, and even video.
• The cable is relatively easy to install.
• Coaxial cable is reasonably priced compared with other cable types.
Disadvantages of coaxial cable
Coaxial cable has the following disadvantages when used for a network:
• It is easily damaged and sometimes difficult to work with, especially in the case
of thick coaxial.
• Coaxial is more difficult to work with than twisted-pair cable.
• Thick coaxial cable can be expensive to install, especially if it needs to be pulled
through existing cable conduits.
• Connectors can be expensive.
Coaxial cable faults
The main problems encountered with coaxial cables include the following:
• Open- or short-circuited cables: The cable may have an open or short circuit due
to badly installed connectors or the cable may get crushed during the installation
process. Check that the terminating resistors are fitted and have the correct value.
• Characteristic impedance mismatches: Obviously, cables must be used with
the correct impedance connectors. RG-62 and RG-58 use similar BNC
connectors but have different characteristic impedances (93 vs 50 Ω).
34
Practical Industrial Data Communications
2.4
Twisted-pair cable
Twisted-pair cable is widely used, inexpensive, and easy to install. Twisted-pair cable
comes in two main varieties namely shielded (STP, FTP, and ScTP) and unshielded
(UTP).
It can transmit data at an acceptable rate, up to 1000 Mbps in some network
architectures. The most common twisted-pair wiring is telephone cable, which is
unshielded and is usually voice-grade, rather than the higher quality data-grade cable used
for networks.
In a twisted-pair cable, two conductor wires are wrapped around each other. Twisted
pairs are made from two identical insulated conductors, which are twisted together along
their length at a specified number of twists per meter, typically forty twists per meter
(twelve twists per foot). The wires are twisted to reduce the effect of electromagnetic and
electrostatic induction.
For full-duplex digital systems using balanced transmission, two sets of screened
twisted pairs are required in one cable; each set with individual and overall screens.
A protective PVC sheath then covers the entire cable.
The capacitance of a twisted pair is fairly low (about 40 to 160 pF/m), allowing a
reasonable bandwidth and an achievable slew rate. A signal is transmitted differentially
between the two conductor wires. The current flows in opposite directions in each wire of
the active circuit, as shown in Figure 2.5.
Figure 2.5
Current flow in a twisted pair
Elimination of noise
The twisting of associated pairs and method of transmission reduces the interference from
the other pairs of wire throughout the cable.
The transmitted signals are in the form of changes of electrical state. An encoding
process turns ones and zeroes into these signals. In a twisted-pair system, the required
signal as well as its mirror image is transmitted simultaneously over the TX+ and TX–
wires, respectively. Since these wires are of the same length and have the same
construction, the signal travels at the same rate. Since the pairs are twisted together, any
outside electrical interference will affect both wires in the same way.
The transmissions of the original signal and its mirror image reach a destination
transceiver and are fed into a differential amplifier. Because the signals on the TX+ and
TX– wires are of opposite polarity but of equal magnitude they add up, but any induced
noise, being of the same magnitude and polarity, will cancel out (Figure 2.6).
Since the currents in the two conductors are equal and opposite, their induced magnetic
fields also cancel each other. This type of cable is therefore self-shielding and is less
prone to interference.
Copper cable
35
Magnetic fields cancel
Insulator
Current flowing out of this conductor
Conductor
Current flowing into this conductor
Figure 2.6
Magnetic shielding of twisted-pair cables
Twisting within a pair minimizes crosstalk between pairs. The twists also help deal
with EMI and RFI as well as balancing the mutual capacitance of the cable pair. The
performance of a twisted-pair cable can be influenced by changing the number of twists
per meter in a wire pair.
2.4.1
Components of twisted-pair cable
A twisted-pair cable has the following components.
Conductor wires
The signal wires for this cable come in pairs that are wrapped around each other. The
conductor wires are usually made of copper. They may be solid (consisting of a single
wire) or stranded (consisting of many thin wires wrapped tightly together). A twisted-pair
cable usually contains multiple twisted pairs; 2, 4, 6, 8, 25, 50, or 100 twisted-pair
bundles are common. For network applications, two- and four-pair cables are most
commonly used.
Shield
The terminology is slightly confusing as different methods are used for shielding. Some
STP (shielded twisted pair) cables such as IBM type 1 cable have a shield in the form of a
woven copper braid. In addition, each pair is individually wrapped in a foil screen. For
Ethernet, screened Cat5 is normally used. Some versions of screened Cat5 have a full foil
screen around all four pairs while some have individual foil screens around each pair.
These are called ScTP (screened twisted pair) or FTP (foil twisted pair).
Sheath
The wire bundles are encased in a sheath made of PVC or, in plenum cables, of a fireresistant material, such as Teflon or Kynar.
UTP cabling most commonly includes four pairs of wires enclosed in a common sheath.
The typical single UTP cable is a PVC or plenum-rated plastic jacket containing four
pairs of wire, and the majority of facility cabling in current and new installations is four
pair cable of this type. The dedicated single connections made using four pair cable are
36
Practical Industrial Data Communications
easier to troubleshoot and replace than the alternative, bulk multipair cable such as
25-pair cable.
The jacket of each wire in a four pair cable will have an overall color: brown, blue,
orange, green, or white. In a four pair UTP cable (the typical UTP used in networking
installations), there is one wire each of brown, blue, green, and orange, and four wires of
which the overall color is white. The white wires are distinguished from one another by
periodically placed (usually within 1/2 in. of one another) rings of the other four colors.
Wires with a unique base color are identified by that base color: blue, brown, green, or
orange. Those wires that are primarily white are identified as white/<color>, where <color>
indicates the color of the rings of the other four colors in the white insulator.
2.4.2
EIA/TIA-568 cable categories
To distinguish between varieties of UTP, the United States Electronic Industries
Association/Telecommunications Industries Association (EIA/TIA) has formulated
several categories. The electrical specifications for these cables are detailed in EIA/TIA568A, TSB-36, TSB-40, and their successor SP2840. These categories are:
Category 1 (Cat1)
Voice-grade, UTP telephone cable. This describes the cable that has been used for years in
North America for telephone communications. Officially, such cable is not considered
suitable for data-grade transmissions. In practice, however, it works fine over short
distances and under ordinary working conditions. Be aware that other telecommunications
providers have often used cable that does not even come up to this minimum standard and
as such is very unacceptable for data transmission.
Category 2 (Cat2)
Voice-grade UTP, capable of supporting transmission rates of up to 4 Mbps. IBM Type 3
cable falls into this category.
Category 3 (Cat3)
Data-grade UTP used extensively for supporting data transmission rates of up to
10 Mbps. An Ethernet 10BASE-T network cabled with twisted pair requires at least this
category of cable. Cat3 UTP cabling must not produce an attenuation of a 10 MHz signal
greater than 98 dB/km at the control temperature of 20 °C. Typically, Cat3 cable
attenuation increases 1.5% per degree Celsius.
Category 4 (Cat4)
Data-grade UTP, capable of supporting transmission rates of up to 16 Mbps. An IBM
Token Ring network transmitting at 16 Mbps requires this type of cable.
Cat4 UTP cabling must not produce an attenuation of a 10 MHz signal greater than
72 dB/km at the control temperature of 20 °C.
Category 5 (Cat5)
Data-grade UTP capable of supporting transmission rates of up to 155 Mbps (but
officially only up to 100 Mbps). Cat5 cable is constructed and insulated such that the
maximum attenuation of a 10 MHz signal in a cable run at the control temperature of
20 °C is 65 dB/km.
Copper cable
37
TSB-67 contains specifications for the verification of installed UTP cabling links that
consist of cables and connecting hardware specified in the TIA-568A standard.
Enhanced Category 5 (Cat5e)
This standard specifies transmission performance that exceeds Cat5. Cat5 supports the
vast majority of existing network types such as 100BASE-T (Fast Ethernet) and ATM
155, but lacks the test parameters, are critical for support of new network technologies
such as 1000BASE-T (Gigabit Ethernet). Cat5e directly supports the needs of Gigabit
Ethernet. Test parameters include wire map, length, propagation delay, delay skew,
attenuation, NEXT (pair-to-pair as well as Power Sum, PSELFEXT, and return loss).
Frequency range: To be measured from 1 through 100 MHz (not 100 Mbps).
Category 6 (Cat6)
Cat6 is aimed at delivering a 100 m (330 ft) channel of twisted-pair cabling providing a
minimum ACR at 200 MHz that is approximately equal to the minimum ACR of a Cat5
channel at 100 MHz.
Cat6 includes all of the Cat5e parameters but extends the test frequency to 200 MHz,
greatly exceeding current Cat5 requirements. (The IEEE has proposed extending the test
frequency to 250 MHz to characterize links that may be marginal at 200 MHz.) It is likely
that Cat6 will be the most demanding standard for four-pair UTP terminating with RJ-45
connectors. Like Cat5e, Cat6 is intended to support emerging networks such as Gigabit
Ethernet. Cat6 provides a mechanism to certify to a level much greater than Cat5,
providing assurance that the cabling infrastructure will support future networks that may
require greater than 100 MHz bandwidth.
Test parameters include all the performance parameters that have been specified for
Cat5e. Cat6 components and links are to be tested to 250 MHz even though the ACR
values for the installed links are negative at 250 MHz.
Note: Several vendors are promoting ‘proprietary Category 6 solutions’. These
‘proprietary Category 6’ cabling systems only deliver a comparable level of performance
if every component of connecting hardware (socket and plug combination) is purchased
from the same vendor and from the same product series.
Specially selected eight-pin modular connector jacks and plugs need to be matched
because they are designed as a ‘tuned pair’ in order to achieve the high level of crosstalk
performance for NEXT and FEXT. If the user mixes and matches connector components,
the system will no longer deliver the promised ‘Cat6-like’ performance.
Category 7 (Cat7)
This is a shielded twisted-pair (STP) standard. This type of cabling may very likely only
exist as the ISO Class F specification (possibly as STP-B in the USA). The cable people
refer to when discussing Cat7 is constructed with individually shielded wire pairs with an
overall shield around the four-shielded pairs. This cable is expensive and very difficult
(i.e. expensive) to install. The cable itself provides a positive ACR value at 600 MHz but
with a very weak signal (attenuation is 64 dB at 600 MHz). The installed link may be
usable to 350 MHz.
Cat7 terminations will be something other than RJ-45 due to the inherent bandwidth
limitations of the RJ-45 style connectors. Cat7 will not only support emerging high-speed
technologies but will meet the bandwidth requirements of future networks that demand a
much higher level of performance from the cabling infrastructure.
38
Practical Industrial Data Communications
Advantages of twisted-pair cable
Twisted-pair cable has the following advantages over other types of cables for networks:
• It is easy to connect devices to twisted-pair cable.
• If an already installed cable system, such as telephone cable, has extra, unused
wires, you may be able to use a pair of wires from that system – but see the
warnings on this above.
• STP does a reasonably good job of blocking interference.
• UTP is quite inexpensive.
• UTP is very easy to install.
• UTP may already be installed (but make sure it all works properly and that it
meets the performance specifications your network requires).
Disadvantages of twisted-pair cable
Twisted-pair cable has the following disadvantages:
• STP is sometimes bulky and difficult to work with.
• UTP is more susceptible to noise and interference than coaxial or fiber-optic
cable.
Skin effect can increase attenuation. This occurs when transmitting data at a fast rate
over twisted-pair wire. Under these conditions, the current tends to flow mostly on the
outside surface of the wire. This greatly decreases the cross-section of the wire being
used, and thereby increases resistance. This, in turn, increases signal attenuation.
2.4.3
Problems with unshielded twisted pairs
There are generally six types of faults with UTP cabling which will be discussed here. They
are: attenuation, crossed pairs, split pairs, connector problems, termination practices, and
crosstalk.
Attenuation
If the cable tester identifies a problem with attenuation, the following items should be
checked:
• Excessive length of the cable (normally 100 m (330 ft) maximum between hub
and node)
• Inferior quality cable (e.g. Cat3 used in place of Cat5)
• Rising temperatures (temperatures higher than 40 °C are not recommended)
• Badly installed connectors and connections.
Crossed pairs
For normal connections between a workstation and a hub, a straight-through cable is
normally used. The hub port then has an X mark indicating that the connector at the hub
actually performs the crossover function. In the total scheme of things, crossover is
required between the network interface card and the hub (similar to a DTE connected to a
DCE device where the transmitter on the one end has to be connected to a receiver at the
other end).
Copper cable
39
Normally, one of the ports on the hub can be set up as a straight port or an X
(crossover) port. This then allows hubs to be daisy-chained together.
Crossed-pair problems can be detected by a simple continuity check using an
inexpensive cable tester or by observing the colors of the wires in the RJ-45 connectors.
Split pair problems
Physical continuity is maintained but conductors from different pairs are combined
(e.g. pin-2 is twisted with pin-6 and pin-3 is twisted with pin-1). This sort of problem is
unlikely to be detected with a simple continuity tester; but can be identified with low
values of NEXT (22 dB or lower). The reason is that the electrical circuits in the pairs
have enhanced coupling between them and do not have equal and opposite currents in
each leg, resulting in greater crosstalk.
Connector problems
Ensure that the right type of connector is used. Stranded wire connectors have one blade,
which pierces the strand; whilst solid wire connectors have two blades in the pin
connector, which makes a firm grip on the conductor.
Termination problems
The checks that can be made here are:
• Ensure that the cable is twisted as far as possible on the connector and maintains
a tight degree of twisting.
• Avoid the use of the old 66 type blocks (which have high attenuation and
crosstalk).
• Ensure that voice and data cables are terminated on different punch blocks.
Crosstalk
If the cable tester indicates a high value of crosstalk check for the following items:
•
•
•
•
2.5
Split pairs
Use of silk line cables
Voice and data cables mixed up
Bad terminations.
Distribution/installation standards
There are various standards dealing with installations. These include (but are not limited to):
• ANSI/EIA/TIA-T568: Commercial Building Telecommunication Standard
• ANSI/EIA/TIA-569: Commercial Buildings, Standards for telecommunications
Pathways and Spaces
• ANSI/EIA/TIA-606: Telecommunications Infrastructure Administration Standard
• ISO 11801: Generic Cabling for Customer Premises
• EIA/TIA-570: Residential and Light Commercial Telecommunications Wiring
Standard
• EIA/TIA TR41.8.4 Outside Plant
• AT&T Zone Wiring Standard
40
Practical Industrial Data Communications
• NEMA (National Electrical Manufacturers Association). Several standards,
e.g. WC63.1 (Twisted-Pair, Premise Wiring Products)
• ICEA (Insulated Cable Engineers Association/Telecommunications Wire and Cable
Standards/Technical Advisory Committee – (ICEA/TWCS/TAC). Several standards,
e.g. S-90-661 Indoor Wiring Standard (Category Cables)
• IEEE 518: Guide for the Installation of Electrical Equipment to Minimize
Electrical Noise Inputs to Controllers from External Sources.
Whereas most of these standards are of US origin, there are also specific standards
originating in other countries, e.g.
• CAN/CSA-T528-93: Design Guidelines for Administration of Telecommunications Infrastructure (Canada)
• AUSTEL TS 008 and TS 009 (Australia)
• JIS X 5150 (Japan).
IEEE 518 deals with the identification of electrical noise in control circuits, the
significance of electrical noise, sources of electrical noise, coupling of electrical noise,
susceptibility of control circuits, electrical noise classification, electrical noise susceptibility
classification, wiring type classification, shielding, filtering, buffering, noise immunity
testing, installation recommendations, and wiring practices.
For situations where there are a large number of cables varying in voltage and current
levels, the IEEE 518 standard provides a useful set of tables indicating separation
distances for various classes of cables. There are four classification levels of
susceptibility for cables. Susceptibility, in this context, is understood to be an indication
of how well the signal circuit can differentiate between the undesirable noise and
required signal. It follows that a data communication physical standard such as RS-232
would have a high susceptibility and a 1000 V, 200 A AC cable would have a low
susceptibility.
The four susceptibility levels defined by the IEEE 518 standard are briefly:
Level 1 (high) This is defined as analog signals less than 50 V and digital signals less than
15 V. This would include digital logic buses and telephone circuits. Data communication
cables fall into this category.
Level 2 (medium) This category includes analog signals greater than 50 V and switching
circuits with voltages less than 50 V.
Level 3 (low) This includes switching signals greater than 50 V and analog signals
greater than 50 V. Currents less than 20 A are also included in this category.
Level 4 (power) This includes voltages in the range 0–1000 V and currents in the range
20–800 A. This applies to both AC and DC circuits.
IEEE 518 also provides for three different situations when calculating the separation
distance required between the various levels of susceptibilities. In considering the specific
case where one cable is a high-susceptibility cable and the other cable has a varying
susceptibility, the required separation distance would vary as follows:
Both cables contained in a separate tray
• Level 1 to Level 2: 30 mm
• Level 1 to Level 3: 160 mm
• Level 1 to Level 4: 670 mm.
Copper cable
41
One cable contained in a tray and the other in conduit:
• Level 1 to Level 2: 30 mm
• Level 1 to Level 3: 110 mm
• Level 1 to Level 4: 460 mm.
Both cables contained in separate conduits:
• Level 1 to Level 2: 30 mm
• Level 1 to Level 3: 80 mm
• Level 1 to Level 4: 310 mm.
Figures are approximate as the original standard is quoted in inches.
A few words need to be said about the construction of the trays and conduits. It is
expected that the trays be manufactured from metal and firmly grounded with complete
continuity throughout the length of the tray. The trays should also be fully covered
preventing the possibility of any area being without shielding.
2.6
Connector standards
There is a wide range of connectors used by data communications equipment and to deal
with all of them here would be impossible. This section will therefore discuss only a few
of the more common and universally used connectors.
One of the older connectors around is the D-Subminiature connector (also called
D-Submin, D-type, DB, or D connectors). They are specified for use with RS-232 and
have recently also been used in Industrial Ethernet equipment, although they are not
mentioned specifically in the Ethernet specification. Although they are fairly rugged, they
are not dust or watertight and can only therefore only be used in IP20 environments, i.e.
inside enclosures, in a plant (Figure 2.7).
Figure 2.7
DB-9 connector
Another type of connector that has become very popular due to Ethernet is the RJ-45
connector. This 8-pin plastic connector resembles a RJ-11 telephone connector; it is just a
little wider. For applications in IP20 environments, requiring mounting and cabling of the
equipment in a protected environment such as an enclosure, the RJ-45 has become a
de facto standard for networking applications. New and improved connectors that fit the
requirements based on the RJ-45 according to EN 60603-7 are also available. The RJ-45
is, in fact, recommended under IP20 conditions by the PNO/PTO (Profibus user
organization) (Figure 2.8).
Ethernet I/O devices have become part of modern control systems. Ethernet makes it
possible to use a variety of TCP/IP protocols to communicate with decentralized
components virtually to sensor level. As a result, Ethernet is now installed in areas that
were always the domain of traditional fieldbus systems. These areas demand IP67 class
protection against dirt, dust, and fluids. This requires that suitable connector technology
42
Practical Industrial Data Communications
meeting IP67 standards have to be defined for transmission speeds up to 100 Mbps.
Two solutions to this problem are emerging. The one is a modified RJ-45 connector
(Figure 2.9) while the other is an M12 (micro-style) connector (Figure 2.10).
Figure 2.8
RJ-45 connector
Figure 2.9
Modified RJ-45 connector (RJ-LNxx) (Courtesy: AMC Inc)
Standardization groups are addressing the problem both nationally and internationally.
User organizations such as IAONA (Industrial Automation Open Networking Alliance),
Profibus user organization, and ODVA (Open DeviceNet Vendor Association) are also
trying to define standards within their organizations.
Network connectors for IP67 are not easy to implement. Three different approaches can
be found. First, there is the RJ-45 connector sealed within an IP67 housing. Then, there is
an M12 (micro-style) connector with either four or eight pins. A third option is a hybrid
connector based on RJ-45 technology with additional contacts for power distribution.
Two of the so-called sealed RJ-45 connectors are in the process of standardization.
Initially the 4-pin M12 version will be standardized in Europe. Connectors will be tested
against existing standards (e.g. VDE 0110) and provide the corresponding IP67 class
protection at 100 Mbps. In the US, the ODVA has standardized a sealed version of the
RJ-45 connector for use with Ethernet/IP.
For the use of the standard M12 in Ethernet, systems are covered in standard EN
61076-2-101. The transmission performance of 4-pin M12 connector for Ethernet up to
100 Mbps is comparable, if not better than standardized office grade Ethernet products.
In office environments, four-pair UTP cabling is common. For industrial applications,
two-pair cables are less expensive and easier to handle. Apart from installation
difficulties, 8-pin M12 connectors may not meet all the electrical requirements described
in EN 50173 or EIA/TIA-568B (Figure 2.10).
Copper cable
43
Figure 2.10
M12 connector (EtherMate) (Courtesy: Lumberg Inc.)
2.7
Earthing/grounding
This is a contentious issue and a detailed discussion laying out all the theory and practice is
possibly the only way to minimize areas of disagreement. The picture is further complicated
by different national codes, which whilst not actively disagreeing with the basic precepts of
other countries, tend to lay down different practical techniques in the implementation of a
good earthing system. There is also a great deal of confusion between what exactly is meant
by earthing and grounding, the two words often (incorrectly) used as synonyms of each other.
‘Ground’ is defined as a common reference point for all signals in equipment, situated
at zero potential. Therefore, the ideal ground is actually the planet Earth since it can
source or sink an almost indefinite amount of energy without changing its potential.
There are essentially three forms of grounding for electronic circuits.
The first is the ‘circuit ground’ or ‘circuit common’, indicated by a triangular shape.
This refers, for example, to the common ground area on a printed circuit board, to which
all circuitry on the board is referenced. Generally speaking, this would be the ground
reference for the electronics of a data communication system (Figure 2.11).
Figure 2.11
Symbol for common ground
A second type of ground is the ‘protective ground’ or ‘chassis ground’, represented by a
rectangular shape. This could be, for example, the metal casing of an instrument or rackmounted device. In the case of a rack-mounted device, it would be bonded to the rack in
which the device is mounted. The circuit ground needs to be connected to the chassis
ground, either directly or through a resistor (e.g. 100 Ω 1/2 W) to limit any currents that
may flow due to potential differences (Figure 2.12).
Finally, there is the ‘green wire ground’, ‘power system ground’ or ‘earth ground’. This
is connected to the actual soil and forms the connection to ‘planet Earth’. This connection
should always be treated with some caution, as it cannot be automatically assumed that it
has been done properly. In fact, the connection to ‘earth’ could very well only exist at the
nearest electrical sub-station (Figure 2.13)!
44
Practical Industrial Data Communications
Figure 2.12
Symbol for chassis ground
Figure 2.13
Symbol for earth ground
The symbols shown so far are not the only ones. ISO 7000 and IEC 60417 define the `
following symbols (Figure 2.14).
Figure 2.14
IEC 60417 symbols
No. 5018, ‘noiseless earth’ is used to identify a noiseless (clean) earth terminal (e.g. of
a specially designed earthing system to avoid equipment malfunction).
No. 5019, ‘protective earth’, is used to identify any terminal intended for connection to
an external conductor (for protection against electrical shock in case of a fault) or the
terminal of a protective earth electrode.
For specific examples of when to use these symbols, refer to EN 204, ‘Safety of
Machinery-Electrical Equipment of Machines’.
A good grounding system serves to minimize the electrical noise in the system, reduce
the effects of fault or ground loop currents on the instrumentation system and minimize
the hazardous voltages on equipment due to electrical faults.
Below 10 MHz, a single point grounding system is the optimum solution. Two key
concepts to be considered when setting up an effective grounding system are:
1. To minimize the effects of impedance coupling between different circuits
(i.e. when three different currents, for example, flow through a common impedance) and
2. To ensure that ground loops are not created, for example, by mistakenly tying
the screen of a cable to earth at two points.
There are three types of grounding systems possible as shown in Figure 2.14. The series
single point is perhaps the most common, while the parallel (multiple) single point is the
Copper cable
45
preferred approach with a separate grounding system for each of the following groups of
signals:
•
•
•
•
2.8
Safety or power ground
Low-level signal (or instrumentation) ground
High-level signal (motor controls) ground
Building ground.
Termination
A piece of wire carrying a signal from point A to point B acts as a transmission line.
Signals propagate along copper wire at approximately two-thirds the speed of light, i.e.
2 × 108 m/s or 200 m/µs. If the furthest end of the cable is unterminated, the signal is
reflected in phase and travels back towards the transmitter. In doing this, it collides with
oncoming signals and the two signals become electrically superimposed, thereby
corrupting the signal being transmitted. The reflected signal, once it reaches the original
point of departure, gets reflected towards the furthest end once more. Fortunately, the
wire has some resistance and due to its attenuating effect on the reflected signal, this will
eventually stop (Figure 2.15).
Figure 2.15
Terminated vs unterminated waveforms
The reflections also lead to the formation of standing waves along the transmission
line, which create problems if the transmission line is a bus with several receivers
attached to it. If a receiver is located at one of the ‘troughs’ it might not be able to
receive anything. If, instead of being open circuit, the furthest end of the cable is short
circuited, the signal is simply reflected back inverted (180° out of phase), but the effect
is the same.
The only way to eliminate this phenomenon is by making the cable infinitely long, so that
the signal travels onwards forever and never gets reflected, or, more realistically, by making
the cable appear infinitely long. This is done by terminating the cable with a resistor (RT)
equal to the characteristic impedance of the cable (Zo). The signal reaching the terminator
now gets transformed into heat (Ohm’s law: P = V2/RT) and does not return, creating the
impression the cable is infinitely long.
It should be obvious that if the bit rate is relatively low and/or the cable is relatively
short, the reflections (although they still exist) will not be much of a problem. In general,
if the transient effect due to reflections dies out in less than 10% of the length of a single
bit, it should not pose much of a problem.
Assuming the network cable is long enough for transmission-line effects to become
relevant, a termination method has to be decided upon. There are quite a few termination
methods available and National Semiconductor has published an application note (AN-903)
that describes several techniques. Some of the simpler ones are discussed here.
46
Practical Industrial Data Communications
It is, of course, possible that an unterminated network is an option. Unterminated
networks are simple to build (no additional terminating components) but the data rates
must be quite slow or the cable length must be short for the network to operate reliably.
Parallel termination (termination resistor at the furthest end) works well but it is limited to
networks that only have one driver (e.g. RS-422). The driver must be located on one end of
the network and the termination resistor must be located at the opposite end. The resistor,
Rp, should have the same value as the characteristic impedance (Zo) of the transmission line.
Zo for various cables are published by manufacturers. The larger the Zo, the less power Rp
must dissipate as heat. A 1/4 W or 1/2 W resistor is usually fine (Figure 2.16).
Figure 2.16
Parallel termination
Most twisted-pair cables have a Zo in the range 100–130 Ω whereas coaxial cables have
Zo in the range 50–100 Ω. The characteristic impedance of a given cable depends on the
diameters and spacing of the conductors, as well as the dielectric used.
The third termination technique is a bi-directional termination, which offers excellent
signal integrity. With this technique, the line drivers can be anywhere on the network.
This technique is probably the most reliable RS-485 termination technique (Figure 2.17).
Figure 2.17
Bi-directional termination
The fourth technique is called AC termination. The idea is to use a capacitor as a DC
blocking element to reduce power consumption in the terminating resistor. This method is
tricky since the RC combination effectively forms a low-pass filter, hence the value of C
has to be chosen so that the signal is not affected adversely. The inverse of τ , the time
constant = RC, should be at least 10 times higher than the maximum signal frequency
component. This type of termination is necessary on buses where the DC power and the
signal is carried on the same pair of wires, as is the case with Foundation Fieldbus.
Typical component values for RS-485 is 100 nF in series with 120 Ω (Figure 2.18).
Figure 2.18
RC termination
Copper cable
2.9
47
Transient protection
Electro static discharge (ESD) and capacitively or inductively coupled transients are a fact
of life often overlooked when designing communication networks. Quite often the receiver
circuits used for data communications (e.g. RS-485) fail for no apparent reason, despite
everything apparently being done by the book. Inspection of damaged chips might reveal
that often only one of several receivers on a chip is blown; the others remain functional.
This might be caused by transient voltages finding their way onto the data lines, despite
no single source or coupling mechanism being apparent. If this is suspected, a method has
to be devised to eliminate the problem at the board level. On new systems, this protection
should be built in from the start.
There are several transient voltage suppressant (TVS) schemes to be used. Figures 2.19
to 2.22 show two specific methods. Both increase the ability of the receivers to tolerate
transient events. Figure 2.22 shows the simplest and most effective method. It protects the
receiving circuitry to about 8 kV. The tradeoff for good transient voltage protection is,
however, a fairly high capacitive loading. The TranZorbs used have an open-circuit
capacitance of around 500 pF. This is to be treated with caution as Figure 2.21 shows the
effect of the protective devices having too large a capacitance (some devices can have a
capacitance of as much as 6000 pF, equivalent to about 400 m of additional cable!). This
effect is, of course, relative to the frequency of the incoming signal. Figure 2.21 shows a
second circuit, which only protects to about 4 kV. The circuit uses a bridge with a low
capacitance (about 13 pF) in series with the TranZorbs. This is a common circuit used to
protect high-speed data lines.
Figure 2.19
Signal at receiving end with capacitance within limits (Courtesy: Rodney Tower)
Figure 2.20
Effect of protective devices with too large capacitance (Courtesy: Rodney Tower)
48
Practical Industrial Data Communications
Figure 2.21
TVSs directly on the data line (Courtesy: National Semiconductor)
Figure 2.22
Alternative protection scheme (Courtesy: National Semiconductor)
A typical protective device will have a peak power rating of 500 W for 1 ms and a surge
current rating of 70 A for 1/120 s. The response time of these devices is almost instantaneous (1 × 10–12 s).
Should it be necessary to test the effectiveness of the protective circuitry in a lab, an
ESD gun such as a Shaffner NSG-435 must be used to simulate transient events on the
transmission lines. A small network can be built in the lab and energy discharged into the
data lines directly. Some devices, e.g. the Texas Instruments TI 75175 quad receiver, a
very common device in RS-485 circuits, can often be destroyed with a single 1 to 2 kV
air-gap discharge into either or both data lines.
A 1 kV air gap discharge seems a lot, but it is right on the edge of human perception.
This means the receiver chips could be destroyed by ESD that may not even be noticeable
to a technician.
TVSs directly on the data line provide the highest level of protection and the highest
capacitive loading of the transmission line.
The second figure is common circuit for protecting high-speed data lines. Both TVS
schemes shown provide significant improvement in the ability of receiver circuits to
withstand transient voltage events.
It might also be prudent to specify receiver chips with a high-published resistance
against static damage. For example, the datasheet for the Maxim MAX3095 claims
a ±15 kV protection using IEC1000-4-2 air-gap discharge, ±8 kV using IEC1000-4-2
contact discharge, and ±15 kV using the Human Body Model.
If there is a possibility of damage by lightning induced transients, the TranZorbs alone
are not sufficient since they cannot absorb all the energy. In this case, it is necessary to
place metal oxide varistors (MOVs) or gas diodes in parallel with the TranZorbs. The
TranZorbs are much quicker, and can clamp the input voltage for a few milliseconds until
Copper cable
49
the MOV or gas diode fires. Care should be taken to supply a solid ground return for the
large currents involved. Please note that the larger protective devices usually have a large
capacitance that can lead to unacceptable signal degradation. The effect of the protective
circuitry should be verified by means of a signal generator, line driver and oscilloscope
before installation.
Fiber optics
3.1
Introduction
Fiber-optic communication uses light signals guided through a fiber core. Fiber-optic
cables act as wave-guides for light, with all the energy guided through the central core of
the cable. The light is guided due to the presence of a lower refractive index cladding
surrounding the central core. None of the energy in the signal is able to escape into the
cladding and no energy is able to enter the core from any external sources. Therefore the
transmissions are not subject to electromagnetic interference.
The core and the cladding will trap the light ray in the core, provided the light ray
enters the core at an angle greater than the critical angle. The light ray will then travel
through the core of the fiber, with minimal loss in power, by a series of total internal
reflections. Figure 3.1 illustrates this process.
Figure 3.1
Light ray traveling through an optical fiber
Little of the light signal is absorbed in the glass core, so fiber-optic cables can be used
for longer distances before the signal must be amplified, or repeated. Some fiber-optic
segments can be many kilometers long before a repeater is needed. Data transmission
using a fiber-optic cable is many times faster than with electrical methods, and speeds of
over 10 Gbps are possible. Fiber-optic cables deliver more reliable transmissions over
greater distances, although at a somewhat greater cost. Cables of this type differ in their
physical dimensions and composition and in the wavelength(s) of light with which the
cable transmits.
Fiber-optic cables offer the following advantages over other types of transmission
media:
• Light signals are impervious to interference from EMI or electrical crosstalk.
• Light signals do not interfere with other signals. As a result, fiber-optic connections can be used in extremely adverse environments, such as in elevator shafts
or assembly plants, where powerful motors produce lots of electrical noise.
Fiber optics 51
• Optical fibers have a much wider, flat bandwidth than coaxial cables and
equalization of the signals is not required.
• The fiber has a much lower attenuation, so signals can be transmitted much
further than with coaxial or twisted-pair cable before amplification is
necessary.
• Optical fiber cables do not conduct electricity and so eliminates problems of
ground loops, lightning damage and electrical shock when cabling in highvoltage areas.
• Fiber-optic cables are generally much thinner and lighter than copper cable.
• Fiber-optic cables have greater data security than copper cables.
• Licensing is not required, although a right-of-way for laying the cable is
needed.
3.2
Fiber-optic cable components
The major components of a fiber-optic cable are the core, cladding, buffer, strength members,
and jacket, as shown in Figure 3.2. Some types of fiber-optic cable even include a conductive
copper wire that can be used to provide power to a repeater.
Figure 3.2
Fiber-optic cable components
3.2.1
Fiber core
The core of a fiber-optic telecommunications cable consists of a glass fiber through
which the light signal travels. The most common core sizes are 50 and 62.5 µm
(microns), which are used in multimode cables. 8.5 micron fibers are used in singlemode systems.
52
Practical Industrial Data Communications
3.2.2
Cladding
The core and cladding are actually manufactured as a single unit. The cladding is a
protective layer with a lower index of refraction than the core. The lower index means
any light that hits the core walls will be redirected back to continue on its path. The
cladding diameter is typically 125 microns.
3.2.3
Fiber-optic buffer
The buffer of a fiber-optic cable is made of one or more layers of plastic surrounding the
cladding. The buffer helps strengthen the cable, thereby decreasing the likelihood of micro
cracks, which can eventually break the fiber. The buffer also protects the core and cladding
from potential invasion by water or other materials in the operating environment. The
buffer typically doubles the diameter of the fiber.
A buffer can be tight or loose, as shown in Figure 3.3. A tight buffer fits snugly around
the fiber. A tight buffer can protect the fibers from stress due to pressure and impact, but
not from changes in temperature. A loose buffer is a rigid tube of plastic with one or more
fibers (consisting of core and cladding) running through it. The fibers are longer than the
tube so that the tube takes all the stresses applied to the cable, isolating the fiber from
these stresses.
Figure 3.3
Fiber housing types
3.2.4
Strength members
Fiber-optic cable also has strength members, which are strands of very tough material
(such as steel, fiberglass, or Kevlar) that provide tensile strength for the cable. Each of
these materials has advantages and drawbacks. For example, steel conducts electricity
making the cable vulnerable to lightning, which will not disrupt an optical signal but may
seriously damage the cable or equipment.
3.2.5
Cable sheath
The sheath of a fiber-optic cable is an outer casing that provides primary mechanical
protection, as with electrical cable.
Fiber optics 53
3.3
Fiber-optic cable parameters
3.3.1
Attenuation
The attenuation of a multimode fiber depends on the wavelength and the fiber construction,
and range from around 3 to 8 dB/km at 850 nm and 1 to 3 dB/km at 1300 nm. The
attenuation of single-mode fiber ranges from around 0.4 to 0.6 dB/km at 1300 nm and
0.25 to 0.35 dB/km at 1550 nm.
3.3.2
Diameter
The fiber diameter is either 50 or 62.5 microns for multimode fiber or 8.5 microns for
single mode.
• Multimode fibers (50 or 62.5 micron): In multimode fibers, a beam of light has
room to follow multiple paths through the core. Multiple modes in a transmission
produce signal distortion at the receiving end, due to the difference in arrival time
between the fastest and slowest of the alternate light paths.
• Single mode fibers (8.5 micron): In a single-mode fiber, the core is so narrow
that the light can take only a single path through it. Single-mode fiber has the
least signal attenuation, usually less than 0.5 dB/km. This type of cable is the
most difficult to install, because it requires precise alignment of the system
components and the light sources and detectors are very expensive. However,
transmission speeds of 50 Gbps and higher is possible.
3.3.3
Wavelength
Fiber-optic systems today operate in one of the three-wavelength bands viz. 850 nm,
1300 nm, or 1550 nm. The shorter wavelengths have a greater attenuation than the longer
wavelengths. Short-haul systems tend to use the 850 or 1300 nm wavelengths with the
multimode cable and light emitting diode (LED) light sources. The 1550 nm fibers are
used almost exclusively with the long-distance systems using single-mode fiber and laser
light sources.
3.3.4
Bandwidth
The bandwidth of a fiber is given as the range of frequencies across which the output
power is maintained within 3 dB of the nominal output. It is quoted as the product of the
frequencies of bandwidth multiplied by distance, for example, 500 MHz km. This means
that 500 MHz of bandwidth is available over a distance of one kilometer, or 100 MHz of
bandwidth over 5 km.
3.3.5
Dispersion
Modal dispersion is measured as nanoseconds of pulse spread per kilometer (ns/km). The
value also imposes an upper limit on the bandwidth, since the duration of a signal must be
larger than the nanoseconds of a tail value. With step-index fiber, expect between 15 and
30 ns/km. Note that a modal dispersion of 20 ns/km yields a bandwidth of less than 50 Mbps.
There is no modal dispersion in single-mode fibers, because only one mode is involved.
Chromatic dispersion occurs in single-mode cables and is measured as the spread of the
pulses in picoseconds for each nanometer of spectral spread of the pulse and for each
kilometer traveled. This is the only dispersion effect in single-mode cables and typical
values are in the order of 3.5 ps/nm km at 1300 nm and 20 ps/nm km at 1550 nm.
54
Practical Industrial Data Communications
3.4
Types of optical fiber
One reason optical fiber makes such a good transmission medium is because the different
indices of refraction for the cladding and core help to contain the light signal within the
core, producing a waveguide for the light. Optical fiber can be constructed by changing
abruptly from the core refractive index to that of the cladding, or this change can be made
gradually (Figure 3.4). The two main types of multimode fiber differ in this respect.
• Step-index cable: Cable with an abrupt change in refraction index is called
step-index cable. In step-index cable, the change is made in a single step.
Single-step multimode cable uses this method, and it is the simplest, least
expensive type of fiber-optic cable. It is also the easiest to install. The core is
usually 50 or 62.5 microns in diameter; the cladding is normally 125 microns.
The core width gives light quite a bit of room to bounce around in, and the
attenuation is high (at least for fiber-optic cable); between 10 and 50 dB/km.
Transmission speeds up to 10 Mbps over 1 km are possible.
• Graded-index cable: Cable with a gradual change in refraction index is called
graded-index cable, or graded-index multimode. This fiber-optic cable type has
a relatively wide core, like single-step multimode cable. The change occurs
gradually and involves several layers, each with a slightly lower index of
refraction. A gradation of refraction indexes controls the light signal better than
the step-index method. As a result, the attenuation is lower, usually less than
15 dB/km. Similarly, the modal dispersion can be 1 ns/km and lower, which
allows more than ten times the bandwidth of step-index cable.
Figure 3.4
Fiber refractive index profiles
Fiber optics 55
• Fiber designations: Optical fibers are specified in terms of their core, cladding,
and coating diameters. For example, a 62.5/125/250 fiber has a core diameter of
62.5 microns, a cladding of 125 microns and a coating of 250 microns.
3.5
Basic cable types
There are four broad application areas into which fiber-optic cables can be classified:
aerial cable, underground cable, sub-aqueous cable, and indoor cable. The special
properties required for each of these applications will now be considered. Note that this
list is not all encompassing, and that some specialized cables need to combine the features
of several of these classes.
3.5.1
Aerial cable
Aerial cables are literally exposed to the elements, more than any other application, and as
such are exposed to many external forces and hazards. Aerial cables are installed between
poles with the weight of the cable continuously supported by usually a steel messenger wire
to which the cable can be directly lashed, or by the strength members integral to the cable
construction. Greatly increased tensile forces can be produced by the effects of combined
wind and ice loadings. Other considerations are the wide variations in temperature to which
the cable may be subjected, affecting the physical properties of the fibers and the
attenuation of the fibers. The longitudinal cable profile is important for reducing the wind
and ice loadings of such cables. Moisture barriers are essential, with jelly-filled, loose
buffered fiber cable configurations being predominant. Any water freezing within the fiber
housings would expand and could produce excessive bending of fibers.
The cable sheath material is required to withstand the extremes of temperature and the
intense ultraviolet light exposure from continuous exposure to sunlight. UV-stabilized
polyethylene is frequently used for this purpose.
The installed span length and sag requirements are important design parameters
affecting the maximum cable tension, and which dictates the type of cable construction to
be used. Short-span cables have less-stringent tension requirements, which can be met by
the use of integral Kevlar layers, whereas long-span cables may need to utilize multiplestranded FRP rods to meet the required maximum tensions.
Advantages of aerial cable
• Useful in areas where it may be very difficult or too expensive to bury the cable
or install it in ducts.
• Also useful where temporary installations are required.
Disadvantages
• System availability is not as high as for underground cables. Storms can disrupt
these communication bearers, with cables damaged by falling trees, storm
damage, and blown debris. Roadside poles can be hit by vehicles and frustrated
shooters seem unable to miss aerial cables!
3.5.2
Underground cable
Underground cables experience less environmental extremes than aerial cables. Cables are
usually pulled into ducts or buried directly in the ground, with the cables being placed in deep
narrow trenches, which are backfilled with dirt or else ploughed directly into the ground.
56
Practical Industrial Data Communications
Cable type
Loose buffering, using loose tube or slotted core construction, is generally used to isolate
fibers from external forces including temperature variations.
Advantages
• Usually the most cost-effective method of installing cables outdoors
• Greater environmental protection than aerial cable
• Usually more secure than aerial cable.
Disadvantages
• Can be disrupted by earthworks, farming, flooding, etc.
• Rodents biting cables can be a problem in some areas. This is overcome with the
use of steel tape armor or steel braid, or the use of a plastic duct of more than
38 mm OD for all dielectric cable installations. Also the use of Teflon coatings
on the sheath make the cable too slippery for the rodent to grip with its teeth.
3.5.3
Sub-aqueous cables
Sub-aqueous cables are basically outdoor cables designed for continuous immersion in
water. While international telecommunications carriers use the most sophisticated cables
for deep ocean communications, there are practical applications for sub-aqueous cables
for smaller users. These include cabling along or across rivers, lakes, water races, or
channels, where alternatives are not cost effective. Sub-aqueous cable is a preferred
option for direct buried cabling in areas subjected to flooding or with a high water table,
where, for example, if the cable were buried at, say, 1 m depth, it would be permanently
immersed in water. These cables are essentially outdoor cable constructions incorporating
a hermetically sealed unit, using a welded metallic layer, and encasing the fiber core.
Advantages
• Cheaper installation in some circumstances.
Disadvantages
• Unit cost of cable is higher.
3.5.4
Indoor cables
Indoor cables are used inside buildings and have properties dictated by the fire codes. Such
cables need to minimize the spread of fires, and must comply with your relevant local fire
codes, such as outlined in the national electrical codes (NEC) in USA. Outdoor cables
generally contain oil-based moisture-blocking compounds like petroleum jelly. These
support combustion and so, their use inside buildings is strictly controlled. Outdoor cables
are frequently spliced to appropriate indoor cables close to the building entry points, to
avoid the expense of encasing long runs of outdoor cable inside metallic conduit.
The fiber in indoor cables and the indoor cable itself is usually tightly buffered, as was
discussed in Section 3.2. The tight buffer provides adequate water resistance for indoor
applications, but such cables should not be used for long outdoor cable runs. The buffered
fibers can be given sufficient strength to enable them to be directly connected to
equipment from the fiber structure without splicing to patch cords.
Fiber optics 57
3.6
Connecting fibers
This section will identify the main issues involved in connecting fibers together and to
optical devices, such as sources and detectors. This can be done using splices or
connectors. A splice is a permanent connection used to join two fibers and a connector is
used where the connection needs to be connected and disconnected repeatedly, such as at
patch panels. A device used to connect three or more fibers or devices is called a coupler.
3.6.1
Connection losses
The main parameter of concern when connecting two optical devices together is the
attenuation – that fraction of the optical power lost in the connection process. This
attenuation is the sum of losses caused by a number of factors, the main ones being:
•
•
•
•
•
•
•
Lateral misalignment of the fiber cores
Differences in core diameters
Misalignment of the fiber axes
Numerical aperture differences of the fibers
Reflection from the ends of fibers
Spacing of the ends of the fibers
End finish and cleanliness of fibers.
The most important of these loss mechanisms involved in connecting multimode fibers
is the axial misalignment of the fibers.
With connectors, the minimum loss across the glass/air interface between them will
always be about 0.35 dB, unless index-matching gel is used.
3.6.2
Splicing fibers
Two basic techniques are used for splicing of fibers: fusion splicing and mechanical splicing.
With the fusion splicing technique, the fibers are welded together, requiring expensive
equipment, but will produce consistently lower loss splices with low consumable costs. With
mechanical splicing, the fibers are held together in an alignment structure, using an adhesive
or mechanical pressure. Mechanical splicers require lower capital cost equipment but have a
high consumable cost per splice.
• Fusion splicing: Fusion splices are made by melting the end faces of the
prepared fibers and fusing the fibers together. Practical field fusion splicing
machines use an electric arc to heat the fibers. Factory splicing machines often
use a small hydrogen flame. The splicing process needs to precisely pre-align
the fibers, then heat their ends to the required temperature and move the
softened fiber ends together sufficiently to form the fusion joint, whilst
maintaining their precise alignment. Fusion splices have consistently very low
losses and are the preferred method for joining fibers, particularly for singlemode systems. Modern single-mode fusion splicers utilize core alignment
systems to ensure the cores of the two fibers are precisely aligned before
splicing.
• Mechanical splicing: Mechanical splicing involves many different approaches
for bringing the two ends of the fibers into alignment and then clamping them
within a jointing structure or gluing them together. Mechanical splices
generally rely on aligning the outer diameters of the fiber cladding and assumes
that the cores are concentric with the outside of the cladding. This is not always
58
Practical Industrial Data Communications
the case, particularly with single mode fibers. Various mechanical structures
are used to align the fibers, including V grooves, sleeves, 3 rods and various
proprietary clamping structures.
3.6.3
Connectors
Connectors are used to make flexible interconnections between optical devices.
Connectors have significantly greater losses than splices since it is much more difficult to
repeatedly align the fibers with the required degree of precision. Active alignment, as was
used to minimize some splice losses, is not possible. Axial misalignment of the fibers
contributes most of the losses at any connection, consequently connector loss can be
expected to be in the range from 0.2 to over 3 dB.
Most connector designs produce a butt joint with the fiber ends as close together as
possible. The fiber is mounted in a ferrule with a central hole sized to closely match the
diameter of the fiber cladding. The ferrule is typically made of metal or ceramic and its
purpose is to center and align the fiber as well as provide mechanical protection to the
end of the fiber. The fiber is normally glued into the ferrule then the end cut and polished
to be flush with the face of the ferrule. The two most common connectors are the SC and
ST, as detailed below. Many new proprietary connectors are now available for different
types of equipment.
• SC connector: This is built with a cylindrical ceramic ferrule which mates with
a coupling receptacle. The connector has a square cross section for high packing
density on equipment, and has a push–pull latching mechanism. The ISO and
TIA have adopted a polarized duplex version as standard and this is now being
used as a low-cost FDDI connector. The SC connector has a specified loss of less
than 0.6 dB (typically 0.3 dB) for both single mode and multimode fibers and a
typical return loss of 45 dB.
• ST connector: The ST connector is shown in Figure 3.5. This is an older
standard used for data communications. This is also built with a cylindrical
ceramic ferrule which mates with a coupling receptacle. The connector has a
round cross section and is secured by twisting to engage it in the spring-loaded
bayonet coupling. Since it relies on the internal spring to hold the ferrules
together, optical contact can be lost if a force of greater than about 1 kg is
applied to the connector.
Figure 3.5
ST connector
Fiber optics 59
3.6.4
Connector handling
Most fiber-optic connectors are designed for indoor use. Connectors for outdoor use
require to be hermetically sealed. It is very important to protect the optical faces of the
connectors from contamination. The optical performance can be badly degraded by the
presence of dirt or dust on the fiber ends. A single dust particle could be 10 microns in
diameter, which would scatter or absorb the light and could totally disrupt a single-mode
system. Connectors and patch panels are normally supplied with protective caps. These
should always be fitted whenever the connectors are not mated. These not only protect
from dust and dirt, but also provide protection to the vulnerable, polished end of the fiber.
Compressed air sprays are available for cleaning connectors and adapters, without
needing to physically touch the mating surfaces.
Take care not to touch the end of the connector ferrules as the oil from your fingers can
cause dirt to stick to the fiber end. Clean connectors with lint-free wipes and isopropyl
alcohol.
Durability of the connectors is important throughout their lifetime. Typical fiber
connectors for indoor use are specified for 500 to 1000 mating cycles, and the attenuation
is typically specified not to change by more than 0.2 dB throughout that lifetime.
Repeated connection and disconnection of the connectors can wear the mechanical
components and introduce contamination to the optical path.
3.6.5
Optical couplers
Optical couplers or splitters and combiners are used to connect three or more fibers or other
optical devices. These are devices which split the input power to a number of outputs.
While the splitting of the light is done passively, active couplers include optical amplifiers,
which boost the signal before or after the splitting process. Coupler configuration depends
on the number of ports and whether each of these are unidirectional, so called directional
couplers, or bi-directional. Most couplers are found within the equipment for monitoring
purposes.
3.7
Splicing trays/organizers and termination cabinets
This section looks at the different types of storage units that are used for housing optical
fiber splices and end of cable terminations.
3.7.1
Splicing tray
Splices are generally located in units referred to as ‘splicing centers’, ‘splicing trays’ or
‘splicing organizers’. The splicing tray is designed to provide a convenient location to
store and to protect the cable and the splices. They also provide cable strain relief to the
splices themselves.
Splicing trays can be located at intermediate points along a route where cables are
required to be joined or at the termination and patch panel points at the end of the cable
runs.
The incoming cable is brought into the splicing center where the sheath of the cable is
stripped away. The fibers are then looped completely around the tray and into a splice
holder. Different holders are available for different types of splices. The fibers are then
spliced onto the outgoing cable if it is an intermediate point or on to pigtails if it is a
termination point. These are also looped completely around the tray and then fed out of
the tray. A typical splicing tray is illustrated in Figure 3.6.
60
Practical Industrial Data Communications
Figure 3.6
A typical splicing tray
The cables are physically attached to the splice tray to provide strain relief. The cables
normally enter the tray on one side only to facilitate moving the tray/joint enclosure to a
more accessible jointing location. The fibers are looped completely around the tray to
provide slack, which may be required to accommodate any changes in the future, and also
to provide tension relief on the splices.
Each splice joint is encased in a splice protector (plastic tube) or in heat shrink before it
is clipped into the holder.
Splicing trays are available that have patching facilities. This allows different fibers to
be cross-connected and to be looped back for testing purposes.
3.7.2
Splicing enclosures
The splicing trays are not designed to be left in the open environment and must be placed
in some type of enclosure. The enclosure that is used will depend on the application. The
following are examples of some enclosures used for splicing trays.
• Direct buried cylinders: At an intermediate point where two cables are joined
to continue a cable run, the splices can be directly buried by placing the splice
trays in a tightly sealed cylindrical enclosure that is generally made from heavy
duty plastic or aluminum. The container is completely sealed from moisture
ingress and contains desiccant packs to remove any moisture that may get in.
A typical direct buried cylinder is illustrated in Figure 3.7. Note that the cables
normally enter the enclosure at one end only to allow the enclosure to be lifted
from the ground for easier splicing access.
Figure 3.7
Direct buried splicing enclosure
Fiber optics 61
• Termination cabinets: At junction points where a lot of cables meet, the
splicing trays are stored in a larger wall-mounted cabinet (approximately 500 ×
500 × 100 mm) with a hinged door. For outdoor use, the cabinets must be
sealed against bad weather conditions. Figure 3.8 illustrates a splicing tray in a
termination cabinet.
• Patch panels and distribution frames: Splice trays can be installed in the back
of patch panels and distribution frames used for connection of patch cords to
the main incoming cable.
3.7.3
Termination in patch panels and distribution frames
There are three main methods of connecting an incoming cable into a patch panel or
distribution frame. Firstly, if the incoming cable contains fibers that have a large
minimum bending radius, then it is recommended to splice each fiber to a fiber patch cord
that has a smaller bending radius. This reduces undue stress on the incoming fibers but
does introduce small losses into the link. This also replaces the more fragile glass of the
incoming cable with the more flexible and stronger glass of the patch cords. This is
illustrated in Figure 3.8.
Figure 3.8
Termination cabinet for splicing trays
The second method is to place the fibers from the incoming cable into a breakout unit.
The breakout unit separates the fibers and allows a plastic tube to be fitted over the
incoming fibers to provide protection and strength as they are fed to the front of the patch
panel. Note there are no splices, which therefore keep losses to a minimum. This is
illustrated in Figure 3.9.
62
Practical Industrial Data Communications
Figure 3.9
Patch panel with break out box
If the incoming cable contains tight buffered fibers that are flexible and strong, then
they can be taken directly to the front of the patch panel. This is referred to as direct
termination, and is illustrated in Figure 3.10.
Figure 3.10
Direct termination of cables in a patch panel
3.8
Troubleshooting
3.8.1
Introduction
This section deals with problems of fiber-optic cables. Problems can be caused by poor
installation practices, where fibers are subjected to excessive tension or bending forces.
This section also deals with the basic methods of testing fibers and how to locate faults on
fiber-optic systems.
Fiber optics 63
3.8.2
Standard troubleshooting approach
The standard approach to troubleshooting fiber-optic systems is as follows:
• Observe the system status indicators and determine whether signals are being
transmitted and received at both terminals.
• Determine whether the appropriate fibers are functional by either a simple
continuity test or a qualitative insertion loss measurement between the patch
panels.
• Once the faulty fiber is identified, clean the optical connectors and repeat the test.
• If the fault remains, swap the system onto a spare fiber by rearranging the patch
cords on the patch panels at both ends.
• Update the records to indicate the faulty fiber.
• When a link does not have sufficient spare fibers available to maintain system
integrity, then attempt fault localization using an OTDR, if appropriate.
• On short lengths of fiber, total replacement may be more cost effective than
expensive location and subsequent repair. This is particularly appropriate
where a spare duct is available for the cable replacement.
3.8.3
Tools required
Continuity tester
This device has a fiber-optic transmitter with a suitable fiber-optic connector that
transmits a visible red light (650 nm). This can transmit visible light over several
kilometers and is used for such applications as continuity testing, finding fractures in
fibers or bad splices by observing light that may be leaking out and for identifying fibers
at the end of a cable that has many fibers in it.
Optical source
This device has a calibrated fiber-optic transmitter with a suitable fiber-optic connector
and is used with an optical power meter for insertion loss testing of fibers.
Optical power meter
This device has a fiber-optic receiver with a suitable fiber-optic connector and displays
the received optical power levels. It is used with an optical source for insertion loss
testing of fibers.
Optical time domain reflectometer (OTDR)
The OTDR sends a short pulse of light down the fiber and measures and records the light
energy that is reflected back up the fiber. A reflection may be caused by the presence of a
connector, splice, crack, impurity, or break in the fiber. By measuring the time it takes for
the reflected light to return to the source, and knowing the refractive index of the fiber, it
is possible to calculate the distance to the reflection point.
3.9
Fiber installation rules
The following section provides general installation rules that should be followed when
installing fiber-optic cabling systems to avoid long-term reliability problems. Fibers can
break at any surface defect if subjected to excessive bending forces while under tension.
64
Practical Industrial Data Communications
3.9.1
Cable bending radius
• The most important consideration when installing fiber-optic cables is to ensure
that at all times during an installation the cable radius is not less than the
manufacturer’s recommended minimum installation bending radius.
• Avoidance of sharp bends along the installation route is absolutely essential.
Sharp bends in cable trays or in conduits can cause macrobends or microbends
in the fibers. This can lead to fiber breakage or adversely affect signal
attenuation.
• Ensure that the conduit or the cable tray is constructed with no sharp edges.
Use curved construction components (ducts or cable trays) and not right-angled
components.
• Ensure the cables are laid on to a flat surface, and that no heavy objects will be
laid on to the cables in the future.
• Avoid putting kinks or twists into the cable. This is best achieved by pulling
the cable directly off the reel and having a member of the installation team
carefully watch any cable slack for possible formation of kinks.
• Cable manufacturers will specify a minimum bending radius that applies to the
long-term final installed cable. The long-term radius is significantly larger than
the installation radius. If this long-term radius is exceeded, the macrobending in
the cable will produce additional attenuation. While not only damaging the fiber,
such unnecessary losses will be detrimental to cable performance. Once the cable
has been installed and the tension has been released, ensure that the cable radius
is not less than the long-term installed radius at any point along the cable.
3.9.2
Cable tension
When a longitudinal tensile force is applied to an optical fiber, it can cause minute
surface defects. These potential weak points can develop into microcracks, which may
cause breakage of the fiber, when it is later subjected to an equal or greater tension.
Optical fibers have some elasticity, stretching under light loads before returning to their
original length when the load is removed. Under heavier loads, the fiber may theoretically
stretch as much as 9% before breaking; however, it is considered advisable to limit this,
in practice, to permanent strains of less than 0.2% to avoid promoting premature failures.
Although modern fiber-optic cables are generally stronger than copper cables, failure
due to excess cable tension during installation is more catastrophic (i.e. fiber snapping
rather than copper stretching). The following guidelines should be observed during
installation.
• The maximum allowable installation cable tension is specified by the manufacturer; the cable tension should not exceed that limit at any time. A general
rule of thumb that is sometimes used is that the maximum allowable cable
tension during installation is approximately the weight of 1 km of the cable
itself.
• When pulling the cable during installation, avoid sudden, short sharp jerking.
These sudden forces could easily exceed the maximum cable tension. The cable
should be pulled in an easy smooth process.
• When pulling cable off a large drum, ensure that the cable drum is smoothly
rotated by one team member to feed off the cable. If the cable is allowed to jerk
the drum around, the high moment of inertia of the drum can cause excessive
tension in the cable.
Fiber optics 65
• It is very important to minimize cable stress after the installation is complete.
A slack final resting condition will help to ensure that the fiber-optic cable has
a long operating life. It is recommended that slack be left in the junction boxes
at the completion of the installation to reduce overall stress in the cable.
• If there are a lot of bends in the cable route, it is recommended to use as many
intermediate junction boxes as possible to reduce cable tension. The cable can
be pulled through at these points, laid out in a large figure ‘8’ pattern on the
ground, and then pulled into the next section. On long cable runs, pulling
assistance can also be done at these intermediate access points. Laying the
cable in a figure ‘8’ pattern naturally avoids kinking and twisting of the cable.
Curved guides or block systems may be used in the junction boxes where the
cable changes direction.
3.10
Cleaning optical connectors
With the core of a single mode fiber being only 9 microns in diameter, a lot of dust
particles can easily cover much of the core area on the end face of the connector. In
addition, connector damage can occur if foreign particles are caught in the end face area
of mated connectors.
It is vital to make sure that cleaning of connectors is completed by all staff handling
them, and that ‘good housekeeping’ practices are employed, such as always installing the
protective dust caps and never allowing any object other than the approved cleaners to
contact the end face.
• The preferred method of connector cleaning is by wiping the end face and
ferrule of the connector with some isopropyl alcohol on a lint-free tissue.
Cleaning tissues can be obtained in sealed packages already impregnated with
alcohol.
• Another method of connector cleaning is the proprietary cassette-type cleaners.
These use a dry tape which is advanced every time the cassette is opened,
ensuring a clean section of tape is available each time.
• Where proprietary connectors have bare fibers exposed such as Volition or
MTRJ, the only acceptable method of cleaning is to use a can of compressed gas.
Adapters are primarily kept clean by always cleaning the connectors prior to insertion,
and fitting of dust caps to all unused patch panel ports. A method of cleaning adapters is
through the careful use of a can of compressed gas. An alternative is a pipe cleaner
moistened with isopropyl alcohol. Remember that most pipe cleaners have a steel wire
center and the end will damage any connector if it is inserted into an adapter, which still
has a connector inserted in the other side.
3.11
Locating broken fibers
3.11.1
Continuity testing
The most fundamental of all fiber-optic cable tests that can be performed is to carry out a
continuity test. The continuity test simply checks that the fiber is continuous from one
end to the other. A light beam is inserted from a light source in one end of the fiber and is
observed coming out of the other end of the fiber. This test provides little information
about the condition of the fiber, other than that there are no complete breaks along the
fiber length. This can be performed by shining a powerful torch beam (or laser pointer)
into one end of the fiber and observing the light coming out of the other end. This is
66
Practical Industrial Data Communications
a cost-effective method for multimode fibers with large core diameters over distances up
to approximately 500 m. It is not reliable over longer distances or with single-mode fibers
because the small diameter core makes penetration into the fiber difficult for the light.
There is specific test equipment available that can be used as a continuity tester. The
device has a fiber-optic transmitter with a suitable fiber-optic connector that transmits a
650 nm visible red light. This will transmit visible light over several kilometers and can
be used for such applications as continuity testing, finding fractures in fibers or bad
splices by observing light that may be leaking out and for identifying fibers at the end of a
cable that has many fibers in it. It can also be used for identifying fibers along the route of
a cable (where it is required to break into a cable for a system extension) by bending the
fiber and watching the light leaking out of the bend. This type of fiber-optic test has
limited application, as it is of no use in finding faults in buried cables or aerial cables.
As a word of warning, the user should not look into fiber groups at the end of cables if
any fibers on the system at any location are connected to lasers. The infra-red light from
lasers cannot be seen by the human eye but can cause permanent eye damage.
3.11.2
Insertion loss testing
The most common qualitative test that is carried out on a fiber-optic system is to measure
the attenuation of a length of fiber. This figure will allow most elements of the system
design to be verified.
Most insertion loss testing is carried out with a power source and a power meter.
Firstly, the power meter is calibrated to the power source by connecting the two
instruments together with a short piece of optic fiber approximately 2 m in length.
Generally, the power source is set to transmit a level of –10 dBm and the power meter
then adjusted accordingly to read –10 dBm. Ensure that the level used to calibrate the
power meter is within the dynamic range of the power meter.
There are four important points to check before commencing insertion loss testing.
Firstly, ensure that the optic fiber type used for calibration purposes is the same optic fiber
type that is to be tested for insertion loss. Secondly, the power meter and the power source
must operate at the same wavelength as the installed system equipment. Thirdly, the power
meter and source must also use the same source and detector types (LED or laser) that the
transmitter and receiver in the installed system are to use. Fourthly, to avoid possible
incorrect calibration, ensure that the same connectors are used for calibration as are used in
the installation.
Once the power meter has been calibrated, then the power meter and source are taken
into the field and connected to the installed cable. The level that is read at the meter can
be used to calculate the insertion loss through the cable section under test. This will
include the losses caused by the optic fiber, splices and the connectors. The test procedure
is illustrated in Figure 3.11.
If the power source and the power meter are calibrated in milliwatts, then the formula
for converting the loss figure to decibels is:
P 
Attenuation (dB) = − 10log  o 
 Pi 
Where
Po = power out of the fiber
Pi = power into the fiber.
Fiber optics 67
To calculate the insertion loss, subtract the dBm reading at the power meter from the input
power source value. For the example shown in Figure 3.11 the insertion loss is 9.3 dB.
It is recommended that the insertion loss measurement be performed in both directions of
an installed cable. The losses measured in each direction tend to vary because connectors
and splices sometimes connect unevenly and because the core diameter of fibers tends to
vary slightly. For example, if the core diameters of two fibers spliced together are 49.5 mm
and 50.5 mm, light waves traveling from the thinner fiber into the thicker fiber will all enter
the thicker fiber. For light traveling from the larger diameter fiber into the smaller diameter
fiber, a small amount will be lost around the edges of the interface between the two cores.
A mismatch of this type could account for a difference in insertion loss in the two directions
of 0.2 dB. If the fiber losses are different in each direction, then that fiber can be used in
whichever direction gives the best system performance.
Figure 3.11
Insertion loss measurement
3.11.3
Optical time domain reflectometer
The only method of analyzing the losses along an individual fiber is to test it with an
optical time domain reflectometer (OTDR). In particular, this allows the location of a
broken fiber to be established. The OTDR sends a short pulse of light down the fiber and
measures and records the light energy that is reflected back up the fiber. A reflection may
be caused by the presence of a connector, splice, crack, impurity, or break in the fiber. By
measuring the time, it takes for the reflected light to return to the source, and knowing the
refractive index of the fiber, it is possible to calculate the distance to the reflection point.
Impurities in the glass will cause a continuous low-level reflection as the light travels
through the glass fiber. This is due to Rayleigh scattering and is commonly referred to as
backscatter. The strength of the backscattered signal received at the source gradually
drops as the pulse moves away from the source. This is seen on an OTDR display as a
near linear drop in the received reflected signal and the slope of this line is the attenuation
of the fiber (dB per km). Figure 3.12 illustrates a typical reflection curve for an OTDR
and notes the backscatter.
An OTDR will generally not provide accurate readings of irregularities and losses in the
fiber for the first 15 m of the cable. This is because the pulse length and its rise time from
68
Practical Industrial Data Communications
the OTDR are comparatively large when compared to the time it takes for the pulse to
travel the short distance to the point of reflection within this 15 m and back.
In general, for shorter local cable running less than 200 m, there is not a lot to be gained
from carrying out an OTDR test unless there are connectors and splices along the cable
route.
With reference to the OTDR plot in Figure 3.12, the Y-axis of the plot shows the
relative amplitude of the light signal that has reflected back to the source, and the X-axis
represents time. The time base is directly translated and displayed as distance by the
OTDR.
Figure 3.12
Trace from an OTDR
The sudden peaks that appear along the slope are the points where reflections have
occurred, and light that has reflected back to the source is stronger than the backscatter.
There are four main reflection points illustrated in Figure 3.12.
In their order of decreasing magnitude they are:
•
•
•
•
•
Reflection from the unterminated end of the fiber
Reflection from a connector
Reflection from a splice
Reflection from a hairline crack in the fiber
Backscatter.
After each of the reflections, the slope of the attenuation curve drops suddenly. This
drop represents the loss introduced by the connector, splice, or imperfection in the fiber.
Point (6) noted in Figure 3.12 illustrates a splice where the cores of the fibers are well
matched for light traveling in the direction away from the source. This splice has no
reflection, just a loss introduced by the splice. The type of drop at point (6) in the
attenuation curve could also be caused by a sharp bend in the fiber where light escapes
out of the fiber at the bend and is not reflected back. Some types of faults in the fiber will
also cause a similar result.
Fiber optics 69
Point (7) noted in Figure 3.12 shows the noise floor of the instrument. This is the lowest
sensitivity of received signal that the device can accept. Measurements made close to this
level are not very accurate.
OTDR testing can provide very accurate fault analysis over long lengths of fiber. On
the better quality instruments, a resolution of 1 m for fault location and 0.01 dB for in line
losses is available. Some instruments will operate with a range up to 200 km.
In general, OTDRs are relatively easy to use and special analysis software packages are
available for downloading the test results and carrying out detailed analysis if required.
The unfortunate downside with OTDR technology is that it is generally very expensive.
Even small reduced feature units can be very expensive.
Care should be taken when interpreting the results from an OTDR. Where different
fibers are joined together, this connection may represent a change in refractive index, core
size, modal properties, and/or material properties of the fiber. For example, after a splice
or connector the OTDR may display what appears to be a signal gain on the screen. What
has probably occurred is that the light has entered a fiber with greater impurities and there
has been an increase in backscatter.
The OTDR test should be carried out on every optical fiber in a cable while it is still on
the reel prior to installation to ensure that faulty fibers are not installed. The results of
these tests should be stored in memory or as a print out. These pre-installation tests are
generally carried out when the custody of the cable is passed from one party to the next;
for example, when the cable is handed down from the purchaser to the installation
contractor.
Once the cable has been installed, the OTDR tests should be carried out again on
every optical fiber. The results of the ‘as installed’ tests can then be compared to the
pre-installation test results to determine if the fibers have been damaged or poorly
installed.
The results of the pre-installation and the post-installation tests should be kept as part of
the commissioning documentation. If there is a fault with the system at a later date, then
the commissioning test results can be used to help determine where the faults are located.
For high-integrity longer-distance systems, it is worth carrying out an audit of the system
after a number of years of operation and performing the OTDR tests again and comparing
them to the commissioning test results to measure any deterioration in the cabling system
since installation.
The OTDR can be used for attenuation measurements but it is generally not recommended,
as OTDR measurements are a relative measurement rather than an absolute measurement.
Also, the OTDR does not account for poor-quality OTDR connector to fiber end-face
connections. Therefore the insertion loss measurement procedure that was described above is
a more appropriate and accurate technique.
Since the OTDR is not used for accurate attenuation measurements but only for relative
measurements, the wavelength at which it operates is not important. Distance readings,
splice losses, and connector losses are not affected by the small changes in wavelength
associated with lasers and LEDs.
It was noted in the previous section that the loss of a connector or splice might be
different when measured from each direction into the optic fiber. For cable sections of
greater than 2 km, it is recommended that the OTDR tests be carried out from each end of
the optical fiber.
Some fiber-optic cables are constructed so that the fibers are laid in a helical
fashion around the center of the cable. In this case, the length of the cable is not going to
be the length of the fibers. This difference will make it inherently difficult to determine the
distance to faults. To overcome this problem, the manufacturer will generally provide
70
Practical Industrial Data Communications
a ratio of fiber length to cable length. The ratio is then used to calculate the exact cable
distance to the fault from the OTDR distance reading. If there is not a ratio available, an
OTDR measurement is performed on a known length of cable (generally 1 km) and the
ratio is calculated.
Length of fiber in 1km of cable
1km
OTDR distance reading
Distance to fault =
Fiber/cable ratio
Fiber/cable ratio =
RS-232 overview
Objectives
When you have completed study of this chapter, you will be able to:
• List the main features of the RS-232 standard
• Fix the following problems:
−
−
−
−
−
−
−
4a.1
Incorrect RS-232 cabling
Male/female D-type connector confusion
Wrong DTE/DCE configuration
Handshaking
Incorrect signaling voltages
Excessive electrical noise
Isolation.
RS-232 interface standard (CCITT V.24 interface standard)
The RS-232 interface standard was developed for the single purpose of interfacing data
terminal equipment (DTE) and data circuit terminating equipment (DCE) employing
serial binary data interchange. In particular, RS-232 was developed for interfacing data
terminals to modems.
The RS-232 interface standard was issued in the USA in 1969 by the engineering
department of the EIA. Almost immediately, minor revisions were made and RS-232C
was issued. RS-232 was originally named RS-232 (Recommended Standard), which is
still in popular usage. The prefix ‘RS’ was superseded by ‘EIA/TIA’ in 1988. The current
revision is EIA/TIA-232E (1991), which brings it into line with the international
standards ITU V.24, ITU V.28, and ISO-2110.
Poor interpretation of RS-232 has been responsible for many problems in interfacing
equipment from different manufacturers. This had led some users to dispute as to whether
it is a ‘standard’. It should be emphasized that RS-232 and other related RS standards
define the electrical and mechanical details of the interface (layer 1 of the OSI model) and
do not define a protocol.
The RS-232 interface standard specifies the method of connection of two devices – the
DTE and DCE. DTE refers to data terminal equipment, for example, a computer or a
printer. A DTE device communicates with a DCE device. DCE, on the other hand, refers
to data communications equipment such as a modem. DCE equipment is now also called
72
Practical Industrial Data Communications
data circuit-terminating equipment in EIA/TIA-232E. A DCE device receives data from
the DTE and retransmits to another DCE device via a data communications link such as a
telephone link (Figure 4a.1).
Figure 4a.1
Connections between the DTE and the DCE using DB-25 connectors
4a.1.1
The major elements of RS-232
The RS-232 standard consists of three major parts, which define:
• Electrical signal characteristics
• Mechanical characteristics of the interface
• Functional description of the interchange circuits.
Electrical signal characteristics
RS-232 defines electrical signal characteristics such as the voltage levels and grounding
characteristics of the interchange signals and associated circuitry for an unbalanced
system.
The RS-232 transmitter is required to produce voltages in the range ±5 to ±25 V as
follows:
• Logic 1: –5 to –25 V
• Logic 0: +5 to +25 V
• Undefined logic level: +5 to –5 V.
At the RS-232 receiver, the following voltage levels are defined:
• Logic 1: –3 to –25 V
• Logic 0: +3 to +25 V
• Undefined logic level: –3 to +3 V.
RS-232 overview 73
Note: The RS-232 transmitter requires a slightly higher voltage to overcome voltage
drop along the line.
The voltage levels associated with a microprocessor are typically 0 to +5 V for
transistor–transistor logic (TTL). A line driver is required at the transmitting end to
adjust the voltage to the correct level for the communications link. Similarly, a line
receiver is required at the receiving end to translate the voltage on the communications
link to the correct TTL voltages for interfacing to a microprocessor. Despite the bipolar
input voltage, TTL-compatible RS-232 receivers operate on a single +5 V supply
(Figure 4a.2).
Figure 4a.2
Voltage levels for RS-232
Modern PC power supplies usually have a standard +12 V output that could be used for
the line driver.
The control or ‘handshaking’ lines have the same range of voltages as the transmission
of logic 0 and logic 1, except that they are of opposite polarity. This means that:
• A control line asserted or made active by the transmitting device has a voltage
range of +5 to +25 V. The receiving device connected to this control line
allows a voltage range of +3 to +25 V.
• A control line inhibited or made inactive by the transmitting device has a
voltage range of –5 to –25 V. The receiving device of this control line allows
a voltage range of –3 to –25 V.
At the receiving end, a line receiver is necessary in each data and control line to reduce
the voltage level to the 0 V and +5 V logic levels required by the internal electronics
(Figure 4a.3).
74
Practical Industrial Data Communications
Figure 4a.3
RS-232 transmitters and receivers
The RS-232 standard defines 25 electrical connections. The electrical connections are
divided into four groups viz.:
1.
2.
3.
4.
Data lines
Control lines
Timing lines
Special secondary functions.
Data lines are used for the transfer of data. Data flow is designated from the perspective
of the DTE interface. The transmit line, on which the DTE transmits and the DCE
receives, is associated with pin 2 at the DTE end and pin 2 at the DCE end for a DB-25
connector. These allocations are reversed for DB-9 connectors. The receive line, on
which the DTE receives, and the DCE transmits, is associated with pin 3 at the DTE end
and pin 3 at the DCE end. Pin 7 is the common return line for the transmit and receive
data lines.
Control lines are used for interactive device control, which is commonly known
as hardware handshaking. They regulate the way in which data flows across the
interface.
The four most commonly used control lines are:
1.
2.
3.
4.
RTS:
CTS:
DSR:
DTR:
Request to send
Clear to send
Data set ready (or DCE ready in RS-232D/E)
Data terminal ready (or DTE ready in RS-232D/E).
It is important to remember that with the handshaking lines, the enabled state means a
positive voltage and the disabled state means a negative voltage.
Hardware handshaking is the cause of most interfacing problems. Manufacturers
sometimes omit control lines from their RS-232 equipment or assign unusual applications
to them. Consequently, many applications do not use hardware handshaking but, instead,
use only the three data lines (transmit, receive, and signal common ground) with some
form of software handshaking. The control of data flow is then part of the application
program. Most of the systems encountered in data communications for instrumentation
and control use some sort of software-based protocol in preference to hardware
handshaking.
RS-232 overview 75
There is a relationship between the allowable speed of data transmission and the length
of the cable connecting the two devices on the RS-232 interface. As the speed of data
transmission increases, the quality of the signal transition from one voltage level to
another, for example, from –25 to +25 V, becomes increasingly dependent on the
capacitance and inductance of the cable.
The rate at which voltage can ‘slew’ from one logic level to another depends mainly on
the cable capacitance, and the capacitance increases with cable length. The length of the
cable is limited by the number of data errors acceptable during transmission. The RS-232
D&E standard specifies the limit of total cable capacitance to be 2500 pF. With typical
cable capacitance having improved from around 160 pF/m to only 50 pF/m in recent years,
the maximum cable length has extended from around 15 m (50 ft) to about 50 m (166 ft).
The common data transmission rates used with RS-232 are 110, 300, 600, 1200, 2400,
4800, 9600, and 19 200 bps. For short distances, however, transmission rates of 38 400,
57 600, and 115 200 can also be used. Based on field tests, Table 4a.1 shows the practical
relationship between selected baud rates and maximum allowable cable length, indicating
that much longer cable lengths are possible at lower baud rates. Note that the achievable
speed depends on the transmitter voltages, cable capacitance (as discussed above) as well
as the noise environment.
In the context of the NRZ-type of coding used for asynchronous transmission on
RS-232 links, 1 baud = 1 bit per second.
Baud Rate Cable Length (meters)
110
850
300
800
600
700
1200
500
2400
200
4800
100
9600
70
19 200
50
115 k
20
Table 4a.1
Demonstrated maximum cable lengths with RS-232 interface
Mechanical characteristics of the interface
RS-232 defines the mechanical characteristics of the interface between the DTE and the
DCE. This dictates that the interface must consist of a plug and socket and that the socket
will normally be on the DCE.
Although not specified by RS-232C, the DB-25 connector (25 pin, D-type) is closely
associated with RS-232 and is the de facto standard with revision D. Revision E formally
specifies a new connector in the 26-pin alternative connector (known as the ALT A
connector). This connector supports all 25 signals associated with RS-232. ALT A is
physically smaller than the DB-25 and satisfies the demand for a smaller connector
suitable for modern computers. Pin 26 is not currently used. On some RS-232-compatible
equipment, where little or no handshaking is required, the DB-9 connector (9 pin, D-type)
is common. This practice originated when IBM decided to make a combined serial/parallel
76
Practical Industrial Data Communications
adapter for the AT&T personal computer. A small connector format was needed to allow
both interfaces to fit onto the back of a standard ISA interface card. Subsequently, the
DB-9 connector has also become an industry standard to reduce the wastage of pins. The
pin allocations commonly used with the DB-9 and DB-25 connectors for the RS-232
interface are shown in Table 4a.2. The pin allocation for the DB-9 connector is not the
same as the DB-25 and often traps the unwary.
The data pins of DB-9 IBM connector are allocated as follows:
• Data transmit pin 3
• Data receive pin 2
• Signal common pin 5.
Pin No.
DTE
DB-9 Connector IBM
Assignment
DB-25 Connector
RS-232 pin Assignment
DB-25 Connector EIA-530
pin Assignment
1
Received line signal
Shield
Shield
2
Received data
Transmitted data
Transmitted data (A)
3
Transmitted data
Received data
Received data (A)
4
DTE ready
Requeste to send
Requeste to send (A)
5
Signal/common ground
Clear to send
Clear to send (A)
6
DCE ready
DCE ready
DCE ready (A)
7
Request to send
Signal/common ground
Signal/common ground
8
Clear to send
Received line signal
Received line signal (A)
9
Ring indicator
+ Voltage (testing)
Receiver signal DCE
element timing (B)
10
– Voltage (testing)
Received line (B)
11
Unassigned
Transmitter signal DTE
element timing (B)
12
Sec received line signal
Detector/data signal
Transmitter signal DCE
element timing
13
Sec clear to send
Clear to send (B)
14
Sec transmitted data
Transmitted data (B)
15
Transmitted signal DCE
element timing
Transmitted signal DCE
element timing (A)
16
Sec received data
Received data (B)
17
Receiver signal DCE
element timing
Receiver signal DCE
element timing (A)
18
Local loopback
Local loopback
19
Sec request to send
Request to send (B)
20
DTE ready
DTE ready (A)
21
Remote loopback/signal
quality detector
Remote loopback
RS-232 overview 77
Pin No.
DTE
DB-9 Connector IBM
Assignment
DB-25 Connector
RS-232 pin Assignment
DB-25 Connector EIA-530
pin Assignment
22
Ring indicator
DCE ready (B)
23
Data signal rate
DTE ready (B)
24
Transmit signal DTE
element timing
Transmit signal DTE
element timing (A)
25
Test mode
Test mode
Table 4a.2
Common DB-9 and DB-25 pin assignments for RS-232 and EIA/TIA-530 (often used for RS-422 and RS-485)
Functional description of the interchange circuits
RS-232 defines the function of the data, timing, and control signals used at the interface
of the DTE and DCE. However, very few of the definitions are relevant to applications
for data communications for instrumentation and control (Table 4a.3).
The circuit functions are defined with reference to the DTE as follows:
• Protective ground (shield): The protective ground ensures that the DTE and
DCE chassis are at equal potentials (remember that this protective ground could
cause problems with circulating earth currents).
• Transmitted data (TxD): This line carries serial data from the DTE to the
corresponding pin on the DCE. The line is held at a negative voltage during
periods of line idle.
• Received data (RxD): This line carries serial data from the DCE to the
corresponding pin on the DTE.
• Request to send (RTS): RTS is the request to send hardware control line. This
line is placed active (+V) when the DTE requests permission to send data. The
DCE then activates (+V) the CTS (clear to send) for hardware data flow control.
• Clear to send (CTS): When a half-duplex modem is receiving, the DTE keeps
RTS inhibited. When it is the DTE’s turn to transmit, it advises the modem by
asserting the RTS pin. When the modem asserts the CTS, it informs the DTE
that it is now safe to send data.
• DCE ready: Formerly called data set ready (DSR). The DTE ready line is an
indication from the DCE to the DTE that the modem is ready.
• Signal ground (common): This is the common return line for all the data
transmit and receive signals and all other circuits in the interface. The
connection between the two ends is always made.
• Data carrier detect (DCD): This is also called the received line signal
detector. It is asserted by the modem when it receives a remote carrier and
remains asserted for the duration of the link.
• DTE ready (data terminal ready): Formerly called data terminal ready
(DTR). DTE ready enables-but does not cause, the modem to switch onto the
line. In originate mode, DTE ready must be asserted in order to auto dial. In
answer mode, DTE ready must be asserted to auto answer.
• Ring indicator: This pin is asserted during a ring voltage on the line.
• Data signal rate selector (DSRS): When two data rates are possible, the higher
is selected by asserting DSRS; however, this line is not used much these days.
78
Practical Industrial Data Communications
Pin No.
CCITT
No.
Circuit
1
2
3
4
5
6
7
8
9
10
11
12
–
103
104
105/133
106
107
102
109
–
–
126
122/112
–
BA
BB
CA/CJ
CB
CC
AB
CF
–
–
See Note
SCF/CI
13
14
15
121
118
114
SCB
SBA
DB
16
18
19
20
21
119
141
120
108/112
140/110
SBB
LL
SCA
CD
RL/CG
22
23
125
111/112
CE
CH/CI
24
113
DA
25
26
142
TM
None
Description
Shield
Transmitted data
Received data
Request to send/ready for receiving
Clear to send
DCE ready
Signal common
Received line signal detector
Reserved for testing
Reserved for testing
Unassigned
Secondary received line signal
Detector/data signal rate selected
Secondary clear to send
Secondary transmitted data
Transmitter signal element timing
(DTE source)
Secondary received data
Local loopback
Secondary request to send
DTE ready
Remote loopback/signal quality
Detector
Ring indicator
Data signal rate selector (DTE/DCE
source)
Transmit signal element timing
(DTE/DCE source)
Test mode
(Alt A connector) No connection at
this time
Circuit
Direction
To DCE
To DCE
From DCE
To DCE
From DCE
From DCE
–
From DCE
–
–
–
From DCE
From DCE
To DCE
From DCE
From DCE
To DCE
To DCE
To DCE
T/F DCE
From DCE
T/F DCE
To DCE
From DCE
Table 4a.3
ITU-T V24 pin assignment (ISO 2110)
4a.2
Half-duplex operation of the RS-232 interface
The following description of one particular operation of the RS-232 interface is based on
half-duplex data interchange. The description encompasses the more generally used fullduplex operation.
RS-232 overview 79
Figure 4a.4 shows the operation with the initiating user terminal, DTE, and its
associated modem DCE on the left of the diagram and the remote computer and its
modem on the right.
Figure 4a.4
Half-duplex operational sequence of RS-232
The following sequence of steps occurs when a user sends information over a telephone
link to a remote modem and computer:
• The initiating user manually dials the number of the remote computer.
• The receiving modem asserts the ring indicator line (RI) in a pulsed ON/OFF
fashion reflecting the ringing tone. The remote computer already has its data
terminal ready (DTR) line asserted to indicate that it is ready to receive calls.
Alternatively, the remote computer may assert the DTR line after a few rings.
The remote computer then sets its request to send (RTS) line to ON.
• The receiving modem answers the phone and transmits a carrier signal to the
initiating end. It asserts the DCE ready line after a few seconds.
• The initiating modem asserts the data carrier detect (DCD) line. The initiating
terminal asserts its DTR, if it is not already high. The modem responds by
asserting its DTE ready line.
80
Practical Industrial Data Communications
• The receiving modem asserts its clear to send (CTS) line, which permits the
transfer of data from the remote computer to the initiating side.
• Data is transferred from the receiving DTE (transmitted data) to the receiving
modem. The receiving remote computer then transmits a short message to
indicate to the originating terminal that it can proceed with the data transfer.
The originating modem transmits the data to the originating terminal.
• The receiving terminal sets its request to send (RTS) line to OFF The receiving
modem then sets its clear to send (CTS) line to OFF.
• The receiving modem switches its carrier signal OFF.
• The originating terminal detects that the data carrier detect (DCD) signal has
been switched OFF on the originating modem and switches its RTS line to the
ON state. The originating modem indicates that transmission can proceed by
setting its CTS line to ON.
• Transmission of data proceeds from the originating terminal to the remote
computer.
• When the interchange is complete, both carriers are switched OFF, and in many
cases, the DTR is set to OFF. This means that the CTS, RTS, and DCE ready
lines are set to OFF.
Full-duplex operation requires that transmission and reception occur simultaneously. In
this case, there is no RTS/CTS interaction at either end. The RTS and CTS lines are left
ON with a carrier to the remote computer.
4a.3
Summary of EIA/TIA-232 revisions
A summary of the main differences between RS-232 revisions C, D, and E are discussed
below.
4a.3.1
Revision D – RS-232D
The 25-pin D-type connector was formally specified. In revision C, reference was made
to the D-type connector in the appendices and a disclaimer was included revealing that it
was not intended to be part of the standard; however, it was treated as the de facto
standard.
The voltage ranges for the control and data signals were extended to a maximum limit
of 25 V from the previously specified 15 V in revision C.
The 15 m (50 ft) distance constraint, implicitly imposed to comply with circuit
capacitance, was replaced by ‘circuit capacitance shall not exceed 2500 pF’. (Standard
RS-232 cable has a capacitance of 50 pF/ft.)
4a.3.2
Revision E – RS-232E
Revision E formally specifies the new 26-pin alternative connector, the ALT A connector.
This connector supports all 25 signals associated with RS-232, unlike the 9-pin connector,
which has become associated with RS-232 in recent years. Pin 26 is currently not used. The
technical changes implemented by RS-232E do not present compatibility problems with
equipment confirming to previous versions of RS-232.
This revision brings the RS-232 standard into line with international standards CCITT
V.24, V.28, and ISO 2110.
RS-232 overview 81
4a.4
Limitations
In spite of its popularity and extensive use, it should be remembered that the RS-232
interface standard was originally developed for interfacing data terminals to modems. In
the context of modern requirements, RS-232 has several weaknesses. Most have arisen as
a result of the increased requirements for interfacing other devices such as PCs, digital
instrumentation, digital variable speed drives, power system monitors, and other peripheral
devices in industrial plants.
The main limitations of RS-232 when used for the communications of instrumentation
and control equipment in an industrial environment are:
• The point-to-point restriction, a severe limitation when several ‘smart’
instruments are used.
• The distance limitation of 15 m (50 ft) end-to-end, too short for most control
systems.
• The 20 kbps rate, too slow for many applications.
• The –3 to –25 V and +3 to +25 V signal levels, not directly compatible with
modern standard power supplies.
Consequently, a number of other interface standards have been developed by the RS to
overcome some of these limitations. The RS-485 interface standards are increasingly
being used for instrumentation and control systems.
RS-232 troubleshooting
4b.1
Introduction
Since RS-232 is a point-to-point system, installation is fairly straightforward and simple,
and all RS-232 devices use either DB-9 or DB-25 connectors. These connectors are used
because they are cheap and allow multiple insertions. None of the 232 standards define
which device uses a male or female connector, but traditionally the male (pin) connector
is used on the DTE and the female-type connector (socket) is used on DCE equipment.
This is only traditional and may vary on different equipment. It is often asked why a
25-pin connector is used when only 9 pins are needed. This was done because RS-232
was used before the advent of computers. It was therefore used for hardware control
(RTS/CTS). It was originally thought that, in the future, more hardware control lines
would be needed hence the need for more pins.
When doing an initial installation of an RS-232 connection it is important to note the
following:
•
•
•
•
•
•
4b.2
Is one device a DTE and the other a DCE?
What is the sex and size of connectors at each end?
What is the speed of the communication?
What is the distance between the equipment?
Is it a noisy environment?
Is the software set up correctly?
Typical approach
When troubleshooting a serial data communications interface, one needs to adopt a
logical approach in order to avoid frustration and wasting many hours. A procedure
similar to that outlined below is recommended:
• Check the basic parameters. Are the baud rate, stop/start bits and parity set
identically for both devices? These are sometimes set on DIP switches in the
device. However, the trend is towards using software, configured from a
terminal, to set these basic parameters.
• Identify which is DTE or DCE. Examine the documentation to establish what
actually happens at pins 2 and 3 of each device. On the 25-pin DTE device, pin
2 is used for transmission of data and should have a negative voltage (mark) in
idle state, whilst pin 3 is used for the receipt of data (passive) and should be
approximately at 0 V. Conversely, at the DCE device, pin 3 should have a
RS-232 troubleshooting
83
negative voltage, whilst pin 2 should be around 0 V. If no voltage can be
detected on either pin 2 or 3, then the device is probably not RS-232
compatible and could be connected according to another interface standard,
such as RS-422, RS-485, etc. (Figure 4b.1).
Figure 4b.1
Flowchart to identify an RS-232 device as either a DTE or DCE
• Clarify the needs of the hardware handshaking when used. Hardware handshaking
can cause the greatest difficulties and the documentation should be carefully
studied to yield some clues about the handshaking sequence. Ensure all the
required wires are correctly terminated in the cables.
• Check the actual protocol used. This is seldom a problem but, when the above
three points do not yield an answer, it is possible that there are irregularities in
the protocol structure between the DCE and DTE devices.
• Alternatively, if software handshaking is utilized, make sure both have
compatible application software. In particular, check that the same ASCII
character is used for XON and XOFF.
4b.3
Test equipment
From a testing point of view, the RS-232-E interface standard states that: ‘The generator
on the interchange circuit shall be designed to withstand an open circuit, a short circuit
between the conductor carrying that interchange circuit in the interconnecting cable and
any other conductor in that cable including signal ground, without sustaining damage to
itself or its associated equipment.’
In other words, any pin may be connected to any other pin, or even earth, without
damage and, theoretically, one cannot blow up anything! This does not mean that the
RS-232 interface cannot be damaged. The incorrect connection of incompatible external
voltages can damage the interface, as can static charges.
If a data communication link is inoperable, the following devices may be useful when
analyzing the problem:
• A digital multimeter: Any cable breakage can be detected by measuring the
continuity of the cable for each line. The voltages at the pins in active and
inactive states can also be ascertained by the multimeter to verify its
compatibility to the respective standards
84
Practical Industrial Data Communications
• An LED: The use of an LED is to determine which are the asserted lines or
whether the interface conforms to a particular standard. This is laborious, and
accurate pin descriptions should be available
• A breakout box
• PC-based protocol analyzer (including software)
• Dedicated hardware protocol analyzer (e.g. Hewlett Packard).
4b.3.1
The breakout box
The breakout box is an inexpensive tool that provides most of the information necessary
to identify and fix problems on data communications circuits, such as the serial RS-232,
RS-422, RS-423, and RS-485 interfaces and also on parallel interfaces (Figure 4b.2).
Figure 4b.2
Breakout box showing test points
A breakout box is connected to the data cable, to bring out all conductors in the cable to
accessible test points. Many versions of this equipment are available on the market, from
the ‘homemade’ using a back-to-back pair of male and female DB-25 sockets to fairly
sophisticated test units with built-in LEDs, switches, and test points.
Breakout boxes usually have a male and a female socket and by using two standard serial
cables, the box can be connected in series with the communication link. The 25 test points can
be monitored by LEDs, a simple digital multimeter, an oscilloscope, or a protocol analyzer. In
addition, a switch in each line can be opened or closed while trying to identify the problem.
The major weakness of the breakout box is that while one can interrupt any of the data
lines, it does not help much with the interpretation of the flow of bits on the data
communication lines. A protocol analyzer is required for this purpose.
4b.3.2
Null modem
Null modems look like DB-25 ‘through’ connectors and are used when interfacing two
devices of the same gender (e.g. DTE to DTE, DCE to DCE) or devices from different
manufacturers with different handshaking requirements. A null modem has appropriate
RS-232 troubleshooting
85
internal connections between handshaking pins that ‘trick’ the terminal into believing
conditions are correct for passing data. A similar result can be achieved by soldering extra
loops inside the DB-25 plug. Null modems generally cause more problems than they cure
and should be used with extreme caution and preferably avoided.
Note that the null modem may inadvertently connect pins 1 together, as in Figure 4b.3.
This is an undesirable practice and should be avoided.
Figure 4b.3
Null modem connections
4b.3.3
Loop back plug
This is a hardware plug which loops back the transmit data pin to receive data pin, and
similarly for the hardware handshaking lines. This is another quick way of verifying the
operation of the serial interface without connecting to another system.
4b.3.4
Protocol analyzer
A protocol analyzer is used to display the actual bits on the data line, as well as the
special control codes, such as STX, DLE, LF, CR, etc. The protocol analyzer can be used
to monitor the data bits, as they are sent down the line and compared with what should be
on the line. This helps to confirm that the transmitting terminal is sending the correct data
and that the receiving device is receiving it. The protocol analyzer is useful in identifying
incorrect baud rate, incorrect parity generation method, incorrect number of stop bits,
noise, or incorrect wiring and connection. It also makes it possible to analyze the format
of the message and look for protocol errors.
When the problem has been shown not to be due to the connections, baud rate, bits,
or parity, then the content of the message will have to be analyzed for errors or
inconsistencies. Protocol analyzers can quickly identify these problems.
Purpose-built protocol analyzers are expensive devices and it is often difficult to justify
the cost when it is unlikely that the unit will be used very often. Fortunately, software has
been developed that enables a normal PC to be used as a protocol analyzer. The use of a
PC as a test device for many applications is a growing field, and one way of connecting a
PC as a protocol analyzer is shown in Figure 4b.4.
Figure 4b.4 has been simplified for clarity and does not show the connections on the
control lines (for example, RTS and CTS).
86
Practical Industrial Data Communications
Figure 4b.4
Protocol analyzer connection
4b.4
Typical RS-232 problems
Below is a list of typical RS-232 problems, which can arise because of inadequate
interfacing. These problems could equally apply to two PCs connected to each other or to
a PC connected to a printer (Table 4b.1).
Problem
Garbled or lost data
Probable Cause of Problem
Baud rates of connecting ports may be different
Connecting cables could be defective
Data formats may be inconsistent (Stop bit/parity/number of
data bits)
Flow control may be inadequate
High error rate due to electrical interference
Buffer size of receiver inadequate
First characters garbled
The receiving port may not be able to respond quickly
enough. Precede the first few characters with the ASCII
(DEL) code to ensure frame synchronization
No data communications Power for both devices may not be on
Transmit and receive lines of cabling may be incorrect
Handshaking lines of cabling may be incorrectly connected
Baud rates mismatch
Data format may be inconsistent
Earth loop may have formed for RS-232 line
Extremely high error rate due to electrical interference for
transmitter and receiver
RS-232 troubleshooting
Problem
87
Probable Cause of Problem
Protocols may be inconsistent/Intermittent communications
Intermittent interference on cable
ASCII data has incorrect Mismatch between ‘LF’ and ‘CR’ characters generated by
spacing
transmitting device and expected by receiving device
Table 4b.1
A list of typical RS-232 problems
To determine whether the devices are DTE or DCE, connect a breakout box at one end
and note the condition of the TX light (pin 2 or 3) on the box. If pin 2 is ON, then the
device is probably a DTE. If pin 3 is ON, it is probably a DCE. Another clue could be the
sex of the connector, males are typically DTEs and females are typically DCEs, but not
always (Figure 4b.5).
Figure 4b.5
A 9-pin RS-232 connector on a DTE
When troubleshooting an RS-232 system, it is important to understand that there are
two different approaches. One approach is followed if the system is new and never been
run before and the other if the system has been operating and for some reason does not
communicate at present. New systems that have never worked have more potential
problems than a system that has been working before and now has stopped. If a system is
new it can have three main problems viz. mechanical, setup, and noise. A previously
working system usually has only one problem, viz. mechanical. This assumes that no
one has changed the setup, and noise has not been introduced into the system. In all
systems, whether having previously worked or not, it is best to check the mechanical
parts first.
This is done by:
•
•
•
•
4b.4.1
Verifying that there is power to the equipment
Verifying that the connectors are not loose
Verifying that the wires are correctly connected
Checking that a part, board or module has not visibly failed.
Mechanical problems
Often, mechanical problems develop in RS-232 systems because of incorrect installation
of wires in the D-type connector or because strain reliefs were not installed correctly.
88
Practical Industrial Data Communications
The following recommendations should be noted when building or installing RS-232
cables:
•
•
•
•
•
Keep the wires short (20 m maximum)
Stranded wire should be used instead of solid wire (solid wire will not flex)
Only one wire should be soldered in each pin of the connector
Bare wire should not be showing out of the pin of the connector
The back shell should reliably and properly secure the wire.
The speed and distance of the equipment will determine if it is possible to make the
connection at all. Most engineers try to stay less than 50 ft or about 16 m at 1 15 200 bps.
This is a very subjective measurement and will depend on the cable, voltage of the
transmitter, and the amount and noise in the environment. The transmitter voltage can be
measured at each end when the cable has been installed. A voltage of at least ±5 V should
be measured at each end on both the TX and RX lines (Figure 4b.6).
Figure 4b.6
Measuring the voltage on RS-232
An RS-232 breakout box is placed between the DTE and DCE to monitor the voltages
placed on the wires by looking at pin 2 on the breakout box (Figure 4b.7). Be careful here
because it is possible that the data is being transmitted so fast that the light on the breakout
box does not have time to change. If possible, lower the speed of the communication at both
ends to something like 2 bps.
Figure 4b.7
A RS-232 breakout box
Once it has been determined that the wires are connected as DTE to DCE and that the
distance and speed are not going to be a problem, the cable can be connected at each end.
The breakout box can still be left connected with the cable and both pin 2 and 3 lights on
the breakout box should now be on.
RS-232 troubleshooting
89
The color of the light depends on the breakout box. Some breakout boxes use red for a
one and others use green for a one. If only one light is on then that may mean that a wire is
broken or there is a DTE to DTE connection. A clue to a possible DTE to DTE connection
would be that the light on pin 3 would be off and the one on pin 2 would be on. To correct
this problem, first check the wires for continuity then turn switches 2 and 3 off on the
breakout box and use jumper wires to swap them. If the TX and RX lights come on, a null
modem cable or box will need to be built and inserted in-line with the cable.
If the pin 2 and pin 3 lights are on, one end is transmitting and the control is correct,
then the only thing left is the protocol or noise. Either a hardware or software protocol
analyzer will be needed to troubleshoot the communications between the devices. On new
installations, one common problem is mismatched baud rates. The protocol analyzer will
tell exactly what the baud rates are for each device. Another thing to look for with the
analyzer is the timing. Often, the transmitter waits some time before expecting a proper
response from the receiver. If the receiver takes too long to respond or the response is
incorrect, the transmitter will ‘time out’. This is usually denoted as a ‘communications
error or failure’.
4b.4.2
Setup problems
Once it is determined that the cable is connected correctly and the proper voltage is being
received at each end, it is time to check the setup. The following circumstances need to be
checked before trying to communicate:
•
•
•
•
Is the software communications setup at both ends for either 8N1, 7E1, or 7O1?
Is the baud rate the same at both devices? (1200, 4800, 9600, 19 200, etc.)
Is the software setup at both ends for binary, hex, or ASCII data transfer?
Is the software setup for the proper type of control?
Although the 8 data bits, no parity, and 1 stop bit is the most common setup for
asynchronous communication, often 7 data bits even parity with 1 stop bit is used in
industrial equipment. The most common baud rate used in asynchronous communications
is 9600. Hex and ASCII are commonly used as communication codes.
If one device is transmitting but the other receiver is not responding, then the next thing
to look for is what type of control the devices are using. The equipment manual may
define whether hardware or software control is being used. Both ends should be set up
either for hardware control, software control, or none.
4b.4.3
Noise problems
RS-232, being a single-ended (unbalanced) type of circuit, lends itself to receiving noise.
There are three ways that noise can be induced into an RS-232 circuit:
• Induced noise on the common ground
• Induced noise on the TX or RX lines
• Induced noise on the indicator or control lines.
Ground-induced noise
Different ground voltage levels on the ground line (pin 7) can cause ground loop noise.
Also, varying voltage levels induced on the ground at either end by high-power
equipment can cause intermittent noise. This kind of noise can be very difficult to reduce.
Sometimes, changing the location of the ground on either the RS-232 equipment or the
high-power equipment can help, but this is often not possible. If it is determined that
90
Practical Industrial Data Communications
the noise problem is caused by the ground it may be best to replace the RS-232 link with
a fiber-optic or RS-422 system. Fiber-optic or RS-422 to RS-232 adapters are relatively
cheap, readily available, and easy to install. When the cost of troubleshooting the system
is included, replacing the system often is the cheapest option.
Induced noise on the TX or RX lines
Noise from the outside can cause the communication on an RS-232 system to fail,
although this voltage must be quite large. Because RS-232 voltages in practice are usually
between ±7 and ±12, the noise voltage value must be quite high in order to induce errors.
This type of noise induction is noticeable because the voltage on the TX or RX will be
outside of the specifications of RS-232. Noise on the TX line can also be induced on the
RX line (or vice versa) due to the common ground in the circuit. This type of noise can
be detected by comparing the data being transmitted with the received communication at
the other end of the wire (assuming no broken wire). The protocol analyzer is plugged
into the transmitter at one end and the data monitored. If the data is correct, the protocol
analyzer is then plugged into the other end and the received data monitored. If the data is
corrupt at the receiving end, then noise on that wire may be the problem. If it is
determined that the noise problem is caused by induced noise on the TX or RX lines, it
may be best to move the RS-232 line and the offending noise source away from each
other. If this does not help, it may be necessary to replace the RS-232 link with a fiberoptic or RS-485 system.
Induced noise on the indicator or control lines
This type of noise is very similar to the previous TX/RX noise. The difference is that noise
on these wires may be harder to find. This is because the data is being received at both ends,
but there still is a communication problem. The use of a voltmeter or oscilloscope would
help to measure the voltage on the control or indicator lines and therefore locate the
possible cause of the problem, although this is not always very accurate. This is because
the effect of noise on a system is governed by the ratio of the power levels of the signal and
the noise, rather than a ratio of their respective voltage levels. If it is determined that the
noise is being induced on one of the indicator or control lines, it may be best to move the
RS-232 line and the offending noise source away from each other. If this does not help, it
may be necessary to replace the RS-232 link with a fiber-optic or RS-485 system.
4b.5
Summary of troubleshooting
4b.5.1
Installation
•
•
•
•
•
•
4b.5.2
Is one device a DTE and the other a DCE?
What is the sex and size of the connectors at each end?
What is the speed of the communications?
What is the distance between the equipments?
Is it a noisy environment?
Is the software set up correctly?
Troubleshooting new and old systems
• Verify that there is power to the equipment
• Verify that the connectors are not loose
RS-232 troubleshooting
91
• Verify that the wires are correctly connected
• Check that a part, board, or module has not visibly failed.
4b.5.3
Mechanical problems on new systems
•
•
•
•
•
4b.5.4
Setup problems on new systems
•
•
•
•
4b.5.5
Keep the wires short (20 m maximum)
Stranded wire should be used instead of solid wire (stranded wire will flex)
Only one wire should be soldered in each pin of the connector
Bare wire should not be showing out of the connector pins
The back shell should reliably and properly secure the wire.
Is the software communications set up at both ends for either 8N1, 7E1, or 7O1?
Is the baud rate the same for both devices (1200, 4800, 9600, 19 200, etc.)?
Is the software set up at both ends for binary, hex, or ASCII data transfer?
Is the software setup for the proper type of control?
Noise problems on new systems
• Noise from the common ground
• Induced noise on the TX or RX lines
• Induced noise on the indicator or control lines.
RS-485 overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe the RS-485 standard
• Remedy the following problems:
–
–
–
–
–
–
–
5a.1
Incorrect RS-485 wiring
Excessive common mode voltage
Faulty converters
Isolation
Idle state problems
Incorrect or missing terminations
RTS control via hardware or software.
The RS-485 interface standard
The RS-485-A standard is one of the most versatile of the RS interface standards. It is an
extension of RS-422 and allows the same distance and data speed but increases the
number of transmitters and receivers permitted on the line. RS-485 permits a ‘multidrop’
network connection on two wires and allows reliable serial data communication for:
•
•
•
•
Distances of up to 1200 m (4000 ft, same as RS-422)
Data rates of up to 10 Mbps (same as RS-422)
Up to 32 line drivers on the same line
Up to 32 line receivers on the same line.
The maximum bit rate and maximum length can, however, not be achieved at the same
time. For 24 AWG twisted-pair cable the maximum data rate at 4000 ft (1200 m) is
approximately 90 kbps. The maximum cable length at 10 Mbps is less than 20 ft (6 m).
Better performance will require a higher-grade cable and possibly the use of active (solidstate) terminators in the place of the 120 resistors.
According to the RS-485 standard, there can be 32 ‘standard’ transceivers on the
network. Some manufacturers supply devices that are equivalent to 1/2 or 1/4 standard
device, in which case this number can be increased to 64 or 128. If more transceivers are
required, repeaters have to be used to extend the network.
The two conductors making up the bus are referred to as A in B in the specification. The
A conductor is alternatively known as A–, TxA, and Tx+. The B conductor, in similar
RS-485 overview 93
fashion, is called B+, TxB, and Tx–. Although this is rather confusing, identifying the
A and B wires is not difficult. In the MARK or OFF state (i.e. when the RS-232 TxD pin is
LOW (e.g. −8 V), the voltage on the A wire is more negative than that on the B wire.
The differential voltages on the A and B outputs of the driver (transmitter) are similar
(although not identical) to those for RS-422, namely:
• –1.5 to –6 V on the A terminal with respect to the B terminal for a binary 1
(MARK or OFF) state
• +1.5 to +6 V on the A terminal with respect to the B terminal for a binary 0
(SPACE or ON state).
As with RS-422, the line driver for the RS-485 interface produces a ±5 V differential
voltage on two wires.
The major enhancement of RS-485 is that a line driver can operate in three states called
tri-state operation:
• Logic 1
• Logic 0
• High-impedance.
In the high-impedance state, the line driver draws virtually no current and appears not
to be present on the line. This is known as the ‘disabled’ state and can be initiated by a
signal on a control pin on the line driver integrated circuit. Tri-state operation allows a
multidrop network connection and up to 32 transmitters can be connected on the same
line, although only one can be active at any one time. Each terminal in a multidrop
system must be allocated a unique address to avoid conflicting with other devices on the
system. RS-485 includes current limiting in cases where contention occurs.
The RS-485 interface standard is very useful for systems where several instruments or
controllers may be connected on the same line. Special care must be taken with the
software to coordinate which devices on the network can become active. In most cases, a
master terminal, such as a PC or computer, controls which transmitter/receiver will be
active at a given time.
The two-wire data transmission line does not require special termination if the signal
transmission time from one end of the line to the other end (at approximately 200 m/µs)
is significantly smaller than one quarter of the signal’s rise time. This is typical with
short lines or low bit rates. At high bit rates or in the case of long lines, proper
termination becomes critical. The value of the terminating resistors (one at each end)
should be equal to the characteristic impedance of the cable. This is typically 120 for
twisted-pair wire.
Figure 5a.1 shows a typical two-wire multidrop network. Note that the transmission line
is terminated on both ends of the line but not at drop points in the middle of the line.
An RS-485 network can also be connected in a four-wire configuration as shown in
Figure 5a.2. In this type of connection it is necessary that one node is a master node and
all others slaves. The master node communicates to all slaves, but a slave node can
communicate only to the master. Since the slave nodes never listen to another slave’s
response to the master, a slave node cannot reply incorrectly to another slave node. This
is an advantage in a mixed protocol environment.
During normal operation there are periods when all RS-485 drivers are off, and the
communications lines are in the idle, high-impedance state. In this condition the lines
are susceptible to noise pick up, which can be interpreted as random characters on the
communications line. If a specific RS-485 system has this problem, it should incorporate
bias resistors, as indicated in Figure 5a.3. The purpose of the bias resistors is not only to
94
Practical Industrial Data Communications
reduce the amount of noise picked up, but to keep the receiver biased in the IDLE state
when no input signal is received. For this purpose the voltage drop across the 120
termination resistor must exceed 200 mV AND the A terminal must be more negative
than the B terminal. Keeping in mind that the two 120 resistors appear in parallel, the
bias resistor values can be calculated using Ohm’s Law. For a +5 V supply and 120
terminators, a bias resistor value of 560 is sufficient. This assumes that the bias
resistors are only installed on ONE node.
Figure 5a.1
Typical two-wire multidrop network
Figure 5a.2
Four-wire network configuration
RS-485 overview 95
Figure 5a.3
Suggested installation of bias resistors
Some commercial systems use higher values for the bias resistors, but then assume that
all or several nodes have bias resistors attached. In this case the value of all the bias
resistors in parallel must be small enough to ensure 200 mV across the A and B wires.
RS-485 line drivers are designed to handle 32 nodes. This limitation can be overcome
by employing an RS-485 repeater connected to the network. When data occurs on either
side of the repeater, it is transmitted to the other side. The RS-485 repeater transmits at
full voltage levels, consequently another 31 nodes can be connected to the network.
A diagram for the use of RS-485 with a bi-directional repeater is given in Figure 5a.4.
Figure 5a.4
RS-485 used with repeaters
The ‘gnd’ pin of the RS-485 transceiver should be connected to the logic reference
(also known as circuit ground or circuit common), either directly or through a 100
1/2 W resistor. The purpose of the resistor is to limit the current flow if there is a
significant potential difference between the earth points. This is not shown in Figure 5a.2.
In addition, the logic reference is to be connected to the chassis reference (protective
96
Practical Industrial Data Communications
ground or frame ground) through a 100 1/2 W resistor. The chassis reference, in turn, is
connected directly to the safety reference (green wire ground or power system ground).
If the grounds of the nodes are properly interconnected, then a third wire running in
parallel with the A and B wires are, technically speaking, not necessary. However, this is
often not the case and thus a third wire is added, as in Figure 5a.2. If the third wire is
added, a 100 1/2 W resistor is to be added at each end, as shown in Figure 5a.2.
The ‘drops’ or ‘spurs’ that interconnect the intermediate nodes to the bus need to be as
short as possible since a long spur creates an impedance mismatch, which leads to
unwanted reflections. The amount of reflection that can be tolerated depends on the bit
rate. At 50 kbps a spur of, say, 30 m could be in order, whilst at 10 Mbps the spur might
be limited to 30 cm. Generally speaking, spurs on a transmission line are ‘bad news’
because of the impedance mismatch (and hence the reflections) they create, and should be
kept as short as possible.
Some systems employ RS-485 in a so-called ‘star’ configuration. This is not really a
star, since a star topology requires a hub device at its center. The ‘star’ is in fact a very
short bus with extremely long spurs, and is prone to reflections. It can therefore only be
used at low bit rates.
The ‘decision threshold’ of the RS-485 receiver is identical to that of both RS-422 and
RS-423 receivers (not discussed as they have been superseded by RS-423) at ± 200 mV
( ± 0.2 V), as indicated in Figure 5a.5.
Figure 5a.5
RS-485/422 and 423 receiver sensitivities
RS-485 troubleshooting
5b.1
Introduction
RS-485 is the most common asynchronous voltage standard in use today for multi-drop
communication systems since it is very resistant to noise, can send data at high speeds
(up to 10 Mbps), can be run for long distances (5 km at 1200 bps, 1200 m at 90 kbps),
and is easy and cheap to use.
The RS-485 line drivers/receivers are differential chips. This means that the TX and RX
wires are referenced to each other. A one is transmitted, for example, when one of the
lines is +5 V and the other is 0 V. A zero is then transmitted when the lines reverse and
the line that was +5 V is now 0 V and the line that was 0 V is now +5 V. In working
systems the voltages are usually somewhere around ±2 V with reference to each other.
The indeterminate voltage levels are ±200 mV. Up to 32 devices can be connected on one
system without a repeater. Some systems allow the connection of five legs with four
repeaters and get 160 devices on one system (Figure 5b.1).
Figure 5b.1
RS-485 Chip
Resistors are sometimes used on RS-485 systems to reduce noise, common mode
voltages and reflections.
Bias resistors of values from 560 Ω to 4 kΩ can sometimes be used to reduce noise.
These resistors connect the B+ line to +5 V and the A-line to ground. Higher voltages
should not be used because anything over +12 V will cause the system to fail.
Unfortunately, sometimes these resistors can increase the noise on the system by allowing
98
Practical Industrial Data Communications
a better path for noise from the ground. It is best not to use bias resistors unless required
by the manufacturer.
Common mode voltage resistors usually have a value between 100 k and 200 kΩ. The
values will depend on the induced voltages on the lines. They should be equal and as high
as possible and placed on both lines and connected to ground. The common mode
voltages should be kept less than +7 V, measured from each line to ground. Again,
sometimes these resistors can increase the noise on the system by allowing a better path
for noise from the ground. It is best not to use common mode resistors unless required by
the manufacturer or as needed.
The termination resistor value depends on the cable used and is typically 120 Ω. Values
less than 110 Ω should not be used since the driver chips are designed to drive a load
resistance not less than 54 Ω, being the value of the two termination resistors in parallel
plus any other stray resistance in parallel. These resistors are placed between the lines
(at the two furthest ends, not on the stubs) and reduce reflections. If the lines are less
than 100 m long and speeds are 9600 baud or less, the termination resistor usually
becomes redundant, but having said that, you should always follow the manufacturer’s
recommendations.
5b.2
RS-485 vs RS-422
In practice, RS-485 and RS-422 are very similar to each other and manufacturers often
use the same chips for both. The main working difference is that RS-485 is used for twowire multi-drop half-duplex systems and RS-422 is for four-wire point-to-point fullduplex systems. Manufacturers often use a chip like the 75154 with two RS-485 drivers
on board as an RS-422 driver. One driver is used as a transmitter and the other is
dedicated as a receiver. Because the RS-485 chips have three states, TX, RX, and high
impedance, the driver that is used as a transmitter can be set to high-impedance mode
when the driver is not transmitting data. This is often done using the RTS line from the
RS-232 port. When the RTS goes high (+voltage) the transmitter is effectively turned off
by putting the transmitter in the high-impedance mode. The receiver is left on all the time,
so data can be received when it comes in. This method can reduce noise on the line by
having a minimum of devices on the line at a time.
5b.3
RS-485 installation
Installation rules for RS-485 vary per manufacturer and since there are no standard
connectors for RS-485 systems, it is difficult to define a standard installation procedure.
Even so, most manufacture procedures are similar. The most common type of connector
used on most RS-485 systems is either a one-part or two-part screw connector. The
preferred connector is the two-part screw connector with the sliding box under the screw
(phoenix type). Other connectors use a screw on top of a folding tab. Manufacturers
sometimes use the DB-9 connector instead of a screw connector to save money.
Unfortunately, the DB-9 connector has problems when used for multidrop connections.
The problem is that the DB-9 connectors are designed so that only one wire can be
inserted per pin. RS-485 multidrop systems require the connection of two wires so that
the wire can continue down the line to the next device. This is a simple matter with screw
connectors, but it is not so easy with a DB-9 connector. With a screw connector, the two
wires are twisted together and inserted in the connector under the screw. The screw is
then tightened down and the connection is made. With the DB-9 connector, the two wires
must be soldered together with a third wire. The third wire is then soldered to the single
pin on the connector.
RS-485 troubleshooting
99
Note: When using screw connectors, the wires should NOT be soldered together
(Figure 5b.2). Either the wires should be just twisted together or a special crimp ferrule
should be used to connect the wires before they are inserted in the screw connector.
Figure 5b.2
A bad RS-485 connection
Serious problems with RS-485 systems are rare (that is one reason it is used) but having
said that, there are some possible problems that can arise in the installation process:
•
•
•
•
•
•
•
The wires get reversed (e.g. black to white and white to black)
Loose or bad connections due to improper installation
Excessive electrical or electronic noise in the environment
Common mode voltage problems
Reflection of the signal due to missing or incorrect terminators
Shield not grounded, grounded incorrectly, or not connected at each drop
Starring or tee-ing of devices (i.e. long stubs).
To make sure the wires are not reversed, check that the same color is connected to the
same pin on all connectors. Check the manufacturer’s manual for proper wire color codes.
Verifying that the installers are informed of the proper installation procedures can reduce
loose connections. If the installers are provided with adjustable torque screwdrivers, then
the chances of loose or over tightened screw connections can be minimized.
5b.4
Noise problems
RS-485, being a differential type of circuit, is resistant to receiving common mode noise.
There are five ways that noise can be induced into an RS-485 circuit.
1.
2.
3.
4.
5.
Induced noise on the A/B lines
Common mode voltage problems
Reflections
Unbalancing the line
Incorrect shielding.
100
Practical Industrial Data Communications
5b.4.1
Induced noise
Noise from the outside can cause communication on an RS-485 system to fail.
Although the voltages on an RS-485 system are small (±5 V), because the output of
the receiver is the difference of the two lines, the voltage induced on the two lines
must be different. This makes RS-485 very tolerant to noise. The communications
will also fail if the voltage level of the noise on the either or both lines is outside of
the minimum or maximum RS-485 specification. Noise can be detected by comparing
the data communication being transmitted out of one end with the received
communication at the other (assuming no broken wire). The protocol analyzer is
plugged into the transmitter at one end and the data monitored. If the data is correct,
the protocol analyzer is then plugged into the other end and the received data
monitored. If the data is corrupt at the received end, then the noise on that wire may
be the problem. If it is determined that the noise problem is caused by induced noise
on the A or B lines it may be best to move the RS-485 line or the offending noise
source away from each other.
Excessive noise is often due to the close proximity of power cables. Another possible
noise problem could be caused by an incorrectly installed grounding system for the cable
shield. Installation standards should be followed when the RS-485 pairs are installed
close to other wires and cables. Some manufacturers suggest biasing resistors to limit
noise on the line while others dissuade the use of bias resistors completely. Again, the
procedure is to follow the manufacturer’s recommendations. Having said that, it is
usually found that biasing resistors are of minimal value, and that there are much better
methods of reducing noise in an RS-485 system.
5b.4.2
Common mode noise
Common mode noise problems are usually caused by a changing ground level. The
ground level can change when a high-current device is turned on or off. This large current
draw causes the ground level as referenced to the A and B lines to rise or decrease. If the
voltages on the A or B line are raised or lowered outside of the minimum or maximum, as
defined by the manufacturer specifications, it can prohibit the line receiver from operating
correctly. This can cause a device to float in and out of service. Often, if the common
mode voltage gets high enough, it can cause the module or device to be damaged. This
voltage can be measured using a differential measurement device like a handheld digital
voltmeter. The voltage between A and ground and then B to ground is measured. If the
voltage is outside of specifications then resistors of values between 100 and 200 kΩ are
placed between A and ground and B and ground. It is best to start with the larger value
resistor and then verify the common mode voltage. If it is still too high, try a lower
resistor value and recheck the voltage. At idle the voltage on the A line should be close to
0 and the B line should be between 2 and 6 V. It is not uncommon for an RS-485
manufacturer to specify a maximum common voltage value of +12 and –7 V, but it is best
to have a system that is not near these levels. It is important to follow the manufacturer’s
recommendations for the common mode voltage resistor value or whether they are needed
at all (Figure 5b.3).
Note: When using bias resistors, neither the A nor the B line on the RS-485 system
should ever be raised higher than +12 V or lower than –7 V. Most RS-485 driver chips
will fail if this happens. It is important to follow the manufacturer recommendations for
bias resistor values or whether they are needed at all.
RS-485 troubleshooting
101
Figure 5b.3
Common mode resistors
5b.4.3
Reflections or ringing
Reflections are caused by the signal reflecting off the end of the wire and corrupting the
signal. It usually affects the devices near the end of the line. It can be detected by placing
a balanced ungrounded oscilloscope across the A and B lines. The signal will show
ringing superimposed on the square wave. A termination resistor of typically 120 Ω is
placed at each end of the line to reduce reflections. This is more important at higher
speeds and longer distances (Figure 5b.4).
Figure 5b.4
Ringing on an RS-485 signal
5b.4.4
Unbalancing the line
Unbalancing the line does not actually induce noise, but it does make the lines more
susceptible to noise. A line that is balanced will more or less have the same resistance,
capacitance, and inductance on both conductors. If this balance is disrupted, the lines then
become affected by noise more easily. There are a few ways most RS-485 lines become
unbalanced:
• Using a star topology
• Using a ‘tee’ topology
102
Practical Industrial Data Communications
• Using unbalanced cable
• Damaged transmitter or receiver.
There should, ideally, be no stars or tees in the RS-485-bus system. If another device is
to be added in the middle, a two-pair cable should be run out and back from the device.
The typical RS-485 system would have a topology that would look something as in
Figures 5b.5 and 5b.6.
Figure 5b.5
A typical RS-485 multi-drop configuration
Figure 5b.6
Adding a new device to a RS-485 bus
The distance between the end of the shield and the connection in the device should be
no more than 10 mm or 1/2 in. The end of the wires should be stripped only far enough to
fit all the way into the connector, with no exposed wire outside the connector. The wire
should be twisted tightly before insertion into the screw connector. Often, installers will
strip the shield from the wire and connect the shields together at the bottom of the
cabinet. This is incorrect, as there would be from 1 to 2 m of exposed cable from the
terminal block at the bottom of the cabinet to the device at the top. This exposed cable
will invariably receive noise from other devices in the cabinet. The pair of wires should
be brought right up to the device and stripped as mentioned above.
RS-485 troubleshooting
5b.4.5
103
Shielding
The choices of shielding for an RS-485 installation are:
• Braided
• Foil (with drain wire)
• Armored.
From a practical point of view, the noise reduction difference between the first two is
minimal. Both the braided and the foil shields will provide the same level of protection
against capacitive noise. The third choice, armored cable has the distinction of protecting
against magnetic induced noise. Armored cable is much more expensive than the first two
and therefore the braided and the foil types of cable are more popular. For most installers,
it is a matter of personal choice when deciding to use either braided or foil shielded wire.
With the braided shield, it is possible to pick the A and B wires between the braids of
the shield without breaking the shield. If this method is not used, then the shields of the
two wires should be soldered or crimped together. A separate wire should be run from the
shield at the device down to the ground strip in the bottom of the cabinet, but only one per
bus, not per cabinet. It is incorrect in most cases to connect the shield to ground in each
cabinet, especially if there are long distances between cabinets.
5b.5
Test equipment
When testing or troubleshooting an RS-485 system, it is important to use the right test
equipment. Unfortunately, there is very little in generic test equipment specifically
designed for RS-485 testing. The most commonly used are the multimeter, oscilloscope
and the protocol analyzer. It is important to remember that both of these types of test
equipment must have floating differential inputs. The standard oscilloscope or multimeter
each has their specific uses in troubleshooting an RS-485 system.
5b.5.1
Multimeter
The multimeter has three basic functions in troubleshooting or testing an RS-485 system.
1. Continuity verification
2. Idle voltage measurement
3. Common mode voltage measurement.
Continuity verification
The multimeter can be used before start-up to check that the lines are not shorted or open.
This is done as follows:
•
•
•
•
•
•
•
•
•
Verify that the power is off
Verify that the cable is disconnected from the equipment
Verify that the cable is connected for the complete distance
Place the multimeter in the continuity check mode
Measure the continuity between the A and B lines
Verify that it is open
Short the A and B at the end of the line
Verify that the lines are now shorted
Un-short the lines when satisfied that the lines are correct.
104
Practical Industrial Data Communications
If the lines are internally shorted before they are manually shorted, then check to see if an A
line is connected to a B line. In most installations the A line is kept as one color wire and the
B is kept as another. This procedure keeps the wires away from accidentally being crossed.
The multimeter is also used to measure the idle and common mode voltages between
the lines.
Idle voltage measurement
At idle the master usually puts out a logical ‘1’ and this can be read at any station in the
system. It is read between A and B lines and is usually somewhere between –1.5 and –5 V
(A with respect to B). If a positive voltage is measured, it is possible that the leads on the
multimeter need to be reversed. The procedure for measuring the idle voltage is as follows:
•
•
•
•
•
Verify that the power is on
Verify that all stations are connected
Verify that the master is not polling
Measure the voltage difference between the A– and B+ lines starting at the master
Verify and record the idle voltage at each station.
If the voltage is zero, then disconnect the master from the system and check the output
of the master alone. If there is idle voltage at the master, then plug in each station one at a
time until the voltage drops to or near zero. The last station probably has a problem.
Common mode voltage measurement
Common mode voltage is measured at each station, including the master. It is
measured from each of the A and B lines to ground. The purpose of the measurement
is to check if the common mode voltage is getting close to maximum tolerance. It is
important therefore to know what the maximum common mode voltage is for the
system. In most cases, it is +12 and –7 V. A procedure for measuring the common
mode voltage is:
• Verify that the system is powered up
• Measure and record the voltage between the A and ground and the B and
ground at each station
• Verify that voltages are within the specified limits as set by the manufacturer.
If the voltages are near or out of tolerance, then either contact the manufacturer or
install resistors between each line to ground at the station that has the problem. It is
usually best to start with a high value such as 200 kΩ 1/4 W and then go lower as needed.
Both resistors should be of the same value.
5b.5.2
Oscilloscope
Oscilloscopes are used for:
• Noise identification
• Ringing
• Data transfer.
Noise identification
Although the oscilloscope is not the best device for noise measurement, it is good for
detection of some types of noise. The reason the oscilloscope is not that good at noise
RS-485 troubleshooting
105
detection is that it is a two-dimensional voltmeter; whereas the effect of the noise is seen
in the ratio of the power of a signal vs the power of the noise. But having said that, the
oscilloscope is useful for determining noise that is constant in frequency. This can be a
signal such as 50/60 Hz hum, motor-induced noise or relays clicking on and off. The
oscilloscope will not show intermittent noise, high-frequency radio waves or the power
ratio of the noise vs the signal.
Ringing
Ringing is caused by the reflection of signals at the end of the wires. It happens more
often on higher baud rate signals and longer lines. The oscilloscope will show this ringing
as a distorted square wave.
As mentioned before, the ‘fix’ for ringing is a termination resistor at each end of the
line. Testing the line for ringing can be done as follows:
• Use a two-channel oscilloscope in differential (A-B) mode.
• Connect the probes of the oscilloscope to the A and B lines. Do NOT use a
single-channel oscilloscope, connecting the ground clip to one of the wires will
short that wire to ground and prevent the system from operating.
• Set up the oscilloscope for a vertical level of around 2 V per division.
• Set up the oscilloscope for horizontal level that will show one square wave of
the signal per division.
• Use an RS-485 driver chip with a TTL signal generator at the appropriate baud
rate. Data can be generated by allowing the master to poll, but because of the
intermittent nature of the signal, the oscilloscope will not be able to trigger. In
this case a storage oscilloscope will be useful.
• Check to see if the waveform is distorted.
Data transfer
Another use for the oscilloscope is to verify that data is being transferred. This is done
using the same method as described for observing ringing, and by getting the master to
send data to a slave device. The only difference is the adjustment of the horizontal level.
It is adjusted so that the screen shows complete packets. Although this is interesting, it is
of limited value unless noise is noted or some other aberration is displayed.
5b.5.3
Protocol analyzer
The protocol analyzer is a very useful tool for checking the actual packet information.
Protocol analyzers come in two varieties, hardware and software. Hardware protocol
analyzers are very versatile and can monitor, log, and interpret many types of protocols.
When the analyzer is hooked-up to the RS-485 system, many problems can be displayed
such as:
•
•
•
•
•
Wrong baud rates
Bad data
The effects of noise
Incorrect timing
Protocol problems.
The main problem with the hardware protocol analyzer is the cost and the relatively
rare use of it. The devices can cost from US$5000 to US$10 000 and are often used only
once or twice a year.
106
Practical Industrial Data Communications
The software protocol analyzer, on the other hand, is cheap and has most of the features
of the hardware type. It is a program that sits on the normal PC and logs data being
transmitted down the serial link. Because it uses existing hardware (the PC), it can be a
much cheaper but useful tool. The software protocol analyzer can see and log most of the
same problems a hardware type can.
The following procedure can be used to analyze the data stream:
•
•
•
•
Verify that the system is on and the master is polling
Set up the protocol analyzer for the correct baud rate and other system parameters
Connect the protocol analyzer in parallel with the communication bus
Log the data and analyze the problem.
5b.6
Summary
5b.6.1
Installation
•
•
•
•
•
•
5b.6.2
Troubleshooting new and old systems
•
•
•
•
5b.6.3
Keep the wires short, if possible
Stranded wire should be used instead of solid wire (stranded wire will flex)
Only one wire should be soldered in each pin of the connector
Bare wire should not be showing out of the pin of the connector
The back shell should reliably and properly secure the wire.
Setup problems on new systems
•
•
•
•
5b.6.5
Verify that there is power to the equipment
Verify that the connectors are not loose
Verify that the wires are correctly connected
Check that a part, board or module has not visibly failed.
Mechanical problems on new systems
•
•
•
•
•
5b.6.4
Are the connections correctly made?
What is the speed of the communications?
What is the distance between the equipment?
Is it a noisy environment?
Is the software setup correctly?
Are there any tees or stars in the bus?
Is the software communications setup at both ends for 8N1, 7E1, or 7O1?
Is the baud rate the same at both devices (1200, 4800, 9600, 19 200, etc.)?
Is the software setup at both ends for binary, hex, or ASCII data transfer?
Is the software setup for the proper type of control?
Noise problems on new systems
• Induced noise on the A or B lines?
• Common mode voltage noise?
• Reflection or ringing?
Current loop and RS-485 converters
overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe current loop hardware
• Describe RS-232/RS-485 interface converters
• Fix problems with cabling and isolation for interface converters.
6a.1
The 20 mA current loop
Another commonly used interface technique is the current loop. This uses a current signal
rather than a voltage signal, employing a separate pair of wires for the transmitter current
loop and receiver current loop.
A current level of 20 mA, or up to 60 mA, is used to indicate logic 1 and 0 mA or
4 mA to indicate logic 0. The use of a constant current signal enables a far greater
separation distance to be achieved than with a standard RS-232-voltage connection.
This is due to the higher noise immunity of the 20 mA current loop, which can drive
long lines of up to 1 km, but at reasonably slow bit rates. Current loops are mainly used
between printers and terminals in the industrial environment. Figure 6a.1 illustrates the
current loop interface.
Figure 6a.1
The 20 mA current loop interface
108
Practical Industrial Data Communications
6a.2
Serial interface converters
Serial interface converters are becoming increasingly important with the move away from
RS-232C to industrial standards such as RS-422 and RS-485. Since many industrial
devices still use RS-232 ports, it is necessary to use converters to interface a device to
other physical interface standards. Interface converters can also be used to increase the
effective distance between two RS-232 devices.
The most common converters are:
• RS-232/422
• RS-232/485 (Figure 6a.2)
• RS-232/current loop.
Figure 6a.3 is a block diagram of an RS-232/RS-485 converter.
Figure 6a.2
RS-232/485 converter
Figure 6a.3
Block structure of RS-232/RS-485 converter
Figure 6a.4 shows a circuit-wiring diagram.
The RS-232/422 and RS-232/485 interface converters are very similar and provide
bidirectional full-duplex conversion for synchronous or asynchronous transfer between
RS-232 and RS-485 ports. These converters may be powered from an external AC source,
Current loop and RS-485 converters overview 109
such as 240 V, or smaller units can be powered at 12 V DC from pins 9 and 10 of the
RS-232 port. For industrial applications, externally powered units are recommended. The
RS-232 standard was designed for communications, not as a power supply unit!
Figure 6a.4
Wiring diagram for RS-232/485 converter
LEDs are provided to show the status of incoming signals from both RS-232 and
RS-485.
When operating over long distances, a useful feature of interface converters is optical
isolation. This is especially valuable in areas prone to lightning. Even if the equipment is
not directly hit by lightning, the earth potential rise (EPR) in the surrounding area may
be sufficient to damage the communications equipment at both ends of the link. Some
specifications quote over 10 kV isolation, but these figures are seldom backed up with
engineering data and should be treated with some caution.
Typical specifications for the RS-232/422 or RS-232/485 converters are:
•
•
•
•
•
•
•
•
Data transfer rate of up to 1 Mbps
DCE/DTE switch selectable
Converts all data and control signals
LEDs for status of data and control signals
Powered from AC source
Optically isolated (optional)
DB-25 connector (male or female)
DB-37 connector (male or female).
Typical specifications for the RS-232/current loop converters are:
•
•
•
•
•
•
•
•
20 or 60 mA operation
DCE/DTE, full/half-duplex selectable
Active or passive loops supported
Optically isolated (optional)
Powered from AC source
Data rates of up to 19 200 kbps over 3 km (10 000 ft)
DB-25 connector (male or female)
Current loop connector – 5 screw.
Current loop and RS-485 converters
troubleshooting
6b.1
Troubleshooting converters
The troubleshooting procedure is very similar to that discussed in the RS-232 and RS-485
sections with a few additional considerations. It should be emphasized that care should
be taken in using non-isolated converters where there are any possibilities of electrical
surges and transients. In addition, loop-powered RS-232/485 converters should also be
avoided, as they can be unreliable. Using one of the handshaking pins of the RS-232
port may sound clever and avoid the necessity of providing a separate power supply but
RS-232 was never intended to be a power supply.
A few suggestions for troubleshooting and the diagnostic LEDs used are outlined in the
following sections.
6b.1.1
Check that the converter is powered up
If there is no power to the converter or the power supply on the converter is damaged the
LED on the converter will not light up. Typically, it should be illuminated solid green.
In addition, on some converters there is provision for an isolated power LED, which
should also be illuminated solid green.
With all the connections made, the converter should have the power light on, DCD,
CTS, RTS LEDs on, and the receive and transmit lights off. If you loop back the transmit
and receive pins of the converter, and use a protocol analysis program such as PAT, you
should see the transmit and receive lights flickering to indicate data flow and characters
transmitted should be received. This will be the quickest test to indicate that all is well
with the converter.
6b.1.2
Network transmit
This LED will be lit up to indicate that the RS-485 converter is active and driving the
network. In a multi-drop network, only one unit should be driving the line at one time
otherwise, there will be contention with no resultant transmission at all.
6b.1.3
Configuration jumpers
Although the configuration jumpers are often set internally, sometimes the user can also
set them.
Current loop and RS-485 converters troubleshooting
111
Choose the RTS transmit polarity correctly. Normally assertion of RTS is used to place
the RS-485 transmitter in transmit mode.
6b.1.4
Failsafe termination
This jumper determines what happens if the RS-232 host is disconnected from the
converter. Conventionally, the RS-485 transmitter should be disabled when the RS-232
host is disconnected. To test this jumper, power up the system and disconnect the RS-232
connection. The network transmit LED should be off.
6b.1.5
DTE/DCE settings
These should be checked that they are correctly set up depending on the application. Note
that no damage will occur if a mistake is made in setting them up. The system simply will
not work.
6b.1.6
Digital ground/shield
Sometimes, noise rejection performance can be improved by grounding the digital logic.
But this is a very rare setting and other issues should be examined first before trying this
configuration.
TCP/IP overview
7a.1
Introduction
TCP/IP is the de facto global standard for the Internet (network) and host-to-host
(transport) layer implementation of internetwork applications because of the popularity of
the Internet. The Internet (in its early years known as ARPANet), was part of a military
project commissioned by the Advanced Research Projects Agency (ARPA), later known
as the Defense Advanced Research Agency or DARPA. The communications model used
to construct the system is known as the ARPA (or DoD) model.
Whereas the OSI model was developed in Europe by the International Organization for
Standardization (ISO), the ARPA model was developed in the USA by ARPA. Although
they were developed by different bodies and at different points in time, both serve as
models for a communications infrastructure and hence provide ‘abstractions’ of the same
reality. The remarkable degree of similarity is therefore not surprising.
Whereas the OSI model has seven layers, the ARPA model has four layers. The OSI
layers map onto the ARPA model as follows:
• The OSI session, presentation and applications layers are contained in the
ARPA process/application layer
• The OSI transport layer maps onto the ARPA host-to-host layer (sometimes
referred to as the service layer)
• The OSI network layer maps onto the ARPA Internet layer
• The OSI physical and data-link layers map onto the ARPA network interface
layer.
The relationship between the two models is depicted in Figure 7a.1.
TCP/IP, or rather the TCP/IP protocol suite, is not limited to the TCP and IP protocols,
but consists of a multitude of interrelated protocols that occupy the upper three layers of
the ARPA model. TCP/IP does NOT include the bottom network interface layer, but
depends on it for access to the medium.
As depicted in Figure 7a.2, an IP transmission frame originating on a specific host
(computer) would contain the local network (for example, Ethernet) header and trailer
applicable to that host. As the message proceeds along the internetwork, this header and
trailer could be replaced depending on the type of network on which the packet finds
itself – be that X.25, frame relay, or ATM. The IP datagram itself would remain
untouched, unless it has to be fragmented and reassembled along the way.
TCP/IP overview 113
Figure 7a.1
OSI vs ARPA models
Figure 7a.2
Internet frame
Note: Any Internet-related specification is referenced as a request for comments
or RFC, RFCs can be obtained from various sources on the Internet such as
www.rfc-editor.org.
7a.1.1
The Internet layer
This layer is primarily responsible for the routing of packets from one host to another.
Each packet contains the address information needed for its routing through the
internetwork to the destination host. The dominant protocol at this level is the Internet
protocol (IP). There are, however, several other additional protocols required at this level
such as:
• Address resolution protocol (ARP), RFC 826: This is used for the translation of an IP address to a hardware (MAC) address, such as required by
Ethernet.
• Reverse address resolution protocol (RARP), RFC 903: This is the complement
of ARP and translates a hardware address to an IP address.
• Internet control message protocol (ICMP), RFC 792: This is a protocol used
for exchanging control or error messages between routers or hosts.
114
Practical Industrial Data Communications
7a.1.2
The host-to-host layer
This layer is primarily responsible for data integrity between the sender host and the
receiver host regardless of the path or distance used to convey the message. It has two
protocols associated with it, namely:
1. User data protocol (UDP), a connectionless (unreliable) protocol used for
higher layer port addressing with minimal protocol overhead (RFC 768).
2. Transmission control protocol (TCP), a connection-oriented protocol that
offers a very reliable method of transferring a stream of data in byte format
between applications (RFC 793).
7a.1.3
The process/application layer
This layer provides the user or application programs with interfaces to the TCP/IP stack.
These include (but are not limited to) file transfer protocol (FTP), trivial file transfer
protocol (TFTP), simple mail transfer protocol (SMTP), telecommunications network
(TELNET), post office protocol (POP3), remote procedure calls (RPC), remote login
(RLOGIN), hypertext transfer protocol (HTTP), and network time protocol (NTP). Users
can also develop their own application layer protocols.
7a.2
Internet layer protocols (packet transport)
This section will deal with the Internet protocol (IP), the Internet control message
protocol (ICMP) and the address resolution protocol (ARP). IPv4 is officially in the
process of being replaced by IPv6 but it will still be some time before IPv6 makes its
appearance. This section will therefore, for the sake of simplicity, only deal with IPv4 and
its associated protocols.
7a.2.1
IP version 4 (IPv4)
IP (RFC 791) is responsible for the delivery of packets (‘datagrams’) between hosts. It is
analogous to the postal system, in that it forwards (routes) and delivers datagrams on the
basis of IP addresses attached to the datagrams, in the same way the postal service would
process a letter based on the postal address. The IP address is a 32-bit entity containing
both the network address (the ‘zip code’) and the host address (the ‘street address’).
IP also breaks up (fragments) datagrams that are too large. This is often necessary
because with LANs and WANs, a datagram may have to traverse on its way to its
destination that may have different frame size limitations. For example, Ethernet can
handle 1500 bytes but X.25 can handle only 576 bytes. IP on the sending side will
fragment a datagram if necessary, attach an IP header to each fragment, and send them off
consecutively. On the receiving side, IP will again rebuild the original datagram.
The IPv4 header
The IP header is appended to the information that IP accepts from higher-level protocols,
before passing it around the network. This information could, within itself, contain the
headers appended by higher-level protocols such as TCP. The header consists of at least
five 32-bit (4 byte) ‘long words’, i.e. 20 bytes total and is made up as in Figure 7a.3.
The ver (version) field is 4 bits long and indicates the version of the IP protocol in use.
For IPv4, it is 4.
TCP/IP overview 115
Figure 7a.3
IPv4 header
This is followed by the 4-bit IHL (Internet header length) field that indicates the length
of the IP header in 32 bit ‘long words’. This is necessary since the IP header can contain
options and therefore does not have a fixed length.
The 8-bit type of service (ToS) field informs the network about the quality of service
required for this datagram. The ToS field is composed of a 3-bit precedence field (which
is often ignored) and an unused (LSB) bit that must be 0. The remaining 4 bits may only
be turned on (set = 1) one at a time, and are allocated as follows:
Bit 3: minimize delay
Bit 4: maximize throughput
Bit 5: maximize reliability
Bit 6: minimize monetary cost.
Total length (16 bits) is the length of the entire datagram, measured in bytes. Using this
field and the IHL length, it can be determined where the data starts and ends. This field
allows the length of a datagram to be up to 216 = 65 536 bytes, although such long
datagrams are impractical. All hosts must at least be prepared to accept datagrams of up
to 576 octets.
The 16-bit identifier uniquely identifies each datagram sent by a host. It is normally
incremented by one for each successive datagram sent. In the case of fragmentation, it is
appended to all fragments of the same datagram for the sake of reconstructing the
datagram at the receiving end. It can be compared to the ‘tracking’ number of an item
delivered by registered mail or UPS.
The 3-bit flag field contains two flags, used in the fragmentation process, viz. DF and
MF. The DF (don’t fragment) flag is set (=1) by the higher-level protocol (for example,
TCP) if IP is NOT allowed to fragment a datagram. If such a situation occurs, IP will not
fragment and forward the datagram, but simply return an appropriate ICMP error message
to the sending host. If fragmentation does occur, MF = 1 will indicate that there are more
fragments to follow, whilst MF = 0 indicates that it is the last fragment to be sent.
The 13-bit fragment offset field indicates where in the original datagram a particular
fragment belongs, for example, how far the beginning of the fragment is removed from
the end of the header. The first fragment has offset zero. The fragment offset is measured
in units of 8 bytes (64 bits); i.e. the transmitted offset is equal to the actual offset divided
by eight.
116
Practical Industrial Data Communications
The TTL (time to live) field ensures that undeliverable datagrams are eventually
discarded. Every router that processes a datagram must decrease the TTL by one and if
this field contains the value zero, then the datagram must be destroyed. Typically, a
datagram can be delivered anywhere in the world by traversing no more than 30 routers.
The 8-bit protocol field indicates the next (higher) level protocol header present in
the data portion of the IP datagram, in other words the protocol that resides above IP in
the protocol stack and which has passed the datagram down to IP. Typical values are
1 for ICMP, 6 for TCP, and 17 for UDP. A more detailed listing is contained in
RFC1700.
The checksum is a 16-bit mathematical checksum on the header only. Since some
header fields change all the time (for example, TTL), this checksum is recomputed and
verified at each point that the IP header is processed. It is not necessary to cover the
data portion of the datagram, as the protocols making use of IP, such as ICMP, IGMP,
UDP, and TCP, all have a checksum in their headers to cover their own header and
data.
Finally, the source and destination addresses are the 32-bit IP addresses of the origin
and the destination hosts of the datagram.
IPv4 addressing
The ultimate responsibility for the issuing of IP addresses is vested in the Internet
assigned numbers authority (IANA). This responsibility is, in turn, delegated to the three
regional Internet registries (RIRs) viz. APNIC (Asia-Pacific network information center),
ARIN (American registry for Internet numbers), and RIPE NCC (Reseau IP Europeans).
RIRs allocate blocks of IP addresses to Internet service providers (ISPs) under their
jurisdiction, for subsequent issuing to users or sub-ISPs.
The IPv4 address consists of 32 bits, e.g. 1100000001100100011001 0000000001.
Since this number is fine for computers but a little difficult for human beings, it is divided
into four octets w, x, y and z. Each octet is converted to its decimal equivalent. The result
of the conversion is written in the format 192.100.100.1. This is known as the ‘dotted
decimal’ or ‘dotted quad’ notation. As mentioned earlier, one part of the IP address is
known as the network ID or ‘NetID’ while the rest is known as the ‘HostID’.
Originally, IP addresses were allocated in so-called address classes. Although the
system proved to be problematic, and IP addresses are currently issued ‘classless’, the
legacy of IP address classes remains and has to be understood.
To provide for flexibility in assigning addresses to networks, the interpretation of the
address field was coded to specify either a small number of networks with a large number
of hosts (class A), or a moderate number of networks with a moderate number of hosts
(class B), or a large number of networks with a small number of hosts (class C). There
was also provision for extended addressing modes, class D was intended for multicasting
whilst E was reserved for future use (Figure 7a.4).
For class A, the first bit is fixed at 0. The values for ‘w’ can therefore only vary
between 0 and 12710. 0 is not allowed and 127 is a reserved number used for testing. This
allows for 126 class A NetIDs. The number of HostIDs is determined by octets ‘x’, ‘y’
and ‘z’. From these 24 bits, 224 = 16 777 218 combinations are available. All zeros and all
ones are not permissible, which leaves 16 777 216 usable combinations.
For class B, the first two bits are fixed at 10. The binary values for ‘w’ can therefore
only vary between 12810 and 19110. The number of NetIDs is determined by octets ‘w’
and ‘x’. The first 2 bits are used to indicate class B and hence cannot be used. This leaves
fourteen usable bits. Fourteen bits allow 214 = 16 384 NetIDs. The number of HostIDs is
TCP/IP overview 117
determined by octets ‘y’ and ‘z’. From these 16 bits, 216 = 65 536 combinations are
available. All zeros and all ones are not permissible, which leaves 65 534 usable
combinations.
Figure 7a.4
Address structure for IPv4
For class C, the first three bits are fixed at 110. The binary values for ‘w’ can therefore
only vary between 19210 and 22310. The number of NetIDs is determined by octets ‘w’,
‘x’ and ‘y’. The first three bits (110) are used to indicate class C and hence cannot be
used. This leaves 22 usable bits. Twenty-two bits allow 222 = 2 097 152 combinations for
NetIDs. The number of HostIDs is determined by octet ‘z’. From these 8 bits, 28 = 256
combinations are available. Once again, all zeros and all ones are not permissible which
leaves 254 usable combinations.
In order to determine where the NetID ends and the HostID begins, each IP address is
associated with a subnet mask, or, technically more correct, a netmask. This mask starts
with a row of contiguous ones from the left; one for each bit that forms part of the NetID.
This is followed by zeroes, one for each bit comprising the HostID.
7a.2.2
Address resolution protocol (ARP)
Some network technologies make address resolution difficult. Ethernet interface boards,
for example, come with built-in 48-bit hardware addresses. This creates several
difficulties:
• No simple correlation, applicable to the whole network, can be created between
physical (MAC) addresses and IP addresses.
• When the interface board fails and has to be replaced the IP address then has to
be remapped to a different MAC address.
• The MAC address is too long to be encoded into the 32-bit IP address.
To overcome these problems in an efficient manner, and eliminate the need for
applications to know about MAC addresses, TCP/IP designers developed the address
resolution protocol (ARP), which resolves addresses dynamically.
When a host wishes to communicate with another host on the same physical network, it
needs the destination MAC address in order to compose the basic frame. If it does not
know what the destination MAC address is, but has its IP address, it broadcasts a special
type of datagram in order to resolve the problem. This is called an address resolution
protocol (ARP) request. This datagram requests the owner of the unresolved Internet
protocol (IP) address to reply with its MAC address. All hosts on the network will receive
the broadcast, but only the one that recognizes its own IP address will respond.
118
Practical Industrial Data Communications
While the sender could, of course, just broadcast the original datagram to all hosts on
the network, this would impose an unnecessary load on the network, especially if the
datagram was large. A small address resolution protocol (ARP) request, followed by a
small address resolution protocol (ARP) reply, followed by a direct transmission of the
original datagram, is a much more efficient way of resolving the problem.
Address resolution cache
Because communication between two computers usually involves transfer of a succession
of datagrams, it is prudent for the sender to ‘remember’ the MAC information it receives,
at least for a while. Thus, when the sender receives an ARP reply, it stores the MAC
address it receives as well as the corresponding IP address in its ARP cache. Before
sending any message to a specific IP address, it checks first to see if the relevant address
binding is in the cache. This saves it from repeatedly broadcasting identical ARP
requests.
To further reduce communication overheads, when a host broadcasts an ARP request it
includes its own IP address and MAC address, and these are stored in the ARP caches of
all other hosts that receive the broadcast. When a new host is added to a network, it can
be made to send an ARP broadcast to inform all other hosts on that network of its
address.
Some very small networks do not use ARP caches, but the continual traffic of ARP
requests and replies on a larger network would have a serious negative impact on the
network’s performance.
The ARP cache holds four fields of information for each device:
1.
2.
3.
4.
IF Index: The physical port
Physical address: The MAC address of the device
Internet protocol (IP) address: The corresponding IP address
Type: The type of entry in the ARP cache; there are four possible types:
•
•
•
•
4 = static – the entry will not change
3 = dynamic – the entry can change
2 = the entry is invalid
1 = none of the above.
ARP datagram
The layout of an ARP request or reply datagram is as in Figure 7a.5.
Figure 7a.5
Address resolution protocol
TCP/IP overview 119
Hardware type
Specifies the hardware interface type of the target:
1 = Ethernet
3 = X.25
7 = ARCnet.
Protocol type
Specifies the type of high-level protocol address the sending device is using. For
example,
2048 = Internet Protocol (IP)
2054 = Address Resolution Protocol (ARP)
3282 = Reverse ARP (reverse address resolution protocol (RARP)).
Hardware address length
The length, in bytes, of the MAC address.
Protocol address length
The length, in bytes, of the IP address.
Operation code
Indicates the type of ARP datagram:
1 = Address resolution protocol (ARP) request
2 = Address resolution protocol (ARP) reply
3 = Reverse address resolution protocol (RARP) request
4 = Reverse address resolution protocol (RARP) reply.
Sender hardware address
The MAC address of the sender.
Sender protocol address
The IP address of the sender.
Target hardware address
The MAC address of the target host.
Target protocol address
The IP address of the target host.
Because of the use of fields to indicate the lengths of the hardware and protocol
addresses, the address fields can be used to carry a variety of address types, making ARP
useful for a number of different types of network.
120
Practical Industrial Data Communications
The broadcasting of ARP requests presents some potential problems. Networks such
as Ethernet are ‘best-effort’ delivery systems, i.e. the sender does not receive any
feedback whether datagrams it has transmitted were received by the target device. If
the target is not available, the ARP request destined for it will be lost without trace and
no ARP response will be generated. Thus the sender must be programed to retransmit
its ARP request after a certain time period and must be able to store the datagram it
is attempting to transmit in the interim. It must also remember what requests it has
sent out so that it does not send out multiple ARP requests for the same address. If
it does not receive an ARP reply, it will eventually have to discard the outgoing
datagrams.
Because it is possible for a machine’s hardware address to change, as happens when an
Ethernet interface fails and has to be replaced, entries in an ARP cache have a limited life
span after which they are deleted. Every time a machine with an ARP cache receives an
ARP message, it uses the information to update its own ARP cache. If the incoming
address binding already exists, it overwrites the existing entry with the fresh information
and resets the timer for that entry.
If a machine is the target of an incoming ARP request, its own ARP software must
organize a reply. It must swap the target and sender address pairs in the ARP datagram,
insert its physical address into the relevant field, change the operation code to 2 (ARP
reply), and send it back to the requester.
7a.2.3
ICMP
When nodes fail, or become temporarily unavailable, or when certain routes become
overloaded with traffic, a message mechanism called the Internet control message
protocol (ICMP) reports errors and other useful information about the performance and
operation of the network.
ICMP communicates between the Internet layers on two nodes and is used by routers as
well as individual hosts. Although ICMP is viewed as residing within the Internet layer,
its messages travel across the network encapsulated in IP datagrams in the same way as
higher-layer protocol (such as TCP or UDP) datagrams. The ICMP message, consisting
of an ICMP header and ICMP data, is encapsulated as ‘data’ within an IP datagram that
is, in turn, carried as ‘payload’ by the lower network interface layer (for example,
Ethernet).
7a.2.4
ICMP datagrams
There are a variety of ICMP messages, each with a different format, yet the first three
fields as contained in the first 4 bytes or ‘Long word’ is the same for all.
The various ICMP messages are shown in Figure 7a.6.
The three common fields are:
1. An ICMP message type (4 bits) which is a code that identifies the type of
ICMP message
2. A code (4 bits) in which interpretation depends on the type of ICMP
message
3. A checksum (16 bits) that is calculated on the entire ICMP datagram.
Table 7a.1 lists the different types of ICMP messages.
TCP/IP overview 121
ICMP messages can be further sub-divided into two broad groups viz. ICMP error
messages (destination unreachable, time exceeded, invalid parameters, source quench or
redirect) and ICMP query messages (echo request and reply messages, timestamp request
and reply messages, and subnet mask request and reply messages).
Figure 7a.6
ICMP message formats
122
Practical Industrial Data Communications
Type Field
0
3
4
5
8
11
12
13
14
15
16
Description
Echo reply
Destination unreachable
Source quench
Redirect (change a route)
Echo request
Time exceeded (datagram)
Parameter problem (datagram)
Time stamp request
Time stamp reply
Address mark request
Address mark reply
Table 7a.1
ICMP message types
7a.2.5
Routing
Unlike the host-to-host layer protocols (for example, TCP), which control end-to-end
communications, IP is rather ‘shortsighted’. Any given IP node (host or router) is only
concerned with routing (switching) the datagram to the next node, where the process is
repeated. Very few routers have knowledge about the entire internetwork, and often the
datagrams are forwarded based on default information without any knowledge of where
the destination actually is.
Direct vs indirect delivery
When the source host prepares to send a message to another host, a fundamental decision
has to be made; namely, is the destination host also resident on the local network or not?
If the NetID portions of the source and destination IP addresses match, the source host
will assume that the destination host is resident on the same network, and will attempt to
forward it locally. This is called direct delivery. If not, the message will be forwarded to
the default gateway (a router on the local network), which will forward it. This is called
indirect delivery. If the router can deliver it directly, i.e. the destination host resides on a
network directly connected to the router, it will. If not, it will consult its routing tables
and forward it to the next appropriate router. This process will repeat itself until the
packet is delivered to its final destination.
Static vs dynamic routing
Each router maintains a table with the format presented in Table 7a.2.
It basically reads as follows: ‘If a packet is destined for network 200.0.0.0 with a
netmask of 255.255.255.0, then forward it to the router IP address 207.194.66.100’, etc. It
is logical that a given router cannot contain the whereabouts of each and every network in
the world in its routing tables, hence it will contain default routes as well. If a packet
cannot be specifically routed, it will be forwarded on a default route, which should (it is
hoped) move it closer to its intended destination.
These routing tables can be maintained in two ways. In most cases, the routing
protocols will do this automatically. The routing protocols are implemented in software
that runs on the routers, enabling them to communicate on a regular basis and allowing
them to share their ‘knowledge’ about the network with each other. In this way they
TCP/IP overview 123
continuously ‘learn’ about the topology of the system, and upgrade their routing tables
accordingly. This process is called dynamic routing. If, for example, a particular router is
removed from the system, the routing tables of all routers containing a reference to that
router will change. However, because of the interdependence of the routing tables, a
change in any given table will initiate a change in many other routers and it will be a
while before the tables stabilize. This process is known as convergence.
Network Address
Net Mask
Gateway Address
Interface
Metric
127.0.0.0
255.0.0.0
127.0.0.1
127.0.0.1
1
207.194.66.0
255.255.255.224
207.194.66.100
207.194.66.100
1
207.194.66.0
255.255.255.255
127.0.0.1
127.0.0.1
1
200.0.0.0
255.255.255.0
207.194.66.100
207.194.66.100
1
224.0.0.0
224.0.0.0
207.194.66.100
207.194.66.100
1
255.255.255.255
255.255.255.255
207.194.66.100
0.0.0.0
1
Table 7a.2
Active routes
Dynamic routing can be further sub-classified as distance vector, link-state, or hybrid,
depending on the method by which the routers calculate the optimum path.
In distance vector dynamic routing, the ‘metric’ or yardstick used for calculating the
optimum routes is simply based on distance, i.e. which route results in the least number of
‘hops’ to the destination. Each router constructs a table, which indicates the number of
hops to each known network. It then periodically passes copies of its tables to its
immediate neighbors. Each recipient of the message then simply adjusts its own tables
based on the information received from its neighbor.
It is also possible for a network administrator to make static entries into routing tables.
These entries will not change, even if a router that they point to is not operational.
Autonomous systems
For the purpose of routing, a TCP/IP-based internetwork can be divided into several
autonomous systems (ASs) or domains. An autonomous system consists of hosts, routers,
and data links that form several physical networks and are administered by a single
authority such as a service provider, university, corporation, or government agency.
Routing decisions that are made within an AS are totally under the control of the
administering organization. Any routing protocol, using any type of routing algorithm,
can be used within an autonomous system since the routing between two hosts in the
system are completely isolated from any routing that occurs in other autonomous systems.
Only if a host within one autonomous system communicates with a host outside the
system, will another autonomous system (or systems) and possibly the Internet backbone
be involved (Figure 7a.7).
There are three categories of TCP/IP routing protocols namely Interior gateway
protocols, exterior gateway protocols, and gateway-to-gateway protocols.
Two routers that communicate directly with one another and are both part of the same
autonomous system are said to be interior neighbors and are called interior gateways.
They communicate with each other using interior gateway protocols such as RIP,
HELLO, IS-IS, or OSPF.
124
Practical Industrial Data Communications
Figure 7a.7
Autonomous systems and routing protocols
Routers in different ASs, however, cannot use IGPs for communication for more than
one reason. Firstly, IGPs are not optimized for long-distance path determination.
Secondly, the owners of ASs (particularly Internet service providers) would find it
unacceptable for their routing metrics (which include sensitive information such as error
rates and network traffic) to be visible to their competitors. For this reason, routers that
communicate with each other and are resident in different ASs communicate with each
other using exterior gateway protocols such as BGP-4.
7a.3
Host-to-host layer: end to end reliability
7a.3.1
TCP
Transmission control protocol (TCP) is a connection-oriented protocol and is said to be
‘reliable’, although this word is used in a data communications context. TCP establishes
a session between two machines before data is transmitted. Because a connection is set
up beforehand, it is possible to verify that all packets are received on the other end and
to arrange retransmission in case of lost packets. Because of all these built-in functions,
TCP involves significant additional overhead in terms of processing time and header
size.
TCP fragments large chunks of data into smaller segments if necessary, reconstructs the
data stream from packets received, issues acknowledgments of data received, provides
socket services for multiple connections to ports on remote hosts, performs packet verification and error control, and performs flow control.
TCP header
The TCP header is structured as in Figure 7a.8.
The source and destination ports (16 bits each) identify the host processes at each side
of the connection. Examples are post office protocol (POP3) at port 110 and simple mail
transfer protocol (SMTP) at port 25. Whereas a destination host is identified by its IP
address, the process on that host is identified by its port number. A combination of port
number and IP address is called a socket.
TCP/IP overview 125
Figure 7a.8
TCP header format
The sequence number (32 bits) ensures the sequential of the data stream. TCP, by
implication, associates a 32-bit number with every byte it transmits. The sequence
number is the number of the first byte in every segment (or ‘chunk’) of data sent by TCP.
If the SYN flag is set, however, it indicates that the sender wants to establish a connection
and the number in the sequence number field becomes the initial sequence number or
ISN. The receiver acknowledges this, and the sender then labels the first byte of the
transmitted data with a sequence number of ISN + 1. The ISN (initial sequence number)
is a pseudo-random number with values between 0 and 232.
The acknowledgments number (32 bits) is used to verify correct receipt of the
transmitted data. The receiver checks the incoming data, and if the verification is positive,
acknowledges it by placing the number of the next byte expected in the acknowledgments
number field and setting the ACK flag. The sender, when transmitting, sets a timer and if
an acknowledgment is not received within a specific time, an error is assumed and the
data is retransmitted.
Data offset (4 bits) is the number of 32-bit words in the TCP header. This indicates
where the data begins. It is necessary since the header can contain options and thus does
not have a fixed length.
Six flags control the connection and data transfer. They are:
1.
2.
3.
4.
5.
6.
URG: Urgent pointer field significant
ACK: Acknowledgments field significant
PSH: Push function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender.
126
Practical Industrial Data Communications
The window field (16 bits) provides flow control. Whenever a host sends an
acknowledgment to the other party in the bidirectional communication, it also sends a
window advertisement by placing a number in the window field. The window size
indicates the number of bytes, starting with the one in the acknowledgments field that the
host is able accept.
The checksum field (16 bits) is used for error control. The urgent pointer field (16 bits)
is used in conjunction with the URG flag and allows for the insertion of a block of
‘urgent’ data in the beginning of a particular segment. The pointer points to the first byte
of the non-urgent data following the urgent data.
7a.3.2
UDP
User datagram protocol (UDP) is a ‘connectionless’ protocol and does not require a
connection to be established between two machines prior to data transmission. It is
therefore said to be ‘unreliable’ – the word ‘unreliable’ used here as opposed to ‘reliable’
in the case of TCP and should not be interpreted against its everyday context.
Sending a UDP datagram involves very little overhead in that there are no
synchronization parameters, no priority options, no sequence numbers, no timers, and no
retransmission of packets. The header is small, the protocol is streamlined functionally.
The only major drawback is that delivery is not guaranteed. UDP is therefore used for
communications that involve broadcasts, for general network announcements, or for realtime data.
The UDP header
The UDP header is significantly smaller than the TCP header and only contains four
fields (Figure 7a.9).
Figure 7a.9
UDP header format
UDP source port (16 bits) is an optional field. When meaningful, it indicates the port of
the sending process, and may be assumed to be the port to which a reply should be
addressed in the absence of any other information. If not used, a value of zero is inserted.
UDP destination port has the same meaning as for the TCP header, and indicates the
process on the destination host to which the data is to be sent.
UDP message length is the length in bytes of the datagram including the header and the
data.
The 16-bit UDP checksum is used for validation purposes.
TCP/IP troubleshooting
7b.1
Introduction
This section deals with problems related to the TCP/IP protocol suite. The TCP/IP
protocols are implemented in software and cover the second (Internet), the third
(host-to-host), and the upper (application) layers of the ARPA model. These protocols
need a network infrastructure as well as a medium in order to communicate. This
infrastructure is typically Ethernet, as dealt with in the previous section.
7b.2
Common problems
TCP/IP is a complex topic and this section cannot really do justice to it. This section
will therefore try to show some common approaches that do not necessarily require an
in-depth knowledge of the protocols involved.
7b.3
Tools of the trade
7b.3.1
TCP/IP utilities
These utilities are DOS programs that form part of the TCP/IP software. They are not
protocols, but simply executable DOS programs that utilize some of the protocols. The
utilities that will be shown here are ping, arp, and tracert. This is not the complete list, but
it is sufficient for the purposes of this chapter.
7b.3.2
Third party utilities
Most of the TCP/IP utilities are included in more recent third-party software packages.
The advantage of these packages such as TJPingPro is that they are windows based and
hence much easier to use.
7b.3.3
Software protocol analyzers
These have already been discussed in the section on Ethernet.
7b.3.4
Hardware-assisted protocol analyzers
These have also been discussed in the section on Ethernet.
128
Practical Industrial Data Communications
7b.4
Typical network layer problems
7b.4.1
TCP/IP protocol stack not properly installed on local host
(host unable to access the network)
The easiest way to confirm this, apart from checking the network configuration via the
control panel and visually confirming that TCP/IP is installed for the particular NIC used
on the host, is to perform a loop-back test by pinging the host itself. This is done by
executing ping localhost or ping 127.0.0.1. If a response is received, it
means that the stack is correctly installed.
7b.4.2
Remote host (e.g. a web server) not reachable
If it needs to be confirmed that a remote host is available, the particular machine can be
checked by pinging it. The format of the command is:
• Ping 193.7.7.3 (for example), where 193.7.7.3 is the IP address of the
remote machine, or
• Ping www.idc–online.com where www.idconline.com is the domain
name of the remote host, or
• Ping john where john (in this example) has been equated to the IP address
of the remote machine in the hosts file of the local machine.
This is an extremely powerful test, since a positive acknowledgment means that the
bottom three layers (OSI) of the local and the remote hosts as well as all routers and all
communication links between the two hosts are operational.
7b.4.3
A host is unable to obtain an automatically assigned IP addresses
When TCP/IP is configured and the upper radio button is highlighted (indicating that an
IP address has to be obtained automatically) the host, upon booting up, will broadcast a
request for the benefit of the local dynamic host configuration protocol (DHCP) server.
Upon hearing the request, the DHCP server will offer an IP address to the requesting host.
If the host is unable to obtain such IP address, it can mean one of two things:
1. The DHCP server is down. If this is suspected, it can be confirmed by pinging
the DHCP server. If the local host supports automatic IP address allocation, it
will temporarily assume an IP address in the range 169.254.x.x. As soon as the
DHCP server is on line again, it will obtain an address from the server.
2. There are no spare IP addresses available. Nothing can be done about this and
the user will have to wait until one of the other logged-in machines are
switched off which will cause it to relinquish it IP address and make it
available for reissue.
7b.4.4
Reserved IP addresses
Reserved IP addresses are IP addresses in the ranges 10.0.0.0/8–10.255.255.255/8,
172.16.0.0/12–172.16.255.255/12, and 192.168.0.0/16–192.168.255.255/16. These IP
addresses are only allocated to networks that will never have access to the Internet. All
Internet routers are pre-programed to ignore these addresses. If a user therefore tries to
access such an IP address over the Internet, the message will not be transported across the
Internet and hence the desired network cannot be reached.
TCP/IP troubleshooting
7b.4.5
129
Duplicate IP addresses
Since an IP address is the Internet equivalent of a postal address, it is obvious that
duplicate IP addresses cannot be tolerated. If a host is booted up, it tries to establish if any
other host with the same IP address is available on the local network. If this is found to be
true, the booting up machine will not proceed with logging on to the network and both
machines with the duplicate IP address will display error messages in this regard.
7b.4.6
Incorrect network ID – different NetIDs on the same physical network
As explained in the chapter on TCP/IP, an IP address consists of two parts, namely, a
network ID (NetID), which is the equivalent of a postal zip code and a HostID, which is
the equivalent of a street, address. If two machines on the same network have different
NetIDs, their ‘zip codes’ will differ and hence the system will not recognize them as
coexisting on the same network. Even if they are physically connected to the same
Ethernet network, they will not be able to communicate directly with each other using
TCP/IP.
7b.4.7
Incorrect subnet mask
As explained in the chapter on TCP/IP, the subnet mask indicates the boundary between
the NetID and the HostID. A faulty subnet mask, when applied to an IP address, could
result in a NetID (zip code) that includes bits from the adjacent HostID and hence looks
different than the NetID of the machine wishing to send a message. The sending host will
therefore erroneously believe that the destination host exists on another network and that
the packets have to be forwarded to the local router for delivery to the remote network.
If the local router is not present (no default gateway specified) the sender will give up
and not even try to deliver the packet. If, on the other hand, a router is present (default
gateway specified), the sender will deliver the packet to the router. The router will then
realize that there is nothing to forward since the recipient does in fact, live on the local
network, and it will try to deliver the packet to the intended recipient. Although the
packet eventually gets delivered, it leads to a lot of unnecessary packets transmitted as
well as unnecessary time delays.
7b.4.8
Incorrect or absent default gateway(s)
An incorrect or absent default gateway in the TCP configuration screen means that a host,
wishing to send a message to another host on a different network, is not able to do so. The
following is an example:
Assuming that a host with IP address 192.168.0.1 wishes to ping a non-existent host on IP
address 192.168.0.2. The subnet mask is 255.255.255.0. The pinging host applies the mask
to both IP addresses and comes up with a result of 192.168.0.0 in both cases. Realizing that
the destination host resides on the same network as the sender, it proceeds to ping
192.168.0.2. Obviously, there will be no response from the missing machine and hence a
time-out will occur. The sending host will issue a time-out message in this regard.
Now consider a scenario where the destination host is 193.168.0.2 and there is no valid
default gateway entry. After applying the subnet mask, the sending host realizes that the
destination resides on another network and that it therefore needs a valid default gateway.
Not being able to find one, it does not even try to ping but simply issues a message to the
effect that the destination host is unreachable.
The reason for describing these scenarios is that the user can often figure out the
problem by simply observing the error messages returned by the ping utility.
130
Practical Industrial Data Communications
7b.4.9
MAC address of a device not known to user
The MAC address of a device, such as a PLC, is normally displayed on a sticker attached
to the body. If the MAC address of the device is not known, it can be pinged by its IP
address (for example, ping 101.3.4.5) whereafter the MAC address can be obtained by
displaying the ARP cache on the machine that did the ping. This is done by means of the
arp – a command.
7b.4.10
IP address of a device not known to user
On a computer, this is not a problem since the IP address can simply be looked up in the
TCP/IP configuration screen. Alternatively, the IP address can be displayed by commands
such as winipcfg (Windows 95/98) or ipconfig /all (Windows 98/NT).
On a PLC, this might not be so easy unless the user knows how to attach a terminal to
the COM (serial) port on the PLC and has the configuration software handy. An easier
approach to confirm the IP address setup on the PLC (if any) is to attach it to a network
and run a utility such as WebBoy or EtherBoy. The software will pick up the device on
the network, regardless of its NetID, and display the IP address.
7b.4.11
Wrong IP address
It is possible that all devices on a network could have valid and correct IP addresses but
that a specific host fails to respond to a message sent to it by a client program residing
on another host. A typical scenario could be a supervisory computer sending a
command to a PLC. Before assuming that the PLC is defective, one has to ascertain that
the supervisory computer is in fact using the correct IP address when trying to
communicate with the PLC. This can only be ascertained by using a protocol analyzer
and capturing the communication or attempt at communication between the computer
and the PLC. The packets exchanged between the computer and the PLC can be
identified by means of the MAC addresses in the appropriate Ethernet headers. It then
has to be confirmed that the IP headers carried within these frames do in fact contain
the correct IP addresses.
7b.5
Transport layer problems
A detailed treatment of the TCP protocol is completely beyond the scope of this course
but there are a few simple things that even a relatively inexperienced user can check.
7b.5.1
No connection established
Before any two devices can communicate using TCP/IP, they need to establish a so-called
triple handshake. This will be clearly indicated as a SYN, ACK / SYN, ACK sequence to
the relevant port. Without a triple handshake the devices cannot communicate at all.
To confirm this, simply try to establish a connection (for example, by using an FTP
client to log into an FTP server) and use a protocol analyzer to capture the handshake.
7b.5.2
Incorrect port number
TCP identifies different programs (processes) on a host by means of so-called port
numbers. For example, an FTP server uses ports 21 and 22, a POP3 server (for e-mail)
uses port 110 and a web server (HTTP) uses port 80. Any other process wishing to
TCP/IP troubleshooting
131
communicate with one of these has to use the correct port number. The port number used
is visible right at the beginning of the TCP header that can be captured with a protocol
analyzer. Port numbers 1–1023 are referred to as ‘well known’ ports. Port numbers are
administered by the Internet Assigned Numbers Authority (IANA). These are detailed in
RFC 1700.
Modbus overview
Objectives
When you have completed study of this chapter, you will be able to:
• List the main Modbus structures and frames used
• Identify and correct problems with:
− No reponse to protocol messages
− Exception reports
− Noise.
8a.1
General overview
MODBUS is an application layer (OSI layer 7) messaging protocol that provides
client/server communication between devices connected to different types of buses of
networks. The MODBUS protocol implements a client/server architecture and operates
essentially in a 'request/ response' mode, irrespective of the media access control used at
layer 2. This client/server model is based on four types of message namely:
• MODBUS requests, the messages sent on the network by the clients to initiate
transactions
• MODBUS confirmations, the response messages received on the client side
• MODBUS indications, the request messages received on the server side and
• MODBUS responses, the response messages sent by the servers.
These messaging services of the client/server model are used to exchange real-time
information between two device applications, between device applications and devices, or
between devices and HMI/SCADA applications (Figure 8a.1).
Figure 8a.1
MODBUS client/server interaction
[Note: Use Figure 19.1 from FE manual]
Modbus overview
133
In an error-free scenario, the exchange of information between client and server can be
illustrated as follows. The client (on the master device) initiates a request. The MODBUS
messaging protocol (layer 7) then generates a protocol data unit or PDU, consisting of a
function code and a data request. At layer 2, this PDU is converted to an application data
unit (ADU) by the addition of some bus or network related fields, such as a slave address
and a checksum for error detection purposes. This process is depicted in Figure 8a.2.
Figure 8a.2
Generic MODBUS frame
[Note: Use Figure 19.2 from FE manual]
The server (on the slave device) then performs the required action and initiates a
response. The interaction between client and server is shown in Figure 8a.3.
Figure 8a.3
MODBUS transaction
[Note: Use Figure 19.3 from FE manual]
The various types of function codes, with their associated requests and responses, are
described in detail in this chapter.
The MODBUS Messaging Protocol (layer 7) needs additional support at the lower
layers in order to get the message across. A popular method is the use of a master/slave
(half-duplex) layer 2 protocol, transmitting the data in serial format over RS-232, RS-485
or Bell 202 type modem links. Other methods include MODBUS+ (half-duplex over RS485), or MAP. A recent addition is the use of TCP/IP and Ethernet to convey data from
client to server. The TCP/IP approach enables client/server interaction over routed
networks, albeit at the cost of additional overheads (processing time, headers, etc.). An
additional sub-layer is required to map the MODBUS application layer on to TCP. The
function of this sub-layer is to encapsulate the MODBUS PDU so that it can be
transported as a packet of data by TCP/IP (Figure 8a.4).
134
Practical Industrial Data Communications
Figure 8a.4
MODBUS communication stack
[Note: Use Figure 19.4 from FE manual]
8a.2
Modbus protocol structure
Table 8a.1 illustrates a typical Modbus message frame format.
Address Field
1 byte
Function Field
1 byte
Data Field
Variable
Error Check Field
2 bytes
Table 8a.1
Format of Modbus message frame
The first field in each message frame is the address field, which consists of a single byte
of information. In request frames, this byte identifies the controller to which the request is
being directed. The resulting response frame begins with the address of the responding
device. Each slave can have an address field between 1 and 247, although practical
limitations will limit the maximum number of slaves. A typical Modbus installation will
have one master and two or three slaves.
The second field in each message is the function field, which also consists of a single
byte of information. In a host request, this byte identifies the function that the target PLC
is to perform.
If the target PLC is able to perform the requested function, the function field of its
response will echo that of the original request. Otherwise, the function field of the request
will be echoed with its most significant bit set to one, thus signaling an exception
response. Table 8a.1 summarizes the typical functions used.
The third field in a message frame is the data field, which varies in length according to
which function is specified in the function field. In a host request, this field contains
information the PLC may need to complete the requested function. In a PLC response,
this field contains any data requested by that host.
The last two bytes in a message frame comprise the error-check field. The numeric
value of this field is calculated by performing a cyclic redundancy check (CRC-16) on the
message frame. This error checking assures that devices do not react to messages that
may have been damaged during transmission.
Modbus overview
135
Table 8a.2 lists the address range and offsets for these four data types, as well as the
function codes that apply to each. The diagram Table 8a.1 also gives an easy reference to
the Modbus data types.
Data Type
Absolute
Addresses
Relative
Addresses
Function
Codes
Description
Coils
00001–09999
0–9998
01
Read coil status
Coils
00001–09999
0–9998
05
Force single coil
Coils
00001–09999
0–9998
15
Force multiple coils
Discrete inputs
10001–19999
0–9998
02
Read input status
Input registers
30001–39999
0–9998
04
Read input registers
Holding registers
40001–49999
0–9998
03
Read holding register
Holding registers
40001–49999
0–9998
06
Preset single register
Holding registers
40001–49999
0–9998
16
Preset multiple registers
–
–
–
07
Read exception status
–
–
–
08
Loopback diagnostic test
Table 8a.2
Modicon addresses and function codes
8a.3
Function codes
Each request frame contains a function code that defines the action expected for the target
controller. The meaning of the request data fields is dependent on the function code
specified.
The following paragraphs define and illustrate most of the popular function codes
supported. In these examples, the contents of the message-frame fields are shown as
hexadecimal bytes.
8a.3.1
Read coil or digital output status (function code 01)
This function allows the host to obtain the ON/OFF status of one or more logic coils in
the target device.
The data field of the request consists of the relative address of the first coil followed by
the number of coils to be read. The data field of the response frame consists of a count of
the coil bytes followed by that many bytes of coil data.
The coil data bytes are packed with one bit for the status of each consecutive coil
(1 = ON, 0 = OFF). The least significant bit of the first coil data byte conveys the status
of the first coil read. If the number of coils read is not an even multiple of eight, the last
data byte will be padded with zeros on the high end. Note that if multiple data bytes are
requested, the low order bit of the first data byte in the response of the slave contains the
first addressed coil.
In Figure 8a.5, the host requests the status of coils 000A (decimal 00011) and 000B
(decimal 00012). The target device’s response indicates both coils are ON.
136
Practical Industrial Data Communications
Figure 8a.5
Example of read coil status
8a.3.2
Read digital input status (function code 02)
This function enables the host to read one or more discrete inputs in the target
device.
The data field of the request frame consists of the relative address of the first discrete
input followed by the number of discrete inputs to be read. The data field of the response
frame consists of a count of the discrete input data bytes followed by that many bytes of
discrete input data.
The discrete-input data bytes are packed with one bit for the status of each consecutive
discrete input (1 = ON, 0 = OFF). The least significant bit of the first discrete input data
byte conveys the status of the first input read. If the number of discrete inputs read is not
an even multiple of eight, the last data byte will be padded with zeros on the high end.
The low order bit of the first byte of the response from the slave contains the first
addressed digital input.
In Figure 8a.6, the host requests the status of discrete inputs with offsets 0000 and 0001
hex, i.e. decimal 10001 and 10002. The target device’s response indicates that discrete
input 10001 is OFF and 10002 are ON.
Figure 8a.6
Example of read input status
Modbus overview
8a.3.3
137
Read holding registers (function code 03)
This function allows the host to obtain the contents of one or more holding registers in the
target device.
The data field of the request frame consists of the relative address of the first holding
register followed by the number of registers to be read. The data field of the response
time consists of a count of the register data bytes followed by that many bytes of holding
register data.
The contents of each requested register (16 bits) are returned in two consecutive data
bytes (most significant byte first).
In Figure 8a.7, the host requests the contents of holding register hexadecimal offset
0002 or decimal 40003. The controller’s response indicates that the numerical value of
the register’s contents is hexadecimal 07FF or decimal 2047. The first byte of the response register data is the high order byte of the first addressed register.
Figure 8a.7
Example of reading holding register
8a.3.4
Reading input registers (function code 04)
This function allows the host to obtain the contents of one or more input registers in the
target device.
The data field of the request frame consists of the relative address of the first input
register followed by the number of registers to be read. The data field of the response
frame consists of a count of the register-data bytes followed by that many bytes of inputregister data.
The contents of each requested register are returned in two consecutive registerdatabytes (most significant byte first). The range for register variables is 0–4095.
In Figure 8a.8, the host requests the contents of input register hexadecimal offset 000 or
decimal 30001. The PLC’s response indicates that the numerical value of that register’s
contents is 03FFH, which would correspond to a data value of 25% (if the scaling of
0–100% is adopted) and a 12-bit D to A converter with a maximum reading of 0FFFH
is used.
8a.3.5
Force single coil (function code 05)
This function allows the host to alter the ON/OFF status of a single logic coil in the target
device.
138
Practical Industrial Data Communications
Figure 8a.8
Example of reading input register
The data field of the request frame consists of the relative address of the coil followed
by the desired status for that coil. A hexadecimal status value of FF00 will activate the
coil, while a status value of 0000H will deactivate it. Any other status value is illegal.
If the controller is able to force the specified coil to the requested state, the response
frame will be identical to the request. Otherwise, an exception response will be returned.
If the address 00 is used to indicate broadcast mode, all attached slaves will modify the
specified coil address to the state required.
Figure 8a.9 illustrates a successful attempt to force coil 11 (decimal) OFF.
Figure 8a.9
Example of forcing a single coil
8a.3.6
Preset single register (function code 06)
This function enables the host to alter the contents of a single holding register in the
target device.
The data field of the request frame consists of the relative address of the holding register
followed by the new value to be written to that register (most significant byte first).
If the controller is able to write the requested new value to the specified register, the
response frame will be identical to the request. Otherwise, an exception response will be
returned.
Modbus overview
139
Figure 8a.10 illustrates a successful attempt to change the contents of holding register
40003 to 3072 (0C00 Hex).
When slave address is set to 00 (broadcast mode), all slaves will load the specified
register with the value specified.
Figure 8a.10
Example of presetting a single register
8a.3.7
Read exception status (function code 07)
This is a short message requesting the status of eight digital points within the slave device.
This will provide the status of eight predefined digital points in the slave. For example, this
could be items such as the status of the battery, whether memory protect has been enabled, or
the status of the remote input/output racks connected to the system (Figure 8a.11).
Figure 8a.11
Read exception status query message
8a.3.8
Loopback test (function code 08)
The objective of this function code is to test the operation of the communications system
without affecting the memory tables of the slave device. It is also possible to implement
140
Practical Industrial Data Communications
additional diagnostic features in a slave device (should this be considered necessary) such
as number of CRC errors, number of exception reports, etc.
The most common implementation will only be considered in this section; namely, a
simple return of the query messages (Figure 8a.12).
Figure 8a.12
Loopback test message
8a.3.9
Force multiple coils or digital outputs (function code 0F)
This forces a contiguous (or adjacent) group of coils to an ON or OFF state. The
following example sets 10 coils starting at address 01 Hex (at slave address 01) to the
ON state. If slave address 00 is used in the request frame, broadcast mode will
be implemented resulting in all slaves changing their coils at the defined addresses
(Figure 8a.13).
Figure 8a.13
Example of forcing multiple coils
8a.3.10
Force multiple registers (function code 10)
This is similar to the preset a single register and the forcing of multiple coils. In
Figure 8a.14, a slave address 01 has 2 registers changed commencing at address 10.
Table 8a.3 lists the most important exception codes that may be returned.
Modbus overview
141
Figure 8a.14
Example of presetting multiple registers
Code
Name
Description
01
02
03
04
05
06
Illegal function
Illegal data address
Illegal data value
Failure in associated device
Acknowledge
Busy, rejected message
Requested function is not supported
Requested data address is not supported
Specified data value is not supported
Slave PLC has failed to respond to a message
Slave PLC is processing the command
Slave PLC is busy
Table 8a.3
Abbreviated list of exception codes returned
An example of an illegal request and the corresponding exception response is shown in
Figure 8a.15. The request in this example is to READ COIL STATUS of points 514
to 521 (eight coils beginning with an offset 0201H). These points are not supported in this
PLC, so an exception report is generated indicating code 02 (illegal address).
Figure 8a.15
Example of an illegal request
Modbus troubleshooting
8b.1
Common problems and faults
No matter what extremes of care you may have taken, there is hardly ever an installation
that boasts of trouble-free setup and configuration. Some of the commonly faced
problems and faults that one comes across in the industry are listed below. This list is
broken into two distinct groups:
1. Hardware-related: This includes mis-wired communication cabling and
faulty communication interfaces.
2. Software-related: Software-related issues arise when the master application
tries to access non-existent nodes or use invalid function codes, address nonexistent memory locations in the slaves, or specify illegal data format types,
which obviously the slave devices do not understand. These issues will be
dealt with in detail under the Modbus plus protocol, as these issues are
common to both the traditional Modbus protocol and the Modbus plus
protocol. They are summarized under software-related problems. These issues
are also applicable to the latest Modbus/TCP protocol.
8b.2
Description of tools used
In order to troubleshoot the problems listed above, one would require the use of a few
tools, hardware and software, to try and decipher the errors. The most important tool of
all would always be the instruction manuals of the various components involved.
The hardware tools that may be required include RS-232 break-out boxes, RS-232 to
RS-485 converters, continuity testers, voltmeters, screw drivers, pliers, crimping tools,
spare cabling, and other similar tools and tackle. These would generally be used to ensure
that the cabling and terminations are proper and have been installed as per the
recommended procedures detailed in the instruction manuals.
On the software front, one would need some form of protocol analyzer that is in a position
to eavesdrop on the communications link between the master and the slave modules. This
could be either a dedicated hardware protocol analyzer, that is very expensive, or a
software-based protocol analyzer that could reside on a computer (Figure 8b.1).
Obviously, this second option is more economical and also requires the relevant hardware component support in order to connect to the network.
Modbus troubleshooting 143
Figure 8b.1
Screen shot of the protocol analysis tool
8b.3
Detailed troubleshooting
8b.3.1
Mis-wired communication cabling
RS-232 wiring for three-wire point-to-point mode
There are various wiring scenarios such as point-to-point, multi-drop two-wire, multidrop four-wire, etc. In a point-to-point configuration, the physical standard used is usually
RS-232 on a 25-pin D connector, which means a minimum of three wires are used. They
are: on the DTE (master) side: transmit (TxD – pin 2), receive (RxD – pin 3), and signal
ground (common – pin 7); and on the DCE (slave) side: receive (RxD – pin 2), transmit
(TxD – pin 3), and signal ground (common – pin 7).
The other pins were primarily used for handshaking between the two devices for data
flow control. Nowadays, these pins are rarely used as the flow control is done via
software handshaking protocols.
With the advent of VLSI technology, the footprint of the various devices have a
tendency to shrink; thereby even the physical connections of communications have been
now been standardized on 9-pin D connectors. The pin assignment that has been adopted
is the IBM standard in which the pin configurations are: on the DTE (master) side:
transmit (TxD – pin 3), receive (RxD – pin 2), and signal ground (common – pin 5); and
on the DCE (slave) side: receive (RxD – pin 3), transmit (TxD – pin 2), and signal ground
(common – pin 5).
It follows that the cabling between the two devices must be straight-through pin-for-pin.
In this manner, the transmit pin of the master is directly connected to the receive pin of
the slave and vice-versa. These cables are standard off-the-shelf products available in
standard predetermined lengths. They can also be fabricated to custom lengths, with ease.
Master devices are usually the present day IBM compatible computers, with a Modbus
application enabled on it, and therefore have the standard IBM RS-232 port provided. The
slave devices usually have a user-selectable option to have either an RS-232 or RS-485 port
for communication. Unfortunately some manufacturers, in order to force the customers to
return to them time-and-again, employ a strategy of modifying these standards to their own
advantage.
Figure 8b.2 illustrates a couple of these combinations.
144
Practical Industrial Data Communications
Figure 8b.2
Typical customizations in RS-232 cabling
As can be seen in case I, the manufacturer has modified the standard pin-out of the
RS-232 connection or has totally replaced the standard IBM RS-232 9-pin DIN connector
with his own standard. In situations like these, it is then imperative to possess the
manufacturer’s manuals of the device being used.
With the aid of the continuity checker, and the use of the RS-232 breakout box, you can
determine the non-standard pin-out of the cable, and fabricate a spare cable to the
required lengths without having to revert to the supplier.
As in case II, the manufacturer has embedded an RS-232 to RS-485 converter into the
cable itself and this would mimic a standard off-the-shelf RS-232 serial cable. This
obviously would not be clearly evident if you were not aware of the modification. With
the aid of the voltage tester, you can determine the operating voltages and therefore be in
a position to decipher the type of standard being employed.
In other implementations, where multi-dropped RS-485 is used, the installations usually
cater for both configurations of two-wire and four-wire communications. In the case of
single-master configurations, either of the two wiring modes could be used. In the case of
multi-master configurations, only the two-wire communication can be used.
RS-485/RS-422 wiring for four-wire, repeat or multi-drop modes
The four-wire configuration is, in actual fact, the RS-422 standard where there is one line
driver connected to multiple receivers, or multiple line drivers connected to one line
receiver. When using four-wire RS-422 communications, messages are transmitted on
one pair of shielded, twisted wires and received on another (Belden part number 9729,
9829, or equivalent). Both multi-drop and repeat configurations are possible when
RS-422 is used.
This is the classic case where the one line driver is the one that belongs to the master
and the remaining receivers belong to the multiple slaves. The slaves receive their
commands via this link and respond via their line drivers that are all connected to each
other and finally connected to the master’s receiver (Figure 8b.3).
RS-485 wiring for two-wire, multi-drop mode only
The two-wire configuration is the actual RS-485 standard where both the line drivers and
the receivers are connected to the same pair of communication cables. When using twowire RS-485 communications, messages are transmitted and received on the same pair of
Modbus troubleshooting 145
shielded, twisted wires (Figure 8b.4). Care must be taken to turn the host driver IC on
when sending messages and off when receiving messages. This can be accomplished
using software or hardware. Only multi-drop wiring configuration is possible when
RS-485 is used.
Figure 8b.3
Typical four-wire RS-485 multi-drop cabling
Typically, the maximum number of physical RS-485 nodes on a network is 32. If more
physical nodes are placed on the network an RS-485 repeater must be used.
This system works inherently similar to the four-wire system, except that the master
transmits requests to the various slaves and the slaves respond to the master over the same
pair of wires.
Figure 8b.4
Typical two-wire RS-422/485 cabling
146
Practical Industrial Data Communications
Note: The shield grounds should be connected to system ground at one point only. This
will eliminate the possibility of ground loops causing erroneous operation.
Grounding
The logic ground of all controllers and any other communication device on the same
network must reference the same ground. If one power supply is used for all controllers,
then they will be referenced to the same ground. When multiple supplies are used, there
are a couple of options to consider:
• Connect the supply output ground to a solid earth ground on all power supplies.
If the grounding system is good, all controllers will be referenced to the same
ground. (Note that the typical PC follows this practice.)
In some older buildings, the earth ground could vary by several volts within
the building. In this case, option 2 should be considered.
• Use the GND position on each controller to tie all controllers to the same
potential. Note that each power supply output must be isolated from the input
lines, otherwise irreparable damage may result if two power supplies are
powered from different phases of an electrical distribution system. It is not
recommended that the shield of a cable be used for this purpose, use a separate
conductor.
• Where it is not possible to get all controllers referenced to the same ground, use
an isolated RS-485 repeater between controllers that are located at different
ground potentials. Isolated repeaters are also an excellent way to clean up
signals and extend distances in noisy environments.
8b.3.2
Faulty communication interfaces
RS-232 driver failed
The RS-232 driver may be tested with a good high-impedance voltmeter. Place the meter
in the DC voltage range. Place the RED probe on the transmit pin (TxD) and the BLACK
probe on the signal ground (common). While the node is not transmitting (TX light off),
there should be a voltage between –5 and –25 V DC across these pins. This is referred to
as the idle state of the RS-232 driver. When the node is transmitting, the voltage will
oscillate through between ±5 and ±25 V DC. It may be difficult to see the deflection at
the higher baud rates and an oscilloscope is suggested for advanced troubleshooting. If
the voltage is fixed at anywhere between –25 and +25 V DC and does not oscillate as the
TX light blinks, then the transmitter is probably damaged and must be returned to the
factory for repair or replacement.
RS-232 receiver failed
This requires the use of an RS-232 loop back plug or an RS-232 breakout box with a
jumper installed between the transmit and the receive pins. After connecting this to the
communication port of the node to be tested, the node is made to transmit data, and when
the TxD light of the node flashes, the RxD light must also flash, then it can be said that
the RS-232 receiver of that node is good. If the RxD light does not flash at the same time
as the TxD, then the RS-232 receiver may be bad.
Alternatively, two nodes may be connected to each other with a tested and working
communication cable and both their RS-232 drivers working. If one of the nodes is
Modbus troubleshooting 147
transmitting to the other, the second node indicates a good reception by flashing its RxD
light as the first node transmits, then the RS-232 receiver on the second node is good.
The same is true when the second node transmits and the first flashes its RxD light.
RS-485 driver failed
The RS-485 driver too may be tested with a good high-impedance voltmeter. Place the
meter in the DC voltage range. Place the RED probe on the non-inverted transmit
terminal (TX+) and the BLACK probe on the inverted transmit terminal (TX–). While
the node is not transmitting (TX light off), there should be approximately –4 V DC
across these wires. When the node is transmitting, the voltage will oscillate through
±4 V DC. It may be difficult to see the deflection at the higher baud rates and an
oscilloscope is suggested for advanced troubleshooting. The minimum voltage obtained
during a full 1200 baud to 19 200 baud sweep will be around –1.6 V DC. If the voltage
is fixed at around +4, 0, or –4 V, and does not oscillate as the TX light blinks, then the
transmitter is probably damaged and must be returned to the factory for repair or
replacement.
RS-485 receiver failed
This requires two nodes connected to each other with a tested and working communication
cable, and both their RS-485 drivers working. If one of the nodes is transmitting to the
other, the second node indicates a good reception by flashing its RX light as the first node
transmits, then the RS-485 receiver on the second node is good. If the RX light flashes
when the first node polls at other baud rates then most likely, the polarity is reversed
between the OUT of the first node and the second node. Reverse the TX+ and TX– wires
at the first node.
8b.3.3
Software-related problems
See the suggestions under Modbus Plus. But the important issues to consider are:
No response to message from master to slave This could mean that either the slave does
not exist or there are CRC errors in the transmitted message due to noise (or incorrectly
formatted message).
Exception responses See the list of potential problems reported by the exception
responses. This could vary from slave address problems to I/O addresses being illegal.
8b.4
Conclusion
We have seen that the master computer sends commands to the various slave units to
determine the status of its various process inputs or to change the status of its outputs
using the Modbus protocol. The commands are transmitted over a single pair of twisted
wires (RS-485) or two pairs of twisted wires (RS-422) at speeds of 9600, 19 200, 38 400,
or 57 600 baud. The addressed slave decodes the commands and returns the appropriate
response. If the master computer is an IBM PC or compatible, inexpensive interface
driver software is available. This software dramatically simplifies sending and receiving
these messages. If you prefer, you can use one of the off-the-shelf graphics-based data
acquisition and control software packages. Many of these packages offer a Modbus
compatible driver.
148
Practical Industrial Data Communications
If you were to follow the recommended installation and startup procedure and took
extra precaution while setting up the following items, then you would achieve troublefree startup:
•
•
•
•
Setting the base address
Setting the protocol and baud rate
Serial communications wiring
Communication wiring termination.
Fundamentals of DNP3
9.1
Fundamental concepts
9.1.1
The OSI seven-layer model
The open systems interconnection model, as defined by the International Standards
Organization (ISO), was a significant step in the development of standardization of data
communications systems. The OSI model presents data communications in a hierarchical
manner, starting at the bottom with the physical layer (layer 1), and moving to the
application layer (layer 7) at the top (Figure 9.1).
Figure 9.1
ISO open systems interconnection (OSI) model
In the arena of SCADA and IED communications, there was a need for a simplified
model. Such a model was created by the International Electro Technical Commission
(IEC) that defined a three-layer model. This is known as the enhanced performance
architecture (EPA) model and it is this model on which DNP3 is based. The functions of
each of the layers used by DNP3 are discussed in the next section.
150
Practical Industrial Data Communications
Before moving to the EPA model, we will use the general OSI reference model to
illustrate message build-up (Figure 9.2). This will demonstrate the general principle and
introduce the idea of message headers. These concepts will assist us in understanding the
details of message construction under DNP3.
Figure 9.2
Generic message build-up using the OSI seven-layer model
Some of the features and terminology illustrated by the message build-up are now
discussed.
Data flow during message construction is from the application level down to the
physical level, then across the physical medium, then at the receiving device up through
the model layers until the original message is regenerated.
The application data is given an application header by the application layer. This forms
the application protocol data unit or APDU. The APDU is the data unit used by the next
layer, the presentation layer. This adds its own header and so on.
The data may be split into smaller size units during the process of message construction.
In this example, the session protocol data unit (SPDU) is broken into multiple data units at
the transport level.
The data-link layer adds both a header, and a trailer section containing an error
detection code, forming the link protocol data unit (LPDU). At the physical layer this is
broken down into 8-bit blocks, each with start and stop bits appended for asynchronous
transmission.
Figure 9.3 shows a distinction in terminology between the service data unit and the
protocol data unit, which is worth being aware of because it will assist in understanding
of terms like TSDU and TPDU when used in relation to DNP3. The TSDU is the data
unit used to communicate with the higher level, that is, to provide the service to the
higher level. The TPDU is the data unit exchanged with the level below.
Figure 9.3 illustrates in this case the transport level breaking down the higher-level data
unit into multiple data units to be passed down to the next level.
9.1.2
Enhance performance architecture (EPA)
The enhanced performance architecture model developed by IEC Technical Committee
57 is a three-layer sub-set of the OSI seven-layer model. The layers used in this model are
the two hardware layers and the top software layer, the application layer (Figure 9.4).
Fundamentals of DNP3 151
Figure 9.3
Definition of service data unit vs protocol data unit
Figure 9.4
Enhanced performance architecture (EPA) model
As for the OSI seven-layer model, during transmission high-level data is passed from
the user application to the application level, and from there down through the hierarchy to
produce a stream of data over the physical communications medium. In the process, the
data may be converted from a single user application data unit into smaller chunks and
ultimately to a bit stream. On reception at its destination, the reverse process is applied,
and leading to regeneration of the original application level data unit.
DNP3 uses these three layers, but adds some transport functions. These are often
referred to as the pseudo-transport layer, and are sometimes depicted as corresponding
to the transport and network layers in a limited manner. This relationship is shown in
Figure 9.5.
This drawing shows the relationship between the three-layer enhanced performance
architecture (EPA), as implemented by DNP3, and the OSI reference model.
In the following section the message, structure will be analyzed in terms of the EPA
model. Before looking at that, a brief review of the general functions of each of the layers
will be made.
152
Practical Industrial Data Communications
Figure 9.5
Relationship of EPA model to OSI seven-layer model
9.1.3
Functions of the model layers
The functions of each layer of the EPA model are described briefly in the following
paragraphs. The network and transport layer functions have included as the pseudotransport layer, as limited transport and network functions are supported by the DNP3
protocol.
Physical layer
The physical layer is the physical media over which the protocol is transmitted. The
definition of this, the physical interface characteristics, in terms of electrical specifications,
timing, pin-outs and so on. The data element at this level is essentially the bit, i.e. it is
concerned with how to pass one bit of data at a time across the physical media.
Definition of the physical layer also includes the functions for controlling the media, such
as details required to establish and maintain the physical link, and to control data flow.
The actual specification of this layer is normally a separate standard such as ITU-T
X.21, or V.24, RS-232 or others.
The physical layer specification for DNP3 is covered in greater detail later in this text,
but it is noted here that RS-232C was originally specified for voltage levels and control
signals, and ITU-T (formerly CCITT) V.24 for DTE–DCE signaling.
Other communications media such as over Ethernet are in the process of being defined
for use by the DNP3 user group technical committee.
Data-link layer
The data-link layer provides for reliable transmission of data across the physical medium.
While the physical layer is concerned with the passage of a signal, or a bit of data, the
data-link layer is concerned with the passage of groups of data. These groups may be
referred to as a frame. Functions provided by the data-link layer include flow control and
error detection.
The pseudo-transport layer
This layer is included in DNP3 to allow for the transmission of larger blocks of data than
could otherwise be handled. Some writers have described this in terms of both network
and transport services, although these are usually simply referred to as pseudo-transport
Fundamentals of DNP3 153
services. Network functions are concerned with routing and flow control of data packets
over networks. Transport functions provide network transparent end-to-end delivery of
whole messages, including disassembly and reassembly, and error correction.
Application layer
The application layer is the level where the data is generated for sending, or requested to
be sent. The layer interfaces to the lower levels in order to transmit the required
information. In turn, the DNP3 application layer provides its services to the user
application programs, such as an HMI system, an RTU, or other system.
9.2
Understanding the DNP3 message structure
In the preceding sections, the build-up of a generic OSI seven-layer model message was
shown, and the Enhanced Performance Architecture was introduced. In this section, the
build-up of a DNP3 message will be examined. This will provide an introduction to the
overall structure of the DNP3 message. At this point we need not be concerned with
the detailed meaning of each part of the message. Detailed descriptions of the meaning of
the data added at each layer will be given in later sections.
9.2.1
DNP3 message build-up
Figure 9.6 shows how the transmitted message is built up in DNP3. Each layer of the
model takes the information passed from the higher layer, and adds information
connected with the services performed by that layer. The additional information is usually
added as a header, that is, in front of the original message.
Figure 9.6
Build-up of DNP3 message
Thus during message assembly, the message will grow in size with each layer that it
passes down through. It is also disassembled in this process into smaller units of data.
The message build-up illustrated in Figure 9.6 is now briefly described for each layer. It
is described here from the highest level, the application level, downwards. This reflects
the sequence of message building during sending. Of course, the sequence at the other
154
Practical Industrial Data Communications
end is in reverse, as the message is passed over the physical medium, into the data-link
layer, and up to the application layer.
9.2.2
Application layer
The user data is the data arising from the user application. The user application can be
visualized as a layer above the application layer, and could for example be a human
machine interface (HMI) program such as ‘Citect’ or ‘Intellution Fix’, or it could be an
embedded program, such as a C+ application program. The data could be alarm and event
data, digital status data, or even a data file such as a configuration file being passed from
a master station to an RTU or an IED. On the other hand, in the case of many types of
command being issued by a master station, there may be no data at all. A key point is that
this data can be of any size. The total data size is not limited by the protocol.
The application layer initially forms the data into manageable sized blocks. These are
called application service data units or ASDUs. The application layer then creates the
application protocol data unit APDU by combining a header with the ASDU data. The
application header is referred to as the application protocol control information, or APCI.
This is either 2 bytes or 4 bytes in length, depending on whether the message is a request
or a response. In the case of a command or other user request requiring no additional
data, there is only a header, and no ASDU.
Depending on the total size of the data to be transmitted (the size of the ASDU), one or
more APDUs are created. In the case that multiple APDUs are required, these are each
termed fragments. Whilst the number of fragments required representing an ASDU is not
limited, the size of each fragment is limited to a maximum of 2048 bytes.
9.2.3
The pseudo-transport layer
The APDU from the application layer may be referred to as the transport service data unit
within the pseudo-transport layer. It is interpreted purely as data to be transported by the
transport layer. The transport layer breaks the TSDU down into smaller units termed
transport protocol data units or TPDUs. These are made up of a 1-byte header, followed
by a maximum of 249 bytes of data.
The overall size of the TPDUs, which is 250 bytes, was determined so that each TPDU
will fit within one ‘Frame’ or LPDU at the data-link layer.
9.2.4
The data-link layer
This layer takes the TPDUs from the pseudo-transport layer and adds a 10-byte header to
each. As the data-link layer is also responsible for providing error detection and correction
functions, error-checking codes are introduced here. A 16-bit cyclic redundancy code
(CRC) is used. Each TPDU is converted to a frame of up to 292 bytes in length.
It is worth noting at this point that the frame format is known as the FT3 frame format,
which was originally described by IEC 870-5-1. This frame format represents a historical
point of common ground between DNP3 and IEC 870-5. This frame format will be
returned to in the next section.
9.2.5
The physical layer
The physical layer converts each frame into a bit stream over the physical media. In the
original DNP3 documentation, a bit-serial asynchronous physical layer is specified. It
calls for 8-bit data, one start bit, one stop bit, no parity, and RS-232C voltage levels and
control signals.
Fundamentals of DNP3 155
9.2.6
Summary of message build-up
The build-up of DNP3 messages has been shown from a top-down perspective to have the
following features:
•
•
•
•
•
•
•
•
Application functions may or may not require the passage of data
Commands will often require no data
The application layer parses the data into APDUs
The APDU maximum size is 2048 bytes
The pseudo-transport layer parses the APDU into smaller TPDUs
TPDU maximum size is 250 bytes
The data-link layer adds headers and CRCs to form the LPDU
LPDU maximum size is 292 bytes, of which 250 bytes are data.
In the following sections, the detailed functioning of each of the model layers will be
examined. Looking at the functionality of each layer it will be necessary to examine and
understand the message header sections applied at each layer, and to see how the
functions of the layer are performed using them.
9.3
Physical layer
The physical layer may be described in terms of the network topology as well as the
characteristics of the communication paths.
As previously noted, the physical layer defines the physical interface characteristics in
terms of electrical specifications, timing, pin-outs and so on. This includes details
required to establish and maintain the physical link. The data element at this level is
essentially the bit, i.e. it is concerned with how to pass one bit of data at a time.
9.3.1
Description of physical layer
The physical layer originally recommended for DNP3 has the following specification:
•
•
•
•
•
•
Bit serial asynchronous
Eight data bits
One start bit, one stop bit
No parity
RS-232C voltage levels and control signals
CCIT V.24 hardware protocol for DTE/DCE communications.
The DNP3 Users Group Technical Committee subsequently produced a standard for
transmission of DNP3 over networks. This provides an alternative definition of the
physical layer for that situation.
9.3.2
Services provided by physical layer
The physical layer must provide these functions:
•
•
•
•
•
Connect
Disconnect
Send
Receive
Status.
156
Practical Industrial Data Communications
9.3.3
Topologies
As previously described, DNP3 supports either master–slave or peer-to-peer communications,
with either one-on-one or multi-drop topologies.
•
•
•
•
Point-to-point
Multi-drop from one master
Hierarchical with intermediate data concentrators
Multiple master.
Point-to-point or direct topology refers to the case of two DNP3 devices connected
together, either directly with a cable, or via modems and a communications path. This could
be via a leased line, via radios, or via the public switched telephone network (PSTN).
A serial bus topology is the alternative to a direct topology. This may also be referred to as
multi-drop. In this case, multiple devices are connected to the same communications path.
9.3.4
Physical layer procedures
Procedures must be provided to support both half-duplex and full-duplex communications.
The specific procedures used will depend on the topology as well as whether half- or fullduplex transmission is available. One particular role of the procedures is to manage the
event of message collision, where it can occur. Because DNP3 supports peer–peer
communications, any station can act as a primary, or message initiator. Because of this,
messages can be sent from two stations simultaneously, causing a collision.
Before discussing the physical layer procedures, a brief review of the terminology is
given in the following table.
Simplex
Communications are in one direction only
Half-duplex
Two-way communication is possible, but only in one direction at a time. This
is because only one communication path or channel is used. To effect
communication a protocol for handing over the channel is required
Simultaneous two-way communication is possible. Two channels are provided
to do this
A two-wire connection will provide one communication channel. A PSTN line
is an example of this. Therefore, half-duplex communications only are
possible via this channel
A four-wire connection will provide two channels, and is capable of
supporting duplex communications
Full-duplex
Two-wire
Four-wire
In the following procedures, the data carrier detect (DCD) indication is used to
determine if the communications medium is free. Generally, a time delay must be used to
prioritize access to the channel once it has become free.
Half-duplex procedures
Direct link
With a direct, or point-to-point link, the delay from DCD clear needs to be just sufficient
to allow the master station to detect loss of DCD and begin transmission. Note that if a
dial-up connection is being used, the DCD signal indicates the establishment of a link
rather than the presence of data. In this case the RTS and CTS signals must be used.
Fundamentals of DNP3 157
DNP3 must assert the RTS signal and await the CTS signal before transmitting each
frame.
Multi-drop link
In the case of a multi-drop link, time delays must be used to ensure that stations get
access to the medium. This is accomplished by providing a back-off time made of a fixed
delay plus a variable delay. These delay times are configurable by the user. Normally the
master station will be given a zero fixed delay time, so that it can always gain access in a
half-duplex environment. Prioritization of slave stations can be accomplished by this
method. Again, the minimum delay must be long enough for the master station to be able
to gain control of the medium before any outstation.
Full-duplex procedures
Direct link
No collision can occur as both master and slave have their own channel to each other.
Multi-drop link
Time delays are used as for half-duplex multi-drop, except that the fixed time delay may
be reduced, as it is not necessary to provide this for the master. In the full-duplex multidrop environment, the master has its own channel to all receivers.
9.4
Data-link layer
9.4.1
Description of data-link layer
The purpose of the data-link layer is to establish and maintain reliable communication of
data over the physical link.
Link establishment involves setting up the logical communications link between sender
and receiver. DNP3 is capable of supporting either connection-oriented or connectionless
operation. Thus, if a channel operates over a PSTN line and requires connection by
dialing before communication can commence, the data-link layer manages this without
any direction from higher levels.
As seen previously, the data unit at the link layer level may be called the frame. The
frame has a maximum size of 292 bytes including CRC codes, and carries a total of
250 bytes of information from the higher levels. The frame includes 16-bit source and
destination addresses in its header. These provide for 65 536 different addresses. The
address range FFF0–FFFF is reserved for broadcast messages, which are intended to be
processed by all receivers. Note that addresses are logical in the sense that it is possible
for one physical device to have more than one logical address. In such a case, the
different addresses would appear as separate devices to the master station.
The frame header also contains a function code. The functions supported by this are
those required to initialize and test the operation of each logical link between sender and
receiver. As an additional security feature, every frame transmitted can request a
confirmation of receipt. This is termed link layer confirmation.
These services and functions are expanded upon in the following sections.
9.4.2
Services provided
The following services are all provided by the data-link layer. Understanding how these
are provided begins with examining the frame format. The frame format includes
158
Practical Industrial Data Communications
necessary control bits for controlling message flow, functions, error detection, and
correction.
Services provided by the data-link layer:
•
•
•
•
•
•
9.4.3
Establish and maintain the communication channel
Report on link status to higher layers
Detect and correct errors that may occur during transmission
Conversion of data between LSDUs and LPDUs
Error notification to higher levels
Sequencing and prioritized delivery of LSDUs.
Some terminology
Communications in a SCADA system will generally have a structure where some stations
may be identified as master stations, and others as slave stations, sub-master stations, or
outstations. In a hierarchical structure, there may be some devices that act both as slave
stations and master stations.
At the data-link level, the terms balanced and unbalanced are used to describe whether
all stations may initiate communications or not. In ‘unbalanced’ systems, only master
stations will initiate communications. That is, only a master station will be a primary, or
originating, station, and slave stations will always be secondary, or responding, stations.
In these systems when a slave station has data that it needs to transmit to the master, it
must wait until the master station polls it.
The DNP3 protocol supports balanced communications at the data-link level. This
provides greater flexibility by allowing non-master stations to initiate communications. In
DNP3 any station can be an originator or primary station.
The terminology ‘balanced’ and ‘unbalanced’ can be confused with the terms
‘master’ and ‘slave’. This confusion can arise because it is easy to imagine that in a
balanced system, all stations must be equal. This is not the case, however. The reason is
that the terms balanced and unbalanced apply only to the lower data-link level
communications, whereas the master/slave distinction applies at the application level.
In DNP3 the terms master and slave have real meaning, and a station is configured as
one or the other. The key to understanding is that only a master station can issue a
request, and only a slave can provide a response. These are each application level
messages.
In the case of an outstation (a slave station) having unsolicited data to send to the
master station, it issues an unsolicited response to the master. At the application level this
is different type of message to a request. At the data-link level, however, the message
frame appears no different from any other message frame whether issued by a master or a
slave. These concepts are illustrated in Figure 9.7.
Figure 9.7 shows how these terms relate to the communication process. It illustrates a
request for data from a master station to a non-master station. This could be a poll for
current data, a ‘static’ poll. It illustrates the communication sequence by showing the
parties on each side, with message directions shown between them. The time sequence is
shown from top to bottom.
In the diagram, the designated master station initiates a communication with a nonmaster station. The request message itself is contained in the application layer
information within the message. The designated master station has initiated this
communication and is therefore the primary station. A response is required to this
message at the data-link level, i.e. a confirmation of receipt is expected. The non-master
Fundamentals of DNP3 159
station sends an acknowledgment. This is a secondary message, i.e. a data-link response
to the primary message. Note that at the data-link level, this transaction is now
completed.
Figure 9.7
Example communication sequence diagram
Because the last transaction contained an application level request for the transmission
of data, the non-master station then initiates a communication with the requested data.
Now the non-master station is initiating the communication, and it is the primary station
for this transaction. This is a new communication sequence, or transaction, at the datalink level. Although it is related to the prior transaction at the application level, it is
unrelated at this level.
It can be seen from this example that the terms primary and secondary relate to the
station initiating a transaction at the data-link level, and not to whether the stations are
master or non-master.
Although the communications are balanced, and therefore any station can be a primary
station and initiate messages, it is incorrect to believe that the terms master and nonmaster have no meaning. In DNP3 stations may be defined as either master or nonmaster.
This information is used at the link level to determine the setting of a message direction
bit, the DIR bit. The direction bit is set for messages from a master, and cleared for
messages from a non-master station.
A final definition that is important to understand is the ‘data link’ or just ‘link’. The link
refers to the logical connection between a primary station and a secondary station. It is
therefore a one-way communication path. To establish two-way communications between
two devices, it is necessary to establish the links in each direction. The logic in this may
be better understood by recognizing that a communication channel between two devices
can be duplex, using two entirely separate physical media, one in each direction. The
simplest example of this is of course a four-wire connection.
Figure 9.8 shows two communication channels between the master and non-master
stations. The first channel uses a data radio, and is full-duplex. The second channel is a
back-up using modems and the public switched telephone network. The second channel is
half-duplex, although parts of it are in fact full-duplex.
In this example, there are four links, as follows:
1. Channel 1 A to B
2. Channel 1 B to A
160
Practical Industrial Data Communications
3. Channel 2 A to B
4. Channel 2 B to A.
Figure 9.8
Example of communication channels
9.4.4
How it does it
DNP3 controls transmission at the link layer level by using defined transmission
procedures. These procedures make use of a control byte contained within the message
frame to control transmission. It is important to realize that neither the control byte nor the
procedures can by themselves do this. The procedures define what actions are taken at each
end, and the control byte provides the coordination between them. It defines what type of
transmission is being sent, i.e. the frame type, and where in the process the frame fits.
In order to understand how the overall process works, it is necessary to examine the
structure of the frame, the meaning of the information in the control byte, and finally the
procedures themselves.
The FT3 frame format
The LPDU or frame format was based on the FT3 format specified by IEC 870-5-1. This
was one of the four possible frame formats specified by IEC 870-5-1. The others are FT1.1,
FT1.2, and FT2. These will be discussed further when comparing DNP3 with IEC 870.
The format specifies a 10-byte header, followed optionally by up to 16 data blocks. The
overall message size is limited to 292 bytes, which provides for a maximum data capacity
of 250 bytes. Thus a fully packed frame will comprise the header plus 16 data blocks,
with the last block containing 10 data bytes (Figure 9.9).
Figure 9.9
FT3 frame format
Fundamentals of DNP3 161
Start:
Length:
2 bytes: 0564 (Hex)
Count of user data in bytes, plus 5, not counting CRC bytes. This
represents all data; excluding CRC codes following the LENGTH count
byte. Its range is 0–255. This is convenient as it is the largest length count
that may be represented by 1 byte (=FF Hex)
Control:
Frame control byte. See later section
Destination: Two-byte destination address (LSB, MSB)
Source:
Two-byte source address (LSB, MSB)
CRC:
Two-byte cyclic redundancy check code
User data:
Each block has 16 bytes of user data. Last block has 1–16 as required. In
case of a full frame the last block will have 10 bytes of user data.
Control byte
The control byte follows the start and length bytes in the frame format. It provides for
control of data flow over the physical link, identifies the type, and indicates the direction.
The interpretation of most of the control byte is dependent on whether the communication
is a primary or a secondary message (Figure 9.10).
Figure 9.10
Control byte
DIR
Direction
1 = A to B
0 = B to A
PRM Primary message
1 = frame from primary (initiating station)
0 = frame from secondary (responding station)
FCB Frame count bit
FCV Frame count bit valid
0 = ignore FCB
1 = FCV is valid
RES Reserved = 0
DFC Data flow control bit. Set to 1 by secondary station if further send of user data
will cause buffer overflow.
FC
Function code
162
Practical Industrial Data Communications
Direction bit
The DIR bit indicates the message direction between master and non-master stations. If a
message is from a designated master station, this bit is set. Otherwise it is cleared.
Primary bit
The PRM bit is set if the frame is primary (initiating) or a secondary (responding). This is
used directly in interpreting the function code. There are six valid function codes for
primary frames, and five valid function codes for secondary frames.
Frame count bits
These are the frame count bit (FCB) and the frame count valid bit (FCV). These bits are
only used for primary messages. The frame count bit is used to detect losses or
duplication of frames to a secondary station. The frame count valid bit enables the use of
the FCB. When the FCV bit is true, the FCB is toggled for each successful SEND–
CONFIRM transaction between the same primary and secondary stations.
How they work is like this:
• Following data-link start-up or a failed transaction, a secondary station will not
accept any primary SEND–CONFIRM frames with FCV = 1 until a reset
transaction is completed. This means it will only accept either a RESET Link
or a RESET user process command.
• After a secondary station receives a RESET link frame from a primary and it
responds with a CONFIRM, that link will be operational until a frame
transmission error occurs.
• The secondary station will expect the next frame to contain FCV = 1 and
FCB = 1.
• The next primary SEND–CONFIRM message will have FCV = 1 and FCB = 1.
The secondary station will accept this as the FCB is valid and is set, as
expected.
• Each subsequent primary SEND–CONFIRM message will have the FCB
cleared or set in turn.
Data-flow control bit
The data flow control bit (DFC) is included in secondary frames. The secondary station
will set DFC = 1 if a further SEND of user data will cause its buffer to overflow. On
receipt of a frame with DFC = 1 a primary station will cease to send data, but will request
link status until DFC = 0.
Data-link function codes
The tables below show the detailed meanings for different values of the function code
byte. The meanings are different depending on whether the message is a primary or a
secondary transmission.
Fundamentals of DNP3 163
Function Codes from Primary Station
Function Code
Frame Type
Service Function
0
1
2
3
4
9
SEND–CONFIRM expected
SEND–CONFIRM expected
SEND–CONFIRM expected
SEND–CONFIRM expected
SEND–NO REPLY expected
REQUEST–RESPOND expected
FCY Bit
Reset of remote link
Reset of user process
Test function for link
User data
Unconfirmed user data
Request link status
0
0
1
1
0
0
Codes 5–8, 10–15 are not used.
Function Codes from Secondary Station
Function Code
0
1
11
14
15
Frame Type
CONFIRM
CONFIRM
RESPOND
Service Function
ACK – positive acknowledgment
NACK – Message not accepted, link busy
Status of link (DFC = 0 or 1)
Link service not functioning
Link service not used or implemented
Codes 2–10, 12–13 are not used.
The following shortened version of the function codes is included in the sequence
diagrams. The codes are preceded with a P or and S, representing primary or secondary
codes.
Function Code Key
P0
P1
P2
P3
P4
P9
Reset link
Reset user process
Test link
User data – confirm
User data – no confirm
Request link status
S0
S1
S11
S14
S15
Confirm–ACK
Confirm–NACK
Link status
Not functioning
Not implemented
The use of the function codes by the transmission procedures is described in the
following sections.
Error control
Error control is effected by the use of an error detection code and by the transmission
procedures.
The first part of error control is detecting when an error has occurred in a message. DNP3
uses a 16-bit cyclic redundancy code or CRC to do this. The CRC is transmitted once in the
frame header, and thereafter once for every block of up to 128 bits of user data being
transmitted. By calculating a CRC for each block of data received, and comparing it to the
transmitted CRC for that block, errors will be detected if they occur. If the CRCs do not
agree, then an error is indicated. This technique is not foolproof, as it is possible to obtain
equal CRCs for different data blocks, but the probability of this is very low.
The second part of error control is the action performed when an error is detected. In
every case where there is an error detected in any part of a frame, the frame is rejected by
the receiving data-link layer.
164
Practical Industrial Data Communications
For all primary frame types except for SEND–NO REPLY, an error will result in a
negative acknowledgment and subsequent resending of the frame by the primary. The
SEND–NO REPLY frame type is used for sending low-priority user data, which does not
require confirmations. This type of data will simply be lost in the event of error.
The calculation of the CRC and processing on sending and receipt is a moderately
complex operation using binary modulo-2 division. Although tricky for people to actually
do, it is accomplished readily by computational devices. The algorithm for CRC
processing is presented as a separate section.
9.4.5
Transmission procedures
Link reset
A link reset is required to enable communications between a primary and secondary station.
A primary station must send the reset frame on power-up, or after a link is placed on-line.
Main points
• Must be used after a start, restart, or any link failure.
• It is the responsibility of the primary station to perform the reset (of its link to a
secondary station).
• Initiates the primary and secondary stations for further SEND–CONFIRM
transactions.
• Synchronizes the FCB so that FCB = 1 is expected by the secondary for the
transaction following the reset.
In the communication sequence diagram in Figure 9.11 the link reset transaction is
represented. Blocks are shown representing the actual frames transmitted. The primary
station is shown on the left and the secondary station is on the right. States are shown for
the FCB (frame control bit) at the primary, and the expected value of the FCB, at the
secondary station. User indications are noted for each station also. These are indications
on link status provided to service users.
Figure 9.11
Reset link
Fundamentals of DNP3 165
In Figure 9.12, the same example is presented showing the actual byte pattern that
would be seen if the transaction was captured by a protocol analyzer.
Primary to Secondary (A to B)
<05 64 05 C0 01 00 0C 00 CF A0>
Link Header
0564
Length
05
Control Byte
C0
; Dir=1, PRI=1, FCV=0, FC=RESET
Destination Address
0001
Source Address
000C
CRC value
CFA0
Secondary to Primary (B to A)
<05 64 05 00 0C 00 01 00 FD E9>
Link Header
0564
Length
05
Control Byte
00
; Dir=1, PRI=0, FC=ACK
Destination Address
000C
Source Address
0001
CRC value
FDE9
Figure 9.12
Data-link transaction detail – reset
It may be noticed that the primary DIR bit is set in the primary frame, and cleared in the
secondary frame. This shows that this transaction is between a master and a non-master.
This is coincidental, as it could equally have been the other way (e.g. as for the return link
between B and A.
Reset user process
The reset user process (Figure 9.13) function code has been discontinued. The original
DNP3 documentation defined the following functionality:
• Used to reset the link user process
• An ACK is sent if accepted by the user process
• Otherwise a NACK response is sent.
Figure 9.13
Reset user process
166
Practical Industrial Data Communications
Test
The test procedure is used by the primary station to test the link. If the FCBs do not
match, the secondary station should resend the last secondary confirm frame.
If the FCBs do match, it should send a CONFIRM–ACK frame and toggle the FCB.
The data flow control bit DFC should be set accordingly in the return frame (Figure 9.14).
Figure 9.14
Test
Summary
•
•
•
•
Used to test the link
Secondary station checks FCB to expected FCB
If FCBs match it sends ACK and toggles expected FCB
If FCB does not match it resends the last confirm frame.
Confirmed send user data
This procedure is used for sending frames with user data that require confirmation of
receipt. The link must be initially reset before the user data function may be used.
Summary
•
•
•
•
The user data function is the most common message.
Link must be reset prior to use.
It carries the user data.
Provides reliable transfer of user data by resending frames with errors.
Figure 9.15 shows the transmission of two consecutive user data frames by the primary
station. On the successful receipt of the first user data frame an ACK (confirmacknowledge) response is sent by the secondary station. The station also indicates the
receipt of user data and toggles the expected FCB. On receipt of the confirmation by the
primary, the FCB flag is toggled and another user data frame is sent.
In the event of an error in the received message, the frame is ignored and therefore no
confirmation is sent. In this case, the primary will resend the message after an appropriate
retry delay time.
Fundamentals of DNP3 167
Figure 9.15
User data
Unconfirmed send user data
This procedure is used for sending user data without requiring confirmation from the
secondary station. It may be used where the data is of low priority. The advantage is that
data can be transmitted at a greater rate, thus providing a better use of bandwidth.
However, the disadvantage is that errors in transmission will result in lost frames.
A frame found to have an error (by CRC checking) will be rejected by the secondary
station, but as no confirmation was expected by the primary there will be no way for the
primary to know that the message was unsuccessful (Figure 9.16).
Summary
• Used for low-priority user data
• No confirmations are sent by secondary
• More efficient use of bandwidth.
Figure 9.16
Unconfirmed send user data
168
Practical Industrial Data Communications
Request link status
The request link status frame is used following receipt of a NACK or and ACK with
DFC = 1 is received during transmission. The DFC (data flow control) bit is set by the
secondary station if its buffers are full or if for any other reason it cannot process further
data. The request link status is then used to determine when the link can again accept
data.
Summary
• Sent to request the status of the secondary station
• Used after a NACK or an ACK with DFC = 1 is received
• Response frame will have DFC either set or cleared to give status.
In Figure 9.17, the secondary station’s buffer is full after the second user data frame.
The secondary responds with an ACK containing the data flow control DFC bit set to 1.
The primary station now waits and at intervals sends a request link status frame. When
this is acknowledged with DFC = 0 the user data transmission is resumed.
Figure 9.17
Request link status
9.4.6
CRC error code
The use of the cyclic redundancy code for detection of errors within transmitted frames
has been introduced in terms of its function. This section examines the detail of the CRC
processing.
DNP3 defines a 16-bit cyclic redundancy check code (or CRC) for error detection. The
16-bit CRC is provided for the 64-bit header data, and for each 128-bit (16-byte) block of
user data transmitted.
The CRC code is generated by the following simple algorithm illustrated in Figure 9.18,
and described following.
Generation of message:
• Start with user data block M of k bits (k = 64 for header, 8–128 for blocks in
body)
• Multiply by 216 (i.e. append 16 zeros to end to form k + 16 bit number)
• Divide by generator polynomial P to obtain quotient Q and remainder R
(module-2 division is used)
Fundamentals of DNP3 169
Figure 9.18
CRC code processing
• Discard Q, keep R
• Invert R to obtain R1
• Append R1 to M forming message T1 to be transmitted.
Where generator polynomial:
P = X 16 + X 13 + X 12 + X 10 + X 8 + X 5 + X 2 + 1
This forms a 17-bit number, which is 13D65 (in Hex).
Processing on receipt of message
•
•
•
•
•
•
Receive T = M followed by R
Invert R and form T = M appended with R
Divide T by P (using modulo-2 division)
Discard quotient QC, keep calculated remainder RC
If RC = 0, result is GOOD, copy M to data output
If RC not = 0, result is BAD, reject message.
It is noted that this CRC has a ‘Hamming distance’ of 6. This means that at least 6 bits
must have been received in error to obtain a false GOOD DATA result.
9.4.7
Summary of data-link operation
Summary of data-link layer
•
•
•
•
•
•
Receives link service data units from higher layer
Forms the LPDU by including addresses, the control byte, and CRC codes
Uses procedures to deliver the frame across the physical layer
Function codes provide for link initialization and error recovery
Reassembles LSDU at secondary station
Provides status indications to higher levels.
170
Practical Industrial Data Communications
9.5
Transport layer (pseudo-transport)
9.5.1
Description of transport layer
The primary function of the transport layer in DNP3 is to implement message disassembly
and reassembly. This allows for the transmission of larger blocks of data than can be
handled by the data-link layer. Because this functionality is fairly limited compared to the
wider OSI definition of the transport layer, the prefix ‘pseudo’ is often used. For simplicity,
the layer is referred to in this text as the transport layer.
9.5.2
The transport protocol data unit
The transport layer takes the user data, the transport service data unit (TSDU) and breaks
it down into one or more transport protocol data units (TPDUs) and sends each to the
data-link layer for sending. The TPDUs become the user data within the data-link layer,
that is the LSDUs.
Recalling that the LSDUs can be a maximum of 250 bytes of user data, this defines the
maximum size of the LPDU, including any transport layer overheads. In fact the transport
functions are accomplished with a single-header byte, leaving 249 bytes for carrying data.
This is pictured in Figure 9.19.
Figure 9.19
Transport protocol data unit
In a receiving, or secondary station, the incoming TPDUs are reformed into the TSDU
for use by the application layer. The transport header bytes are stripped off and the TSDU
is reformed from the multiple TPDMs. The transport layer is responsible for ensuring that
the TSDU is reassembled in the correct sequence.
9.5.3
The transport header
The single byte transport header contains two bits that identify the start and end of a
sequence of frames (TPDUs), plus a six-bit sequence counter (Figure 9.20).
Rules
• If a secondary station receives a frame with the FIR bit set, any previous
incomplete sequence of frames is discarded.
• If a frame is received without the FIR bit set, and no sequence is in progress,
then this frame is ignored.
Fundamentals of DNP3 171
• Sequence number may be any value 0–63 for first frame, and must increment
for each frame in the sequence thereafter.
• Sequence number rolls over from 63 to 0.
Figure 9.20
Transport header byte
9.5.4
Summary of transport layer
Summary
•
•
•
•
•
•
•
Takes TSDU Fragments up to 2048 bytes and breaks them multiple TPDUs
Includes a one-byte header
TPDU can carry 249 bytes of user data
The TPDU can fit into one FT3 frame format (LSDU)
Header has FIR and FIN bits and six-bit sequence number
Passes TPDUs to and from data-link layer
Assembles incoming TPDUs into TSDU.
9.6
Application layer message handling
9.6.1
Description the application layer
The application layer is the highest level of the protocol, and communicates directly with
the user application program. Master stations generate application level request messages
for transmission to outstations, and outstations generate application level responses for
transmission to the master station. These messages may contain commands, or requests
for provision of data, or they may perform other functions such as time synchronization.
Summary
•
•
•
•
•
•
•
•
•
Forms request messages to outstations
Request messages may be commands, send data, or request data
Accepts data from user application to send to master or to outstation
Breaks user data down into multiple ASDUs
Adds control information (APCI) to form APDUs
Multiple APDUs are called fragments
Fragments do not exceed 2048 bytes
Fragments contain data objects
Each fragment must be processable on its own.
172
Practical Industrial Data Communications
The fragment size and data content requirements have been specified so that a fragment
is always processable on its own. This means that a fragment buffer size of 2048 bytes
should be sufficient to allow receipt of a fully processable piece of information. The
buffer can then be left empty for receipt of the next fragment.
9.6.2
Application message sequences
At the application level, there are two basic message types in DNP3. These are requests
and responses. Only a master station can send a request, and only an outstation can send a
response. However, there is a special class of response called an ‘unsolicited response’.
This allows an outstation to send information to a master station or to a peer (another
non-master station).
The message sequences are shown in Figures 9.21 and 9.22.
Figure 9.21
Application message sequences
Figure 9.22
Unsolicited response message
In the sequence in Figure 9.21, the outstation receives a request from the master station.
If the master station has set a ‘confirm’ bit, the outstation will send a confirm message.
It will at the same time commence processing the request. Once it has assembled the
necessary information, it will send its response. The master station will send a
confirmation if the confirm bit is set in the response.
Fundamentals of DNP3 173
The case of an unsolicited response is shown in Figure 9.22. An event or defined
process change is detected, causing the generation of a message by the outstation.
Whilst the message sequences appear simple enough, complexities arise from various
causes. This includes the need to generate multi-fragment messages, and the need to
handle crossovers in communications such as might arise if both a master and an
outstation send messages simultaneously. These complexities are catered for by ‘flow
control’ and the APCI, discussed in the following sections.
9.6.3
Application message formats
As noted previously, application messages are of two types, request and response. Each
of these appears as one or more APDUs. Each APDU is made up of a header, which is
termed the application protocol control information, and optional data, the ASDU.
The request and response headers differ by one field. The response header includes an
additional two-byte field designated internal indications (IIN). The content of the ASDU
is not depicted in Figure 9.23, but it is made up of data objects, each with an object
header. These will be presented in detail later. For now the ASDU may be thought of
merely as data.
Figure 9.23
Application message format
9.6.4
Application control field
The application control field (Figure 9.24) is the first byte of the control information, or
APCI. It is used to control the communication flow. It has three bits or flags, plus a
sequence number that is used to sequentially number fragments.
Key
FIR
FIN
CON
First fragment of multi-fragment message
Final fragment of multi-fragment message
Confirmation required
Sequence fragment sequence number
0–15
Master station requests and outstation responses
16–31 Outstation unsolicited responses.
174
Practical Industrial Data Communications
Figure 9.24
Application control field
Flow control is implemented by:
•
•
•
•
The FIR and FIN bits
The CON bit
Master and outstation response timeouts
Master and outstation retry counts (optional).
Using the following rules:
•
•
•
•
•
•
•
•
If CON = 1 a confirmation must be sent.
Sequence number is incremented for each fragment.
Sequence number rolls over from 15 to 0, or 31 to 16.
Response fragment has same sequence number as request fragment.
If multiple request fragments, response starts at last request fragment number.
Increment sequence number for each extra response fragment.
A retransmitted response uses the original sequence number.
Outstation must process one request before starting a second request.
These are further detailed in the following sub-sections.
Message confirmations
• If the CON bit is set, then a confirmation is required.
• A confirmation response to a request has the same sequence number as the
request.
• If two confirmation messages are received with same sequence number, ignore
the second.
Sequence numbers
• Sequence number is 0–15 for requests, outstation responses, and the associated
confirmations.
• Number rolls over from 15 to 0.
• Sequence number is 16–31 for unsolicited responses and confirmations.
• Confirmations have same sequence number as request or response.
• The first fragment of a response has the same sequence number as the request,
or as the last fragment of a multi-fragment request.
Fundamentals of DNP3 175
• For a single-fragment request retry, the retry has the same sequence number as
the original request.
• For a multi-fragment request retry, the retry commences with the sequence
number of the last fragment of the previous request.
Identical sequence numbers
• If two messages are received with the same sequence number, it usually means
that the response message was not received by the other station. Retransmit
response.
• If two confirmation responses are received with the same sequence number,
ignore the second response.
9.6.5
Message transaction flow diagrams
The DNP3 basic four documentation includes a number of diagrams illustrating typical
message transactions. These are very useful in understanding the transaction sequences.
These are represented in the following diagrams, although the layout has been altered.
In these diagrams, the transaction sequence is represented by showing the application
control (AC) information in each message passing between master station and outstation.
The time sequence in the diagrams is from top to bottom.
The AC information is shown in the form:
[FIR FIN CON SEQ Number]
Receipt of a message fragment is depicted by an equals sign (=) in the diagrams,
indicating the received message was as sent. Where a message is not received, a timeout
may subsequently occur.
It should be remembered that at this point the diagrams are concentrating on only the
AC part of the message, which is the first byte of the message, as this is the part that is
controlling the message flow. Of course, in the whole message the AC byte is followed
by the function code FC, the internal indications for response messages, and the data if
present. Nevertheless, it is only the AC information that is of importance to the flow
control.
Typical message transaction
Case One shows an initial master request without the confirmation bit set. The outstation
responds without a prior confirmation. As the outstation’s response has CON set, the
master sends a confirmation on receipt. Later, the outstation makes an unsolicited
response. It uses a sequence number in the range 16–31. The master station responds
using the same sequence number.
Case One: Typical Message Transaction
Master AC
Outstation AC
[110 7]
=
=
[111 7]
[110 7]
=
=
[111 24]
[110 24]
=
[FIR FIN CON SEQ]
Description
Request, no confirm required
Response to master request
Confirm
Unsoliciated response
Confirm
176
Practical Industrial Data Communications
Case Two shows a typical request–response with confirmation sequence. Notice that
the sequence number is unchanged for the whole transaction.
Case Two: Request with Confirm
Master AC Outstation AC
[111 2]
=
=
[110 2]
=
[111 2]
[110 2]
=
[FIR FIN CON SEQ]
Description
Request
Confirm
Response to master request
Confirm
Multi-fragment response
Case Three shows a single-fragment request, without confirmation, requiring a multifragment response. Two response fragments were required. The master station increments
it sequence number for the subsequent transaction, shown as the ‘new request’.
Case Three: Multi-fragment Response
Master AC
[110 2]
=
[110 2]
=
[110 3]
[110 4]
Outstation AC
=
[101 2]
=
[011 3]
=
=
Description
Request, no confirm required
Response, fragment 1, first
Confirm
Response, fragment 2, last
Confirm
Request (new request)
[FIR FIN CON SEQ]
Confirmation timeouts
In Case Four the outstation response was not received by the master station and so no
confirmation was issued by the master. After the confirmation timeout period at the
outstation, it resends its response. Note that it has used the same sequence number.
Case Four: Outstation Confirmation Timeout
Master AC
[110 3]
Outstation AC
=
[111 3]
Timeout
=
[111 3]
[110 3]
=
[FIR FIN CON SEQ]
Description
Request
Response to master request, not received by master
Confirmation not received
Resend response
Confirm
Case Five appears identical to Case Four at the outstation. In this case the master station
did receive its response, but the confirmation it sent was garbled and lost. After its
confirmation timeout operates, the outstation resends its response. The master station on
receiving a second response with the same sequence number just resends the
confirmation. It does not reprocess the information as it already has that.
Fundamentals of DNP3 177
Case Five: Confirmation Lost
Master AC
[110 5]
=
[110 5]
Outstation AC
=
[111 5]
Timeout
=
[111 5]
[110 5]
=
[FIR FIN CON SEQ]
Description
Request
Response to master request
Confirm sent, not received
Confirmation timeout
Resend response
Confirm
Case Six shows the loss of a master station request. After a request timeout period, the
master station resends its request using the same sequence number.
Case Six: Master Request Timeout
Master AC
Outstation AC
[110 8]
Timeout
[110 8]
=
=
[101 8]
[110 8]
=
=
[011 9]
[110 9]
=
[FIR FIN CON SEQ]
Description
Request, not received
Master station timeout
Request resent by master
Response sent
Confirm
Second fragment
Confirm
Effects of network delays
In the following cases delays in the network cause transmissions to arrive after timeouts
have operated. Such cases might occur for example when a store and forward device
becomes heavily loaded.
In Case Seven the outstation response has been delayed in the network. The master station
times out and then resends its request using the same sequence number. The outstation
resends its response. The first response is received by the master and a confirmation is sent.
When the master receives a second response it does not process the data, but sends a
confirmation. The outstation receives the second confirmation and discards it.
Case Seven: Master Station Timeout
Master AC
[110 12]
Timeout
[110 12]
=(1st)
[110 12]
=(2nd)
[110 12]
Outstation AC
=
[111 12]
=
[111 12]
=
=
[FIR FIN CON SEQ]
Description
Request
Response to master delayed
Master station timeout
Request resent
Resend response
First response received
Confirm
Second response received
Confirmation resent
Confirmation ignored by RTU
178
Practical Industrial Data Communications
In general, message delays lead to timeouts and resending of either requests or
messages. When the delayed message subsequently arrives it is recognized as being part
of the current transaction because of its sequence number. If it is a repeat message
requiring a confirmation, the confirmation is sent but the message itself is not acted on
further. If it is a delayed confirmation that is received, this is simply discarded.
Case Eight shows a repeat message being sent by the master after the confirmation is
delayed. When the confirmation to the first request is received it is discarded by the
master station.
Case Eight: Delayed Confirmation from Outstation
Master AC
[111 12]
Timeout
[111 12]
=
=(1st)
Outstation AC
Description
=
[110 12]
Request, No Response Expected e.g. freeze command
Confirm sent, delayed in network
=
[110 12]
Request resent and received
Confirm
Delayed first confirm received, ignored by master
[FIR FIN CON SEQ]
In Case Nine the unsolicited message is delayed in the network. After confirmation
timeout, the outstation resends the message. This time it is received directly and a
confirmation is sent. If the master subsequently receives the original message it ignores it
but sends a confirmation. This confirmation is ignored by the outstation. The situation is
no different if the messages are received in the opposite order, i.e. the delayed 1st
message arrives first, but after the outstation confirmation timeout.
Case Nine: Master Request Timeout
Master AC
Outstation AC
=
[110 28]
=(1st)
[110 28]
[111 28]
Timeout
[110 28]
=
=
Description
Unsolicited message sent, but delayed in network
Confirmation timeout
Message resent
Confirm
Delayed first message received
Confirm, outstation ignores this
[FIR FIN CON SEQ]
Collision with unsolicited response
If outstations are configured and enabled to provide unsolicited responses, then there will
be the possibility of these occurring at the same time as a master station request. Special
features are provided to handle the cases of these transactions becoming intermingled.
The first feature is the use of different sequence numbers for unsolicited response
transactions. Because these use only the numbers 16–31, these transactions are
differentiated by the sequence number.
A second feature is provided to ensure that data is not lost or duplicated arising from an
inappropriate sequence of actions. This feature is termed process mode, for which two
modes are defined. These are the ‘immediate mode’, and the ‘process after confirm’
mode. Immediate mode is used for all transactions except for a master station read request
to an outstation, when an unsolicited response transaction is in progress. When an
Fundamentals of DNP3 179
outstation is waiting for confirmation for an unsolicited response, it will delay processing
a read request until the confirmation is received. This is the process after confirm mode.
Rules for unsolicited message transactions
• Master station always processes an unsolicited response immediately, even if
waiting for a response to a previous message.
• If a confirmation for the unsolicited response is requested it is issued
immediately by master.
• Outstation will process most requests immediately, even if waiting for a
response to a previous unsolicited response. This is ‘immediate mode’.
• Outstation will not process a master station read request if it is waiting for
confirmation of a previous unsolicited response.
• This is ‘process after confirm’ mode.
These rules are illustrated in the following transactions.
In Case Ten both master and outstation transmits messages at the same time. The
outstation responds immediately to the master station. Because it responded immediately,
it may be deduced that this was an immediate mode request, that is, the master station had
not made a read request.
Case Ten: Simultaneous Transactions
Master AC
Outstation AC
Description
[110 24]
=
[111 7]
=
=
Both master and outstations transmit simultaneously
[110 7]
=
=
[110 7]
[110 24]
Response to master station
Confirmation to outstation response
Confirmation to unsolicited message
[FIR FIN CON SEQ]
Note that the order of receipt of confirmations at the outstation is unimportant.
In Case Eleven a normal message sequence is shown in which an unsolicited message
does not require a confirmation. A simultaneous master station request is serviced by the
outstation.
Case Eleven: Unsolicited Response without Confirm
Master AC
[111 2]
=
=
=
[110 2]
Outstation AC
Description
[110 22]
=
Both master and outstations transmit simultaneously
[110 2]
[111 2]
Master stores and processes unsolicited response. No
confirm was requested
Confirmation to master station request
Response to master station request
Confirm
[FIR FIN CON SEQ]
Note that sending unsolicited responses without requesting confirmation is not
recommended, as this may lead to data loss. Considering this situation leads to
understanding when the use of non-confirmed messages is appropriate. This is when
180
Practical Industrial Data Communications
timeouts will function to ensure that a missed transaction is reattempted. For example,
requests from the master station requiring responses will be repeated if a response is not
received. In this case the message response timeout will cause resending of the message
when a response is not obtained.
In Case Twelve the unsolicited response is confirmed during the course of a multifragment transaction.
Case Twelve: Simultaneous Transactions Multi-fragment
Master AC
[110 2]
=
=
[110 2]
=
[110 18]
[110 3]
Outstation AC
[111 18]
=
[101 2]
=
[011 3]
=
=
Description
Master and outstation send messages simultaneously
Fragment 1 of response
Confirm for fragment 1
Fragment 2 of response
Confirm for unsolicited response
Confirm for fragment 2
[FIR FIN CON SEQ]
In Case Thirteen an unsolicited response from the outstation is lost or garbled. The
outstation responds to a separate transaction from the master station, but resends the
unsolicited response after its confirmation timeout expires.
Case Thirteen: Simultaneous Transactions
Master AC
[110 3]
=
[110 3]
=
[110 28]
Outstation AC
Description
[111 28]
=
[111 3]
=
Timeout
[111 28]
=
Master and outstation send messages simultaneously
Unsolicited response not received
Response to request from master
Confirm
Confirmation timeout
Unsolicited message resent
Confirm
[FIR FIN CON SEQ]
Case Fourteen shows the process after confirm mode. The outstation receives a master
station read request after it has commenced an unsolicited response. It waits until it has
received a confirmation for its unsolicited response before proceeding.
Case Fourteen: Process After Confirm Mode (1)
Master AC
Outstation AC
Description
[110 2]
=
[111 18]
=
[110 18]
=
=
[110 2]
[111 2]
=
Master and outstation send messages simultaneously
Request is for read data
Outstation waits for confirmation
Confirm for unsolicited response
RTU now processes master request
Response
Confirm
[FIR FIN CON SEQ]
Fundamentals of DNP3 181
Case Fifteen is similar to Case Fourteen except that the unsolicited response is not
received. Again the outstation completes the unsolicited response transaction prior to
responding to the master station’s read request.
Case Fifteen: Process After Confirm Mode (2)
Master AC
[110 3]
Outstation AC
[111 28]
=
=
[110 28]
Timeout
[111 28]
=
=
[110 3]
[111 3]
=
Description
Master and outstation send messages simultaneously
Request is for read data. Master does not receive
unsolicited response
Outstation waits for confirmation
Outstation confirmation timeout
Resends unsolicited response
Confirm for unsolicited response
RTU now processes master request
Response
Confirm
[FIR FIN CON SEQ]
9.7
Application layer message functions
9.7.1
General introduction
In the previous section entitled ‘Application layer message handling’, we have been
concerned purely with the flow control of application layer messages. We have seen how
the first byte of the application protocol control information (APCI) is used in managing
the sequence of application level messages between master and outstation.
In doing this, almost no reference has been made to the actual content of the messages.
In this and the following sections, we turn the focus on to the actual messages and the
information they carry.
In general terms, as we are dealing with SCADA systems, we expect that messages will
be capable of sometimes carrying commands, and other times carrying data, which may
be generated in response to commands. This is, after all, the high-level functionality
provided by a SCADA system – control and data acquisition. Not surprisingly then, the
messages contain both function codes and data objects. Function codes define exactly
what the meaning and purpose of a message is. They may, for example, tell an outstation
to perform a particular operation, like freeze some counters, or to send some data. The
data objects define the structure and interpretation of the data itself.
The reader might at this point wonder why the data should need any predefined formats,
that is to have any specific ‘data object’ definition. Why cannot the outstation simply
send data in any format that will be understood by the particular user application? The
answer to this lies in the purpose of the protocol in the first place. The protocol is not just
intended to provide a means for communication between devices from different
manufacturers, but it is intended to make that data meaningful across different platforms.
The data is not just unspecified data therefore, but it is specific data types like digital
inputs, analog values and so on, each of which has a defined type under DNP3. For all
this to work therefore, very specific definitions of the purposes and meaning of the
message content are necessary.
We will examine these now. We will begin with the function codes, and then look at the
data objects defined under DNP3.
182
Practical Industrial Data Communications
9.7.2
Application function codes
The function code is the second byte of the application protocol control information. It
follows the application control byte in both request headers from master stations, and
response headers from outstations. The function code indicates what function is required
to be performed.
Overview of function codes
The specific actions required for each function code is included in the following tables.
The information in these is reproduced directly from that presented in the DNP3 basic
four documentation, DNP3 application layer, Section 3.5 Function codes.
Many of the functions need identify specific data on which they operate. They do this
with data object references that are included in the data within the ASDU, which follows
the APCI header. In this section only the function codes themselves will be examined,
with the referencing of the data objects being presented in following sections. In the
function descriptions these data objects are referred to as ‘specified data objects’.
Request Function Codes
Code
0
1
2
3–6
7–2
13–8
19–22
23
24–128
General Type
Transfer function
Function
Confirm
Read
Write
Control function
Freeze function
Application control function
Configuration function
Time synchronization
Reserved
Response Function Codes
Code
0
129
130
General Type
Response function
Function
Confirm
Read
Write
Request function codes
Transfer functions are those concerned with transferring defined data objects. These are
the functions that acquire data from the outstation, or write control information to it.
Transfer Functions
Code
Function
0
Confirm
1
Read
2
Write
Action
Message fragment confirmation used by
both requests and responses
Store specified data objects in outstation
Response
No further response required
Respond with requested data
objects
Respond with status of operation
Fundamentals of DNP3 183
The control functions are used to operate or change control points in an outstation.
These may be control relay outputs, i.e. digital points, or analog setpoint values. These
may be operated directly, with or without acknowledgment, or with the select before
operate sequence.
Control Functions
Code
Function
3
Select
4
Operate
5
Direct operate
6
Direct operate, no
acknowledgment
Action
Response
Select or arm points, but do not produce
any control action, or change of set
point. The operate function code must
be subsequently received to activate
these outputs
Set or produce the output actions on the
points previously selected by the
Select function
Select and operate the specified outputs
or control points
Respond with the status
of the selected control
points
Select and operate the specified outputs
or control points
Respond with the status
of the control points
following action
Respond with the status
of the control points
following action
No response message to
be sent
The select before operate sequence is a security feature that is well known in the
electrical supply industry. It provides an additional level of security to ensure against
miss-operation arising from a corrupted message, or a human error such as selecting the
wrong device from a group of similar devices shown on an HMI screen.
Select before operate sequence requires the following to occur:
Selection of the control points is made.
The outstation responds with the identity and status of the selected points.
The outstation starts a select-operate timer.
The HMI displays the selected points differently, showing they have been
selected.
• The operate function is sent for the selected points.
• The outstation will reject the operate message if the point identities do not
match those of the previously selected points.
• The outstation will de-select the points if the select-operate timer expires before
the operate function is received.
•
•
•
•
Freeze functions are typically used for the following:
• Record system-wide state data at a common time (e.g. midnight)
• Record status or value of specific point at regular intervals (e.g. for trending a
flow rate).
The application control functions are coded in order of decreasing effect. The cold restart
is a complete restart of the outstation, as if it had been de-powered and then repowered up.
The warm restart may be used to reset the DNP3 application, but not necessarily do reset
other application programs. Typically the warm restart is used to initialize its configuration
and clear events stored in its buffers. Once either a cold or warm restart has been initiated,
the master station should not attempt to communicate with the outstation until the restart
time interval returned in the outstation’s response has elapsed.
184
Practical Industrial Data Communications
Freeze Functions
Code
Function
Action
Response
7
Immediate freeze
Copy specified objects to a
freeze buffer
Respond with status of
operation. Note that the data
themselves are not returned
8
Immediate freeze – no
acknowledgment
Copy specified objects to a
freeze buffer
No response to be sent
9
Freeze and clear
Copy specified objects to a
Respond with status of
freeze buffer, then clear the
operation. Note that the data
objects
themselves are not returned
10
Freeze and clear – no
acknowledgment
Copy specified objects to a
No response to be sent
freeze buffer, then clear the
objects
11
Freeze with time
Copy specified objects to a
freeze buffer at the
specified time and
thereafter at intervals (if)
specified
Respond with status of
operation. Note that the data
themselves are not returned
12
Freeze with time – no
acknowledgment
Copy specified objects to a
freeze buffer at the
specified time and
thereafter at intervals (if)
specified
No response to be sent
Application Control Functions
Code
Function
Action
Response
13
Cold restart
Restart application on
completion of communication
sequence
Respond with time until outstation
availability following restart
14
Warm restart
Partial restart on completion of
communication sequence
Respond with time until outstation
availability following restart
15
Initialize data to
defaults
Initialize specified data to initial
of default settings
Respond with status of operation
16
Initialize
application
Initialize specified applications
Respond with status of operation
17
Start application
Start specified applications
Respond with status of operation
18
Stop application
Stop specified applications
Respond with status of operation
The ‘initialize data to defaults’ could, for example, set up standard set points, and clear
counters. The specific data objects to be initialized are identified in the request, but not
the default values themselves. These are stored locally in the outstation, as fixed readonly data, or as parameters in non-volatile memory, for example.
Fundamentals of DNP3 185
The configuration referred to by these functions is the state of the parameters and settings
that collectively determine the behavior of the outstation. It is not referring to a complete
program or ‘configuration’ download. The save configuration function causes storing in a
specified non-volatile memory location the settings that define the system’s configuration.
Configuration Functions
Code
Function
Action
Response
19
Save configuration
Save the specified configuration to
non-volatile memory
Respond with time outstation
availability
20
Enable unsolicited
messages
Enable spontaneous reporting of
the specified data objects
Respond with status of
operation
21
Disable unsolicited
messages
Disable spontaneous reporting of
the specified data objects
Respond with status of
operation
22
Assign class
Assign specified data objects to a
particular class
Respond with status of
operation
The enable and disable of unsolicited messages allows for a live change to this
configuration aspect.
The assign class function allows for live assignment of data objects to classes. These
are discussed later in the text. The effect of classes is to provide a broad means of
referencing data, for example, by allowing a read of all data in a particular class.
Time Synchronization Function
Code
Function
Action
Response
23
Delay measurement
Allows measurement of the path
delay between the master and the
outstation. The value determined
is used as an offset when setting
the system time at the outstation
Response message with
time delay between
receipt and response, i.e.
outstation processing
delay time
24
Record current time
Causes outstation to record the time
of its clock on completion of
receipt of this command message
Respond with status
(internal indications bits)
The delay measurement function is used immediately prior to performing a time setting
by writing to the outstation clock. It is used over asynchronous serial links, for which the
message transmission time is significant in comparison to the 1 ms clock resolution. The
outstation measures the time in milliseconds that it takes to turnaround the message, that
is, the time from message receipt to sending the response to the master. The master
station subtracts this time from the turnaround time it sees, to determine the time the
messages spent in the communication system. It then divides that time by two to obtain
an approximation of the one-way message delay from master station to outstation.
The record current time function is used when performing a time setting over a LAN. In
this case the transmission occurs essentially instantaneously, so no turnaround time
measurement is needed. Both the master station and the outstation record the current
values of their own clocks at the time this message is sent. Subsequently, the master sends
186
Practical Industrial Data Communications
its recorded time to the outstation, which calculates the its time error from the difference
between the recorded values.
The file function codes provide for file operations with configuration and other files in
outstation devices. Some devices may implement security features to control file access.
Where required, these make use of an authentication step, where the master station
submits a user name and password with the authenticate function. The outstation responds
with a one-time use key, which is then used by the master in conjunction with file change
operations.
File Functions
Code
Function
Action
25
Open file
The required file is locked to prevent
use by other processes, and opened
26
Close file
The file is closed and released for use
by other processes
27
Delete file
28
Get file
information
The required file is deleted, provided
that it is not open. If it is open the
command is not executed
Provide file information: type, size,
time of creation, permissions
29
Authenticate
30
Abort
Requests an authentication key from
the outstation, if required, to allow
file change operations
Immediately abort a file transfer
operation in progress, and close
the associated file without saving
changes
Response
Respond with File Command
Status (Object Group 70,
Variation 4)
Respond with File Command
Status (Object Group 70,
Variation 4)
Respond with File Command
Status (Object Group 70,
Variation 4)
Respond with File Descriptor
Object (Object Group 70,
Variation 7)
Respond with Authentication
Object (Object Group 70,
Variation 2)
Respond with File Command
Status (Object Group 70,
Variation 4)
Response functions
The response function codes apply for all messages from outstations, i.e. stations that are
not designated as master stations. The only response these messages require (of the
master station) is an optional confirmation on receipt by the master station.
Response Functions
Code
0
Function
Confirm
129
Response
130
Unsolicited
message
Action
Message fragment confirmation Used
by both requests and responses
Respond to request
Unsolicited message from outstation
Response
No further response required
Master station will respond with
confirm if CON bit set
Master station will respond with
confirm if CON bit set
Fundamentals of DNP3 187
9.7.3
Internal indications
The internal indications (IIN) field (Figure 9.25) is a two-byte field that follows the
function code in all responses. It is by using the internal indications that the outstation
status is reported to the master following a request. Each bit in the field has a specific
meaning, in accordance with the table. An outstation would have defined flags within its
dynamic memory to correspond to the IIN field bits. These flags would then be copied in
each response message. Descriptions of the detailed meaning of each bit are included in
the following table.
Figure 9.25
Internal indications
First Byte
Bite
Meaning When Set
Notes
0
All stations message received
Set when an address of FFFF is received. Cleared after
next response. Used to confirm that a broadcast
message was received by this station
1
2
3
4
Class 1 data available
Class 2 data available
Class 3 data available
Time synchronization required
Indicates that date of specified class is available. Master
should request this data
5
6
Points in local
Device trouble
7
Device restart
After a restart this will be set. It is cleared during time
synchronization, or by a direct write to the outstation
II object
Set when one or more points are inaccessible to DNP
Used when an abnormal condition is present, and this
cannot be described by the other II bits
Set when the user application restarts. Will be cleared
when the master writes a 0 directly to this bit of the
outstation II object
188
Practical Industrial Data Communications
Second Byte
Bit
Meaning When Set
Notes
0
1
2
Function code not implemented
Requested objects unknown
Parameters invalid
3
4
5
Buffer overflow
Operation already in progress
Corrupt configuration
6
7
Reserved (0)
Reserved (0)
This function code is not available at this outstation
There are no objects as specified or in the specified class
Parameters in the qualifier, range, or data fields are not
valid or are out of range
Event or other application buffers have overflowed
This operation is already executing
This is a specific problem indication showing that the
master will have to download a new configuration
Always to return 0
Always to return 0
9.7.4
The object header
In DNP3, the data is always comprised of two parts, object headers and data objects. The
object headers identify the data object types, and specific instances of those data that are
being referenced by the message. These data may not necessarily be contained in the
message; for example, a read request carries only the references identifying which data is
required. The response to the read request will contain both the data identification and the
data itself. The position of the object headers and data objects within the application
protocol data unit, or message frame, is shown in Figure 9.26.
Figure 9.26
The object header
The ASDU is made up of one or more object header and data object fields, up to a
maximum frame size of 2048 bytes including the frame header (the APCI).
Fundamentals of DNP3 189
The object header is between three and eleven bytes in length, and is made up of the object,
qualifier and range fields. The object field is further sub-divided into two bytes, which are the
object group and object variation respectively. The range is between 0 and 8 bytes.
The object field
The object field (Figure 9.27) is made up of the object group and the object variation
bytes. The group specifies the general type of data, such as analog inputs. The variation
specifies a particular variation of that type of data. A list of the object groups is shown in
the table below. Full details of these and the object variations for each group is included
under Section 5.8 Data object library.
Figure 9.27
The object field
Table of Object Groups
Group Range
0–9
10–19
20–29
30–39
40–49
50–59
60–69
70–79
80–89
90–99
100+
Object Group Description
Binary input objects
Binary output objects
Counter objects
Analog input objects
Analog output objects
Time objects
Class objects
File objects
Device objects
Application objects
Alternate numeric objects
It can be seen from the table that the object groups are organized into decade ranges.
Within each decade, there may be more than one object group. Each of these groups has
its variations, making altogether a substantial number individual objects.
Object variation zero
Object variation zero (0) is not used within any group, because this is reserved for a
special purpose. When a master station specifies object variation zero in a request
message, this defined to mean all variations of that particular object group. This allows
the master station to obtain all object variations without identifying each one.
190
Practical Industrial Data Communications
9.7.5
The qualifier and range fields
In the following sub-sections the operation of the qualifier and range fields are presented.
After looking at the structure of these fields, and the meaning of the codes they contain, the
different means of referencing data are presented. In the discussion, use is made of
referencing ‘modes’. It is noted that the mode labels used here are not included within the
DNP3 documentation, but are applied here to assist understanding of this complicated area.
It is noted that the reader may find point referencing in DNP3 somewhat difficult to
grasp. This arises from the flexibility of referencing that has been provided for by the
protocol. At the same time, it may be observed that although a number of different
referencing modes and variations for each are defined in DNP3, in practice only a small
number are generally used.
Purpose and structure
The qualifier and range fields follow the object field in the object header. These fields are
used to identify the specific data points of each object group and variation that are being
referred to. For example, this may simply be a range of consecutive points, in which case
the start and stop point indexes will be included in the range field. Alternatively, a list of
non-consecutive points may be required, and in this case the list of points must be
provided separately. These cases, and others are all handled by the qualifier and range
field values.
The qualifier and range fields are used both in request messages from master stations,
and in response messages from outstations. For request messages, only identification of
the data may be required. In response messages the data objects themselves may be
included in the message.
The structure of the qualifier field is shown in Figure 9.28.
Figure 9.28
The qualifier field
The qualifier field is made up of two sub-fields. These are the qualifier code (or
Q-code) and the index size (or I-size). The primary meaning of the qualifier field is
provided by the qualifier code value. The index-size sub-field provides additional
information required for some qualifier code values. These sub-fields, together with the
following range field, act to fully identify the data object that follows each object header.
Fundamentals of DNP3 191
Qualifier codes
In the following table, a summary of the qualifier codes is presented. Understanding the
meaning of the table hinges on understanding the different modes of referencing that are
used. These are explained in the sub-sections following the table.
From the qualifier code table it may be seen that there are five different referencing
modes, or ways of referencing or identifying the data objects contained within messages.
Also, for some of the referencing modes there are three Q-codes applicable. These allow
for three different bit-sizes for the numbers in the range field.
Qualifier Code Table
Q
Code
Interpretation of Qualifier Code
Range Value
Size (Bits)
Referencing Mode
0
1
2
8
16
32
Range-index mode
The range field contains start and
stop indexes points are I1 to I2.
I-size = 0, or 4–6 for individual
object size prefixes
3
4
5
8
16
32
Range-absolute
mode
The Range field contains start and
stop absolute memory addresses
in outstation. Data are Bytes B1
to B2. I-size = 0, or 4–6 for
individual object size prefixes
(response messages only)
6
–
All object mode
Specifies all objects in the
referenced group/variation. Only
used for requests. There is no
range field with this mode. I-size
must = 0
7
8
9
8
16
32
Non-ranged mode
Specifies a list of unrelated points.
The Range field contains the
number of points referenced
I-size = 0: No indexes, points only
I-size = 1: Indexes are 8 bit
I-size = 2: Indexes are 16 bit
I-size = 3: Indexes are 32 bit
Object identifier
mode
The Range field contains the
number of Object Identifiers
following. Each Object Identifier
is preceded by a ‘Size’ field. The
size of the Size and Range fields
depends on I-size:
I-size = 1: Fields are 8 bit
I-size = 2: Fields are 16 bit
I-size = 3: Fields are 32 bit
11
As for I-size
Notes
192
Practical Industrial Data Communications
The index-size sub-field
For most of the referencing modes, additional information is provided by the I-size
sub-field. The specific meaning of the sub-field is dependent on the referencing mode. In
each case where the I-size sub-field is used (i.e. is non-zero), it gives the size of
additional fields that are given for each data object. Depending on the referencing mode,
these extra fields contain either an identifier, an index, or a number giving the object size
in bytes.
The I-size codes are shown in the table below. The use of these is demonstrated in the
following sub-sections for each referencing mode.
Table of Index-Size Codes
I-size
Code
Size in
Bits
0
No index
1
2
3
4
5
6
8
16
32
8
16
32
Meaning in Request
with Q-code = 11
Meaning in Request or
Response Containing Data
Objects
No indexing. Objects are packed
directly
Identifier field size
Objects are prefixed with an
index of this length
Objects are prefixed with an
object size field of this length
Range-index mode
The range-index mode is indicated by Q-codes 0, 1, and 2.
In this mode the range field contains two numbers. These are the start and stop index
values for the data objects.
Index values imply that in the outstation there is an index giving a one-to-one
correspondence between the value of the index, and the actual locations within the device
memory of the data objects. The use of indexes simplifies referencing of data objects
because regardless of the object’s size, each consecutive number represents successive
data objects. If the start and stop numbers are I1 and I2, then there are a total of I2 − I1+1
data objects being referenced.
In the diagrams following, Q-code 0 is used, defining the range-index mode with
eight-bit indexes. These can reference a maximum of 256 data objects. In the request
message only the header is present. In the response message, the data objects follow the
message.
In the response message only, it is possible to prefix each data object returned with
an object size identifier. When this is desired, the I-size field is used to specify the
size of the object size fields. This would only be required if the object sizes
were variable within an object variation group. When object size fields are not
required, the I-size field is set to zero, meaning the objects are packed in sequence
without size fields. The diagrams following show the structure of the messages for
each of these cases.
Fundamentals of DNP3 193
REQUEST MESSAGE OBJECT GROUP
GROUP
G
1
VARIATION
V
1
QUALIFIER
0 0
1
RANGE
START
I1
1
I-Size
Q-code = 0
STOP
I2
1
=0
=>
8 Bit start and stop values
RESPONSE MESSAGE OBJECT GROUP
GROUP
G
1
VARIATION
V
1
QUALIFIER
0 0
1
RANGE
START
I1
1
STOP
I2
1
DATA OBJECTS
DO
DO
DO
I1+1
I1+1
I1+2
x
x
x
…
Note: Size of data objects x = determined by object group/variation definition
Qualifier Codes for Range-Index Mode
Q Code
0
1
2
•
•
•
•
Interpretation of Qualifier Code
Range Value
Size (Bits)
8
16
32
Referencing
Notes
Mode
Range-index The Range field contains start and
mode
stop indexes. Points are I1 to I2.
I-size = 0, or 4–6 for individual
object size prefixes
Non-zero index-size is valid only with response messages (for this mode)
It is used when object size prefixes are required before each data object
The I-size codes 4, 5, and 6 are valid
They define 8-, 16-, and 32-bit object size fields respectively.
RESPONSE MESSAGE OBJECT GROUP
RANGE
DATA OBJECT
GROUP VARIATION QUALIFIER START STOP SIZE DO SIZE
DO
G
V
4 0
I1
I2
I1
I1
I1+1
I1+1
1
1
1
1
1
1
x
x
x
…
SIZE
I2
1
Note: Size of data objects is determined individually by each object size prefix
Qualifier Codes for Range-Index Mode
Q Code
Interpretation of Qualifier Code
0
1
2
Range Value Referencing Mode
Notes
Size (Bits)
Range-index mode
The Range field contains start
8
and stop Indexes. Points are I1
16
to I2. I-size = 0, or 4–6 for
32
individual object size prefixes
(response messages only)
194
Practical Industrial Data Communications
Range-absolute mode
The range-absolute mode is indicated by Q-codes 3, 4, and 5.
This mode is similar to the range-index mode in that the range field contains start and
stop values. However, these values represent absolute memory addresses rather than
index numbers. This mode is intended for use as a diagnostic tool during manufacturing.
This is because use of this mode requires detailed knowledge of the memory structure to
interpret the information returned.
In the example following, Q-code 4 is used, defining the absolute-index mode with
16-bit indexes. These can reference a maximum of 65 536 data objects. It is noted when
multi-byte indexes are used, they are given in order of lowest significant byte to highest
significant byte, i.e. LSB, MSB.
As for the range-index mode, it is also possible to use object size prefixes in the
response message. To do this the I-size code is set to 4, 5, or 6 to select 6-, 16-, or 32-bit
object size prefixes. These have not been illustrated, as the concept has already been
demonstrated for the range-index mode.
Fundamentals of IEC 60870-5
10.1
The IEC 60870-5 standard
10.1.1
Overall structure of the standard
The standard IEC 60870-5 was produced by the International Electrotechnical
Commission Technical Committee 57, Working Group 03, and published progressively
from 1988.
The structure of IEC 60870-5 was introduced in the preview Section 8.1. This showed
how the standard is structured in a hierarchical manner, and illustrated how the
companion standards relate to the other sections making up Part 5. Table 10.1 extends the
preview information by showing the full structure of IEC 60870, together with the years
of publication of the component parts and sections.
The sections IEC 60870-5-1 to IEC 60870-5-5 are the core specification documents for
Part 5, the Transmission Protocols part of the standard. The companion standard sections,
or simply companion standards, IEC 60870-5-101 to IEC 60870-5-104, are each separate
application protocols intended for specific purposes. They provide the definitions of
application level data objects and functions to completely define a working protocol.
They are also referred to as profiles, and may sometimes be given the shorthand
references T101, T102, T103, and T104, the T standing for telecontrol.
As shown in the preview, IEC 60870-5-101 provided the first complete working
SCADA protocol under IEC 60870-5. This defines all the necessary application level
functions and data objects to provide for telecontrol applications operating over
geographically wide areas, using low bandwidth bit-serial communications. It covers
general communications with RTUs, including data types and services that are suitable
for electrical and substation systems. The data types are generic and suitable for wider
SCADA applications.
The IEC 60870-5-102 and IEC 60870-5-103 companion standards provide data types
and functions to support electrical protection systems. These include distance protection,
line differential protection, and transformer differential protection.
As explained in the preview, the IEC 60870-5-104 companion standard has special
significance. This defines operation of the transmission protocol over networks using
standard transport profiles specifying the TCP and IP protocols. This companion
standard is not really independent of IEC 60870-5-101, but replaces the message
transport sections with a network version, leaving the application level functions
largely unaltered.
196
Practical Industrial Data Communications
Main Parts
Reference
IEC 60870-1
IEC 60870-2
IEC 60870-3
IEC 60870-4
IEC 60870-5
IEC 60870-6
Description
General considerations
Operating conditions
Interfaces (electrical characteristics)
Performance requirements
Transmission protocols
Telecontrol protocols compatible with
ISO and ITU-T recommendations
Year
1988
1995
1989
1990
1990
1995
Sections of IEC 60870-5
Reference
IEC 60870-5-1
IEC 60870-5-2
IEC 60870-5-3
IEC 60870-5-4
IEC 60870-5-5
Description
Transmission frame formats
Link transmission procedures
General structure of application data
Definition and coding of application
information elements
Basic application functions
Year
1990
1992
1992
1993
1995
Companion Standards of IEC 60870-5
Reference
IEC 60870-5-101
IEC 60870-5-102
IEC 60870-5-103
IEC 60870-5-104
Description
Companion standard for basic telecontrol
tasks
Companion standard for transmission of
integrated totals
Companion standard for protection
communication
Network access using standard transport
profiles
Year
1995
1996
1997
2000
Table 10.1
Full structure of IEC 60870 standard
10.1.2
Development of standards
IEC standards are subject to review and the issue of amendments from time to time, and
since the publication of IEC 60870-5-101, this profile has had two amendments issued.
The first amendment added a small number of information object definitions. The second
amendment added a significant amount of clarifying detail, the purpose of which was to
remove ambiguities and so better provide for interoperability. It is anticipated that during
2002 the standard will be reissued as one document including the two amendments.
The complete version of this profile with amendments is as shown in Table 10.2.
10.1.3
Obtaining standards
Should access to the standards be required, they may be purchased on-line from the IEC (at
www.iec.ch). As an alternative that may be less expensive, it is worth checking if they have
been published as a national standard, as is often the case. In Australia, IEC 60870-5
sections 1 to 5 are available as AS 60870.5.1 to AS 60870.5.5 and are available online
Fundamentals of IEC 60870-5
197
(at www.standards.com.au). However, the companion standards are not presently available
and must be obtained from the IEC.
Reference
IEC 60870-5-101
IEC 60870-5-101-am1
IEC 60870-5-101-am2
Description
Companion standard for basic telecontrol tasks
Companion standard for basic telecontrol tasks
amendment 1
Companion standard for basic telecontrol tasks
amendment 2
Year-Month
1995–11
2000–04
2001–10
Table 10.2
IEC 60870-5-101 including amendments
An alternative to purchase of the standards is to view them at a public or university
library.
10.1.4
Description of contents
In this section the contents of each section of IEC 60870-5 are briefly described. This is
intended to provide a guide to assist the reader should it be necessary to refer to them.
IEC 60870-5-1 1990: transmission frame formats
This describes the operation of the physical and data-link layers in terms of the services
provided to higher layers. It provides a choice of four data-link frame types identified as
FT1.1, FT1.2, FT2, and FT3, each with a different level of security against data errors.
Fixed- and variable-length versions of the frames are described, and a set of transmission
rules is provided for each. Two single-control character transmissions are provided as an
efficient means of transmitting control information such as acknowledgments.
IEC 60870-5-1 1990: link transmission procedures
This section represents the four frame formats from IEC 60870-5-1 and then describes the
internal processes in terms of the service primitives and transmission procedures. The
service primitives are the control indications passed between the link layer and its higher
level user, and the transmission procedures describe the sequence of events occurring
over the physical communications link. A control field is described that is transmitted
over the link and is used by the link layer procedures of each side of the link in
controlling the transmission process. The terms unbalanced and balanced transmissions
are presented and used to describe whether transmission can be initiated only by a master
station or by any station. Services and transmission procedures are presented in detail for
both unbalanced and balanced transmissions.
IEC 60870-5-3 1992: general structure of application data
This section presents two models of the structure of data at the application level. The
reference model 2 version shows how application user data, for example, point
information to be transmitted, is encapsulated within an application protocol data unit,
with or without application protocol control information added, and then passed to the
underlying link layer for transmission. It also describes a general structure for application
data, and rules for forming application data units in general.
198
Practical Industrial Data Communications
IEC 60870-5-4 1993: definition and coding of application
information elements
This section provides rules for the definition of information elements, and defines a
common set of information elements that may be used for transmission of information in
telecontrol applications. These include generic elements such as signed and unsigned
integers, fixed and floating-point numbers, bit-strings, and time elements. The intention
of this section is to provide a set of information building blocks from which a companion
standard, or profile can utilize selectively to build a complete set of application level
information objects. No such set of information objects is selected by this section,
however.
IEC 60870-5-5 1995: basic application functions
This section describes the highest level functions of the transmission protocol, which
are those application functions above layer 7 of the OSI model. Application service
primitives are the request and response indications passed between the application layer
and the application user. Application functions are described.
A set of basic application functions and associated service primitives, or requests and
indications, are presented. These cover the highest level functions that would be required
to carry out telecontrol operations. They include station initialization, methods of
acquiring data, clock synchronization, transmission of commands, totaliser counts, and
file transfer. Again, it is stated in this section that it would be the role of a specific
companion standard to select from these functions and possibly add to them in defining a
complete working protocol.
IEC 60870-5-101 1995: companion standard for basic telecontrol tasks
(including amendments 1 and 2)
The companion standard defines a complete telecontrol protocol by detailing selections
from options described in sections 1 to 5 of IEC 60870 Part 5, and by defining a complete
set of application service data units (ASDUs). Its main subsections are listed below, and a
short description of the contents of these follows this list.
•
•
•
•
•
General rules
Physical layer
Link layer
Application layer and user process
Interoperability.
General rules
This is really an introduction that states the main selections made for the physical layer,
link layer, application layer, and user process. They in effect provide a brief overview of
each of the following sections.
Physical layer
This describes the requirements for the interface with external data communication
equipment using standard ITU-T V.24/V.28 (RS-232) or balanced X.24/X.27 (RS-485)
signals. The types of fixed network configurations such as point-to-point, and multi-point,
are described.
Fundamentals of IEC 60870-5
199
Link layer
Selections of the frame format F1.2, and the fixed and variable frame lengths are made. The
detailed operation of the link layer is specified, making selections from IEC 60870-5-2.
Operation of the link is described for both unbalanced and balanced operations, which
corresponds to whether only the master station, or any station, may initiate message
transmissions. Amendment 2 contains significant additional information clarifying the
procedures with the use of state transition diagrams.
Application layer and user process
This defines the overall structure of the application level data, the ASDU, defines the set
of ASDUs available, and makes selections of application functions from IEC 60870-5-5.
The definition of ASDUs is carried out in two subsections. The first subsection defines
the available information elements. These are used in the second subsection as building
blocks in constructing the full set of ASDUs. The user process subsection makes
selections from IEC 60870-5-5 and includes additional detail for some of the functions.
Interoperability
This provides a check-box method for specifying the particular features supported by a
specific product, under the following type headers:
•
•
•
•
•
System or device
Network configuration
Physical layer
Link layer
Application layer.
10.2
Protocol architecture
10.2.1
EPA and OSI reference models
As for DNP3, IEC 60870-5 is based on the three-layer enhanced performance architecture
or EPA model for data communications. These models are described in detail in Section 2
of this text, which shows how the EPA model is a simplified form of the OSI seven-layer
reference model to provide optimum performance for telecontrol applications.
In Figure 10.1, the relationship between the OSI model and the EPA model is
represented. This shows that the EPA model basically omits the presentation, session, and
transport layers of the OSI model.
The structure of the EPA model is appropriate for a continuously operating system that
operates over a single network. One layer normally added to the top of the basic EPA
model representation is identified as the user layer. This is included to represent the various
functions or processes that must be defined to provide telecontrol system operations. These
must be defined to provide for the interoperability between equipment that will result in a
fully operable telecontrol system, rather than merely a data communication system.
For the first defined companion standard IEC 60870-5-101 or T101 profile, a four-layer
model, as illustrated at the right-hand side of Figure 10.1, provides an accurate
representation of the architecture of the protocol. In the case of the networked version
IEC 60870-5-104, or the T104 profile, additional layers of the OSI model must be included
to provide for transport of messages over networks using standard network protocols. These
are the transport and network layers corresponding to the use of the TCP and IP protocols.
The two architectures are shown in Figure 10.2.
200
Practical Industrial Data Communications
Figure 10.1
Relationship of EPA model to OSI seven-layer model
Figure 10.2
Architectures for T101 and T104
As is clear from Figure 10.2, the operation of the lower layers of the networked version,
IEC 60870-5-104 is completely different from that of the non-networked version, IEC
60870-5-101. These layers correspond to all the layers below the application layer, which
for these architectures are the layers concerned with message transport.
10.2.2
Selections from standards
The benefit of the OSI reference model, and the EPA models derived from it and shown
in Figure 10.2, is that they provide a framework for description of protocol operation.
Describing the operation of the protocol is a matter of specifying the functions of each
layer, and specifying the structure of information passing between the layers.
Under the IEC standard, it is the companion standard IEC 60870-5-101 that completely
specifies the protocol. It does this by referring to the main sections of the IEC 60870-5
standard, and by making particular selections from options that may be available within
those sections.
Table 10.3 shows how the different sections of the IEC 60870-5 set of standards
correspond with the layers of the model.
Fundamentals of IEC 60870-5
Layer
User process
Application
Link
Physical
Source
IEC 60870-5-5
IEC 60870-5-4
IEC 60870-5-3
IEC 60870-5-2
IEC 60870-5-1
ITU-T
201
Selections
Application functions
Application information elements
ASDUs
Transmission procedures
Frame formats
Interface specification
Table 10.3
Standards selections for IEC 60870-5-101
For comparison the corresponding information for the networked version IEC 60870-5-104
is shown in Table 10.4. This illustrates how the lower layers of the IEC 60870-5-101
companion standard have been completely replaced by the standard TCP/IP transport profiles.
Layer
User process
Application
Transport
Network
Link
Physical
Source
IEC 60870-5-101
IEC 60870-5-101
Selections
Application functions
ASDUs and application
information elements
TCP/IP transport and network protocol suite
Table 10.4
Standards selection for IEC 60870-5-104
10.3
Physical layer
The physical layer is concerned with the transmission and reception of data over the
physical medium. This level is concerned with the transmission of bits and bytes, but not
with the meaning of those bytes. The physical interface is defined in terms of the
electrical characteristics, and individual signals passing over the interface.
The definition of the physical layer includes specification of the signal interface
between the IEC 60870-5 and the communications devices to external world, and the
network configurations that are attached to these. These are illustrated in Figure 10.3.
This shows a SCADA master station server connected to a radio modem via a serial port
operating at 9.6 kB/s. The radio modems form a multipoint-star configuration in which
the master communicates with both outstations simultaneously, and either outstation can
communicate back to the master.
10.3.1
Communications interface
To allow the use of standard data communications equipment, the standard utilizes
existing widely used standards covering the exchange of between data terminal
equipment (DTE) and data communication equipment (DCE). These communication
interface standards are the ITU-T equivalents to the well-known Electrical Industries
Association RS-232 and RS-485 standards. These provide for unbalanced and balanced
full-duplex serial data transmission between the data device and communications
equipment such as a modem.
202
Practical Industrial Data Communications
Figure 10.3
Interfacing to communications network
The use of this interface is illustrated in Figure 10.3, at the connection between the
server of a SCADA master station, and a radio modem. The DTE–DCE interface is
simply an RS-232 cable between a serial port of the computer, and a similar port on the
radio modem. It is also used between the RTUs and their radio modems.
The data transmission speeds are defined as follows (Table 10.5):
Interface Type
V.24/V.28 FSK interface
V.24/V.28 Modern interface
X.24/X.27 Synchronous
Transmission Speed (bits/s)
100, 200, 300, 600, 1200
300, 600, 1200, 2400, 4800, 9600
2400, 4800, 9600, 19200, 38400
56000,64000
Table 10.5
DTE–DCE interface transmission speeds
In addition to interfaces using the specified standards, it should be noted that the T101
profile does allow the use of other physical interfaces by agreement between vendor and
user.
10.3.2
Network configurations
The T101 profile specifies support for the following network configurations or topology:
•
•
•
•
•
Point-to-point
Multiple point-to-point
Multipoint-star
Multipoint-party line
Multipoint-ring.
These are defined by IEC 60870-1-1, and are depicted in Figure 10.4. In the figure the
square symbols represent controlling or master stations, and the triangles represent
controlled, or outstations. The small circles at the points of connection are the ports.
From the diagram it may be seen that these fall into two basic types: point-to-point and
multi-point. A point-to-point link has one master station and one outstation. A multi-point
network has a master station connected to a number of outstations. The ring configuration
is only different in that it incorporates redundancy by providing a second port on the
master station that can be used for communications should the ring be broken.
Fundamentals of IEC 60870-5
203
Figure 10.4
Network configurations
In the point-to-point configurations either the master or the outstation(s) can transmit
messages, provided that a full-duplex channel is used. In the multi-point configurations
the master communicates in parallel to all connected outstations. The outstations share a
return communications channel, and therefore only one may transmit at a time.
Note that combinations of links may form a hierarchical network where intermediate
RTUs may act as local master stations to RTUs connected to them. These are sometimes
referred to as sub-master stations.
10.4
Data-link layer
The data-link layer is responsible for the passing of data across the communications
channel, and ensuring that the data is received in full and uncorrupted by errors. It does this
using a unit of data known as a frame, combined with procedures to govern its transmission
and reception. The frame is made up of an amount of data that is large enough to carry
control information such as a destination address, checking information used to detect
errors, and a payload of data, if required. It is also an amount of data that is not too large, so
that a transmission error will not cause the loss of too much data, or so that timing
discrepancies between transmitter and receiver can lead to loss of synchronism.
IEC 60870-5-101, or T101, specifies the operation of the data-link layer by referring to
and making selections from the standards identified in Table 10.1, repeated in the
following extract:
Link Layer
IEC 60870-5-2
Transmission procedures
IEC 60870-5-1
Frame formats
In this section the operation of the data-link layer is explained in detail, commencing
with the data frame structure and then looking at the transmission procedures.
10.4.1
Frame format
The frame format used by T101 is referred to as the FT1.2 format. There are two forms of
this, one of fixed length and the other of variable length. The fixed-length frame is restricted
to use for frames carrying no user data, and therefore is used only for data-link control
command and acknowledgment frames. In addition to the fixed- and variable-length
204
Practical Industrial Data Communications
frames, there is a ‘single control character’ frame, which consists of a single byte. This may
be used for acknowledgment only.
These frames are shown in Figures 10.5 and 10.6. Figure 10.5 shows the actual bit
pattern that would be seen on the physical channel, interpreted from left to right. This
representation includes the start and stop bits that are transmitted with every byte or octet
of the frame. The overall frame construction is shown in Figure 10.6. This does not
represent the bit pattern, but shows only the information content down to the octet level.
For consistency with the applicable standards, these are presented vertically in octet
order. Thus, the first octet is shown at the top, and following octets are shown below.
Clarification of the order of bits and octets is included in the following section.
Figure 10.5
Bit-sequence representation of FT1.2 frame
Figure 10.6
FT1.2 frame options under IEC 60870-5-101
From the bit-sequence representation in Figure 10.5, it is possible to see that the
maximum data rate of the frame is approximately eight-elevenths of the bit transmission
rate. This is reduced further when the frame overheads such as addressing, start and stop
characters, checksum, and control information are accounted for. These overheads can be
seen in Figure 10.6, which shows the overall frame structure.
The following points are noted about the data-link frames:
• Only the variable length frame can carry user data.
• The variable length frame can carry up to 253 octets of link user data.
• The length L is repeated twice, and the two values of L must be equal for the
frame to be accepted as valid.
Fundamentals of IEC 60870-5
205
• The maximum frame length is 261 octets. However, a lower maximum frame
length may be specified by a manufacturer or by the system user as a system
parameter.
• The fixed length frame is 5 or 6 octets long.
• Address field A may be 1 or 2 octets, determined by a fixed system parameter.
• A broadcast address is defined as 0xFF or 0xFFFF for 1 and 2 octet addresses,
respectively.
• The checksum is the modulo-256 sum of the frame user data (not the link user
data). This is the data between the last start character and the checksum, L
octets for the variable frame.
• Rules state that no more than one bit-time idle interval is allowed between
characters within the frame, and that an idle interval of 33 bit-times must be
allowed after detection of a frame error by the receiver.
10.4.2
Order of information
One technical detail that can be difficult to find in the standards is the ordering of bits and
bytes. Under IEC 60870-5, as for DNP3, the following ordering is standard.
Bits are transmitted starting with the least significant byte (LSB) and ending with the
most significant byte (MSB). When a bit-sequence representation as in Figure 10.5 is given,
the bits are shown in this order. However, when the structure of a message in terms of bytes
or octets is depicted, the msb is at the left, and the lsb is at the right, which is consistent
with the numerical weighting of the bits. A good way to mentally resolve this is to envisage
the octet being right-shifted out of a UART register onto the communication channel.
Similarly, for bytes, the least significant byte (LSB) is transmitted first, and the most
significant byte (MSB) is transmitted last. The ordering of bits and bytes is illustrated in
Figure 10.7.
Figure 10.7
Order of information
206
Practical Industrial Data Communications
10.4.3
Link layer concepts
This section presents some concepts that are important for understanding the operation of
the link layer transmission procedures. The detailed operation of link layer is then
discussed in terms of these concepts.
These are:
•
•
•
•
•
Primary and secondary
Unbalanced and balanced
Service procedures
Service primitives
Transmission procedures.
Primary and secondary
The terms primary and secondary refer to the ability of a station to initiate communications on
a communication channel. Only a primary station can initiate communications. Secondary
stations must wait until they are polled by the primary station before they can transmit data.
More accurately these terms are applied to individual communications ports of stations,
because in a hierarchical system, an intermediate station will be both a controlled and a
controlling station. This is illustrated in Figure 10.8. This shows a hierarchical network
configuration with primary and secondary ports marked P and S.
Figure 10.8
Primary and secondary stations
In Figure 10.8 RTU 3 is both a controlling and controlled station. Note that each of the
two communications links has only one primary station.
Unbalanced and balanced transmission
The terms ‘unbalanced transmission’ and ‘balanced transmission’ are related to the terms
‘primary’ and ‘secondary’. Unbalanced transmission refers to the configuration where the
controlling station acts as a primary on the link, and one or more controlled stations act
Fundamentals of IEC 60870-5
207
as secondary stations. The stations are not peer-to-peer at the link level, and so are
unbalanced in their functionality.
This is the situation in Figure 10.8 for each of the two communications links. In this
configuration the controlling station must acquire data from the controlled stations by
polling each in turn for data. This is because they cannot initiate transmissions on their
own. The advantage of unbalanced communication is that there is no possibility of
collisions between controlled stations attempting to transmit information at the same
time.
Balanced transmission refers to the configuration where any station on a link may act
as a primary, which means it can initiate communications. This configuration is also
known as peer-to-peer communications.
Under IEC 60870-5-101, only point-to-point (that is two station) links can be balanced.
Multi-point links must be unbalanced. This is in contrast to DNP3, which uses balanced
transmission only, and therefore has to have procedures to overcome collisions which can
occur when more than one outstation commences communications simultaneously.
A balanced communications link is shown in Figure 10.9. In this case a master station is
connected via a point-to-point link to a sub-master station. Note that each station can act
as a primary and a secondary at the ports for this link. These may in fact be thought of as
two separate processes within each station, which in fact is how they are logically within
the stations. Station A has a primary process and a secondary process operating
simultaneously for that link, and Station B has the same.
Figure 10.9
Balanced communications
Figure 10.10 shows the primary and secondary processes for unbalanced transmission.
The processes are represented by circles identified as primary (P) or secondary (S). The
primary station implements the primary process only, and the secondary stations each
implement a secondary process only. Note that in effect there is a separate logical link for
each secondary station and it is necessary for the primary to keep a record of the state of
each link.
Figure 10.11 shows the processes for balanced transmission. In this case there is a
primary and secondary process for each station.
Service procedures
The term ‘services’ is descriptive of the function of the link layer, it provides specific
services to the user, which is the application layer, to carry out the transmission of data.
There are three main types of services provided by the data-link layer, which have the
following names:
1. Send/No Reply
2. Send/Confirm
3. Request/Respond.
208
Practical Industrial Data Communications
Figure 10.10
Balanced transmission processes
Figure 10.11
Balanced transmission processes
The Send/No Reply service is used to send a message or command for which no reply
is required from the addressed station. It is used for sending broadcast messages and for
messages for which receipt confirmation is not important. The Send/Confirm service is
used to send a command or data which must be reliably transmitted. For this service a
confirmation response is required. The Request/Respond service is used to obtain data
from the controlled station. In this case the controlled station responds not with a
confirmation, but with the required data.
These services are illustrated in Figure 10.12. This shows the link layer of two stations
on a communication channel, and a time-sequence diagram of the service interactions.
At each side are shown the commands, data, and responses that are passed between
the link layer and the service user, which is the application layer. These are termed the
‘service primitives’. In the center the message transmission is labeled with the
transmission procedure name.
The transmission procedures are a set of rules that ensure that transmissions are
successfully carried out in response to link user requests. They must be able to cope with
errors on the transmission channel that may introduce errors, or cause information to be
lost. The transmission procedures are different for unbalanced and balanced links, and
described for each in the following sections.
Fundamentals of IEC 60870-5
209
Figure 10.12
Link layer services
Link initialization
Link initialization is a data-link service carried out after a station has been off-line and
first becomes available again. While the slave station is off-line the master periodically
sends link status request functions until a status of link response is obtained. The
sequences are shown following for both unbalanced and balanced modes.
Station/link initialization, unbalanced mode:
•
•
•
•
Master sends link status request until status of link received
Master sends link reset
Link is active on receipt of ACK
Slave generates station initialization complete event.
Station/link initialization, balanced mode:
•
•
•
•
10.4.4
Each station sends link status request until status of link is received
Each station sends link reset
Link is active on receipt of ACK at each station
One or both may generate station initialization complete event.
Unbalanced transmission procedures
Unbalanced transmission procedures are required for links other than point-to-point, that
is multi-point links. For these links the controlling station must control the data traffic by
polling the outstations for data. Only when the controlling or primary station on a link
polls a particular secondary station may that station respond.
210
Practical Industrial Data Communications
The control field
The control field of the data frame is central to the operation of the transmission procedures.
This field is almost identical to that used by the DNP3 protocol because it was derived from
the same source document, IEC 60870-5-2 1992. The interpretation of the control field is
dependent on whether the communication is a primary or secondary message. Figure 10.13
shows the control field for unbalanced transmission procedures, including the short
descriptions of the meanings of the function codes for primary and secondary messages.
Figure 10.13
Control field – unbalanced transmission
Code
PRM
PCB
FCV
Meaning
Primary message
Frame count bit
Frame count valid
RES
DFC
Reserved
Data flow control bit
ACD
Access demand bit
Description
1 => Frame from primary or initiating station
Alternates between 0 and 1 for sequential frames
1 => FCB is valid
0 => ignore FCB
=0
Set to 1 by secondary station if further send of
user data will cause buffer overflow
Set to 1 if there is Class 1 data available
Table 10.6
Control field bit meanings – unbalanced transmission
Table 10.7 shows the detailed meanings for the function codes in the control field
(Table 10.6). The function codes are also shown in short form in Figure 10.11. The
meanings are different depending on whether the message is a primary or a secondary
transmission. The frame count bit is used in the primary direction only, and is only valid
for certain functions. This is indicated by the state of the frame count valid bit.
Function Codes from Primary Station
Function
Codes
0
1
Frame Type
SEND–CONFIRM expected
SEND–CONFIRM expected
Service Function
Reset of remote link
Reset of user process
FCY
Bit
0
0
Fundamentals of IEC 60870-5
Function
Frame Type
Codes
2
SEND–CONFIRM expected
3
SEND–CONFIRM expected
SEND–NO REPLY expected
4
8
REQUEST–RESPOND expected
9
REQUEST–RESPOND expected
10
REQUEST–RESPOND expected
11
REQUEST–RESPOND expected
Codes 5–7, 12–15 are reserved.
Service Function
Reserved
User data
User data (unconfirmed)
Request for access demand
Request status of link
Request user data class 1
Request user data class 2
211
FCY
Bit
–
1
0
0
0
1
1
Function Codes from Secondary Station
Function
Frame Type
Service Function
Code*
0
CONFIRM
ACK – positive acknowledgment**
CONFIRM
NACK – Message not accepted, link busy
1
8
RESPOND
User data
9
RESPOND
NACK – Requested data not available
11
RESPOND
Status of link (DFC = 0 or 1) or access demand
14
Link service not functioning
15
Link service not used or implemented
*Codes 2–7, 10, 12–13 are reserved.
**Note that control code 0 x E5 may be used in place of the FC0 or FC9 frames.
Table 10.7
Function codes – unbalanced transmission
The functions of the control bits are explained in more detail in the following
paragraphs.
Primary bit
The PRM bit is set if the frame is primary (initiating) or a secondary (responding). This is
used directly by the link layer in interpreting the function code.
Frame count bits
These are the frame count bit (FCB) and the frame count valid bit (FCV). These bits are
only used for primary messages. The frame count bit is used to detect losses or
duplication of frames to a secondary station. The frame count valid bit enables the use of
the FCB. When the FCV bit is true, the FCB is toggled for each successful SEND–
CONFIRM transaction between the same primary and secondary stations.
How they work is like this:
• Following data-link start-up or a failed transaction, a secondary station will
not accept any primary SEND–CONFIRM frames with FCV = 1 until a reset
transaction is completed. This means it will only accept either a RESET link or
a RESET user process command.
• After a secondary station receives a RESET link frame from a primary and
it responds with a CONFIRM, that link will be operational until a frame
transmission error occurs.
212
Practical Industrial Data Communications
• The secondary station will expect the next frame to contain FCV = 1 and
FCB = 1.
• The next primary SEND–CONFIRM message will have FCV = 1 and FCB = 1.
The secondary station will accept this as the FCB is valid and is set, as expected.
• Each subsequent primary SEND–CONFIRM message will have the FCB
cleared or set in turn.
Data flow control bit
The data flow control bit (DFC) is included in secondary frames. The secondary station
will set DFC = 1 if a further SEND of user data will cause its buffer to overflow. On
receipt of a frame with DFC = 1 a primary station will cease to send data, but will request
link status until DFC = 0.
Access demand bit
There are two classes of data defined, class 1 and class 2. Class 1 data has higher priority
than class 2 data. The ACD bit is a means for the secondary station to indicate to the
primary that there is class 1 data available.
Address field
The address field of the link layer frame is one or two octets in length, set as a fixed
system parameter. This field contains the link address of the secondary station. A frame
transmitted by the primary station on a link contains the link address of the secondary
station to which the message is directed. A frame transmitted by a secondary station to
the primary contains its own link address. By this means the primary station can identify
which secondary station the message is from.
Transmission procedures
In this section the procedures are discussed briefly. The procedures are very similar to
those used by the DNP3 protocol. This may be referred to for further illustration of the
concepts presented.
Note that in the following procedures, no new procedure is commenced until the previous
procedure is terminated. It is for this reason that a single-bit frame count bit is sufficient for
protection against frame sequence number errors; there is a frame window of exactly one.
SEND/NO REPLY procedures
The frame is transmitted, and a minimum line-idle time of 33 bit-times is required before
any further transmissions by the primary. On receipt of the message at the secondary, it is
checked for error by comparison with the checksum octet, and if valid it is notified to the
service user.
SEND/CONFIRM procedures
The primary station will transmit the message. If a confirmation is not received from the
secondary station within a configured time-out period, it will re-transmit the message up
until a configured number of retries.
If the secondary station receives the message, it will respond with either a positive or
negative confirmation, function code 0 or 1. FC = 0 means the message is correctly
received and accepted, and in this case the procedure terminates. FC = 1 means the
secondary cannot accept the message, because its buffer is full or some other reason.
In this case the primary will retry, up until a configured number of retries.
Fundamentals of IEC 60870-5
213
This procedure makes use of the frame count bit (FCB) to ensure that the message
sequence is not disturbed. The FCB is toggled with each SEND/CONFIRM transmission
from the primary station, and an expected FCB flag is maintained by the secondary
station. If a message sent by the primary is not confirmed by the secondary, it is
retransmitted with the FCB unchanged. Thus a message lost or corrupted in the primary
to secondary transmission direction would when retransmitted still have the expected
FCB value. Alternatively, if the problem were that the confirmation from the secondary
station was lost or corrupted, the secondary station would be able to recognize receipt of a
retransmitted message from the primary by the unchanged FCB bit. In this case it
retransmits the original confirmation message.
As an alternative to sending a confirmation frame, which is a minimum of six octets in
length (that is if single-octet address is used), the single-control character response
(hexadecimal 0 × E5) is allowed. This option may be used to improve transmission efficiency
when there is no need to transmit any other information back to the primary station.
REQUEST/RESPOND procedures
This procedure is similar to the SEND/CONFIRM except instead of receiving a
confirmation back from the secondary station, a frame containing data is returned, or a
negative response is returned indicating that no data is available. In the case of a negative
response, either a frame with FC = 9, or the single-control character (0´ E5) may be
returned.
As for the SEND/CONFIRM procedures, the frame count bit is toggled at each end for
each message transmission, and this is used to detect errors in the transmission in either
direction. Basically, when both primary and secondary have incremented their frame
count bits and they agree, the procedure is complete.
Philosophy of transmission
For unbalanced transmission only the controlling or primary station on a link can initiate
transmissions. Because of this it is necessary that a polling system is implemented by the
primary station in order to determine if there is change data available at each secondary
station.
To accomplish this the controlling station will poll each secondary station on a cyclic
basis for data. It will typically poll for class 2 user data using the Request–Respond
function code 10. The secondary station will then return any class 2 data that it has
available, and at the same time it will indicate if there is any class 1 data available by
setting the access demand bit (ACD). Typically analog values will be assigned to class 2
and be updated during the cyclic scan, and all other data such as events will be assigned
to class 1.
Note that although the polling operation is carried out by the link layer, the polling
sequence itself is implemented by a higher level of the protocol, the user process level.
The higher level generates services requests using service primitives to specify the
polling actions to be carried out by the link layer.
10.4.5
Balanced transmission procedures
Balanced transmission procedures may only be used for point-to-point links equipped
with duplex channel communication. Under balanced transmission each station can act as
both a primary and a secondary station simultaneously. The control field and transmission
procedures are modified slightly from the unbalanced case to accommodate this
operation.
214
Practical Industrial Data Communications
The control field
There are two modifications to the data-link control field for balanced transmission. The
first is that the access demand (ACD) bit is not required, because the either station can
initiate transmissions. The second is the inclusion of a direction bit (DIR). This indicates
the direction of transmission of a message between the two stations. The control field for
balanced transmission is shown in Figure 10.14 (see also Table 10.8).
Figure 10.14
Control field – balanced transmission
Code
DIR
Meaning
Direction of message
PRM
FCB
FCV
Primary message
Frame count bit
Frame count valid
RES
DFC
Reserved
Data flow control bit
Description
1 => A to B
0 => B to A
1 => Frame from primary or initiating station
Alternates between 0 and 1 for sequential frames
1 => FCB is valid
0 => Ignore FCB
=0
Set to 1 by secondary station if further send of user
data will cause buffer overflow
Table 10.8
Control field bit meanings – balanced transmission
Table 10.9 shows the detailed meanings for the function codes in the control field. The
changes from the codes for unbalanced transmission are shown in italics.
Function Codes from Primary Station
Function Code
0
1
2
3
4
Frame Type
SEND–CONFIRM expected
SEND–CONFIRM expected
SEND–CONFIRM expected
SEND–CONFIRM expected
SEND–NO REPLY expected
Service Function
Reset of remote link
Reset of user process
Test for function of link
User data
User data (unconfirmed)
FCV Bit
0
0
1
1
0
Fundamentals of IEC 60870-5
Function Code
Frame Type
8
9
REQUEST–RESPOND expected
10
11
Codes 5–7, 12–15 are reserved
Service Function
User by unbalanced only
Request status of link
Used by unbalanced only
Used by unbalanced only
215
FCV Bit
–
0
1
1
Table 10.9
Function codes – balanced transmission
The changes in functions from unbalanced transmission are:
• A test link primary function code FC = 2 has been added.
• Requests for access demand and for user data class 1 or 2 have been removed
(primary function codes FC = 8, 10, 11).
• Secondary respond function codes FC = 8, 9 have been removed.
These changes reflect the changes to the way data is transmitted. Under balanced
transmission the request/respond service is not used for transmission of user data, but
only for checking the status of the link. User data is transmitted directly by the link layer
using the send service.
Address field
The address field for balanced communications may be zero, one, or two octets. Thus
there is the option of having no address field under balanced communications. This is the
case because as there is only one station at each end of the link, there is no need for the
secondary process at either end to verify the address.
In effect, for a balanced link, the direction (DIR) bit substitutes for the address, making
the inclusion of the link address redundant.
Transmission procedures
The transmission procedures for balanced transmission are the same as those for
unbalanced transmission. The only difference is that there are both primary and secondary
processes operating simultaneously at each station. This is similar to two unbalanced
links operating in parallel. These processes maintain a separate frame count bit sequence
for each primary–secondary link and use these to detect and recover from errors in the
same way as a single unbalanced primary–secondary link operates.
Philosophy of transmission
For balanced transmission either station on the link can initiate transmission. Therefore
there is no need for a station to be polled for data as it can send it directly when it is
available. This changes the services used at the link layer from the case for unbalanced
transmission. Where unbalanced transmission makes use of the request/respond service to
obtain user data from the secondary station, for balanced transmission the station with the
data just uses the send/confirm service directly. Comparison of Figures 10.13 and 10.14
shows that the secondary codes for responding with user data FC8 and FC9 are not
available for balanced transmission. Instead, primary function codes FC3 or FC4 are used
to send the data directly.
216
Practical Industrial Data Communications
10.4.6
Data-link security
IEC 60870-5-101 and DNP3 utilize the FT1.2 and FT3 frame formats specified by
IEC 60780-5-1, respectively. These formats differ in their security provisions as shown in
Table 10.10. IEC 60870-5-101 uses an 8-bit checksum and a maximum frame size of 255
bytes. DNP uses a 16-bit cyclic redundancy code (CRC) for every 16 bytes of user data
contained in the body of its frame.
Comparison of Security Features
Protocol
IEC 60870-5101
DNP 3.0
IEC 60870-5
Frame Type
FT1.2
FT3
Security
Method
8-bit
checksum
16-bit CRC
Hamming Distance
Error Bits
4
Maximum Secured
Length in Bytes
255
6
16
Table 10.10
Link layer error security
The effect of these differences in security is often quoted in terms of the ‘hamming
distance’. This is equal to the minimum number of single bit errors that are required to
allow an incorrect message to be mistakenly accepted as a good message that is for the
security system to fail. These are 4 and 6 for methods used by FT1.2 and FT3. However,
these figures ignore the effect of the ratio of security code bits to message bits, which in
the case of DNP is higher due to the inclusion of CRC codes within the body section of
the FT3 frame format.
10.4.7
Link vs application data
There is a distinction between link data and application level data that is subtle and can be
difficult to grasp at first sight. It derives from the fact that the application level messages
are treated simply as data at the link layer level, and may be best illustrated by example.
Take the case where an unbalanced link is operating between primary station A and
secondary station D. Suppose that the application level of station D requires some
particular data from A. This is in the reverse direction to most data traffic, which is
generally from the secondary station to the primary. In this case the application level
request from D will generate a class 1 access demand at the link level. This will have to
wait until that station is polled for data by the primary station A. At this point, if class 2
data has been polled for it will respond with any class 2 data, or with an FC9 ‘NACK –
no data’ frame, but in either case with the ACD (access demand) bit set. The primary
station A will then poll for the class 1 data from station D, and at this point D will be able
to transmit its class 1 data. This will contain the application level request by D, for the
primary station to send the application level data that it requires.
The link layer frame that was requested by polling from primary A, carried an
application level request from station D. This resulted in the application layer of A using
the send service to return the requested data to station D. Note that a link layer request in
direction A to D (the poll) has resulted in an application level request from D to A.
In the following sections dealing with the application layer and above, the operation of
the link layer can be generally seen as a message transport mechanism that can be
assumed to just transport the application level messages as required. However, awareness
of the transport mechanism is important to understanding the response times, as these will
depend on the transmission mode and polling frequency for the unbalanced mode.
Fundamentals of IEC 60870-5
10.5
217
Application layer
The remaining sections of Chapter 11 describe the operation of the application layer and
above of the IEC 60870-5 protocol. Most of this section is applicable to both the nonnetworked and networked versions of the protocol, IEC 60870-5-101 and IEC 608705-104. The areas that are different are discussed in the following chapter under Advanced
considerations of IEC 60870-5.
This information is presented in the following sequence:
•
•
•
•
•
10.5.1
Overall message structure
ASDU structure
Message addressing and routing
Information elements
Set of ASDUs.
Overall message structure
In the preview, the overall message structure under IEC 60870-5 was presented. This is
represented in Figure 10.15. This shows the application layer ASDU, and shows how this is
carried as link user data by the data-link layer under IEC 60870-5-101. For the networked
version IEC 60870-5-104 the ASDU is carried by the TCP/IP protocols instead of the T101
link layer and so the link frame shown in Figure 10.15 does not apply to this case.
Figure 10.15
Message structure under IEC 60870-5-101
The only important point to note about the relationship between the ASDU and the link
layer frame is that a maximum of just one ASDU is allowed per frame. This sets an upper
limit for the size of the ASDU at 255 octets, minus 2–3 octets for the link control and
address fields.
10.5.2
ASDU structure
The structure of the ASDU is in two main sections. These are the data unit identifier, and
the data itself, made up of one or more information objects. The data unit identifier
218
Practical Industrial Data Communications
defines the specific type of data, provides addressing to identify the specific identity of
the data, and it includes additional information in the cause of transmission field. The
fields of the ASDU are now discussed in turn.
Type identification
The type identification field is a single-octet unsigned integer field. Its content is
interpreted as a code in the ranges shown in Figure 10.16.
Figure 10.16
Type identification field
The following notes apply to these codes:
• The value <0> is not used.
• The range <128..255> is not defined by the standard, and may be used by
particular vendors for system specific roles. However this has implications for
interoperability.
In the range of the standard type definitions, there are presently 58 specific types
defined. These are grouped as shown in Table 10.11, which shows the overall groups and
numbers of type identification codes that are defined.
Defined Type Codes
<1..40>
<45..51>
<70>
<100..106>
<110..113>
<120..126>
Group
Process information in monitor direction
Process information in control direction
System information in monitor direction
System information in control direction
Parameter in control direction
File transfer
Table 10.11
Defined type code groups
It is important to note that the type identification applies to the whole ASDU, therefore if there are multiple information objects contained in the ASDU, they are all of the
same type.
Table 10.13(a–g) provides a full list of the ASDU types. The table is broken into the
code groups shown in Table 10.11. Full details of each individual ASDU type are given
under ‘Set of ASDUs’ later in this text.
Table 10.13 also includes information code references that may be encountered. These
references are defined by IEC 60870-5-5. They provide a hierarchical reference system
using the following structure (Table 10.12).
Fundamentals of IEC 60870-5
Level
Symbol
1
M_
C_
P_
F_
2
3
Various
_Nx
_Tx
_xA
_xB
_xC
_xD
219
Description
Monitored information
Control information
Parameter
File transfer
See actual usages
Not time tagged
Time tagged
Type A: status and normalized, with quality
Type B: scaled, with quality
Type C: short floating point, with quality
Type D: normalized without quality
Table 10.12
Reference information code structure
For example, M_ME_TA_1 is monitored information, a measured value, with time tag,
and type A, which is a normalized value with quality.
Type No.
<0>
<1>
<2>
<3>
<4>
<5>
<6>
<7>
<8>
<9>
<10>
<11>
<12>
<13>
<14>
<15>
<16>
<17>
<18>
<19>
<20>
Description
Not defined
Single-point information
Single-point information with time tag
Double-point information
Double-point information with time tag
Step position information
Step position information with time tag
Bitstring of 32 bit
Bitstring of 32 bit with time tag
Measured value, normalized value
Measured value, normalized value with
time tag
Measured value, scaled value
Measured value, scaled value with time tag
Measured value, short floating point number
Measured value, short floating point number
with time tag
Integrated totals
Integrated totals with time tag
Event of protection equipment with time tag
Packed start events of protection equipment
with time tag
Packed output circuit information of
protection equipment with time tag
Packed single-point information with status
change detection
Reference
M_SP_NA_1
M_SP_TA_1
M_DP_NA_1
M_DP_TA_1
M_ST_NA_1
M_ST_TA_1
M_BO_NA_1
M_BO_TA_1
M_ME_NA_1
M_ME_TA_1
M_ME_NB_1
M_ME_TB_1
M_ME_NC_1
M_ME_TC_1
M_IT_NA_1
M_IT_TA_1
M_EP_TA_1
M_EP_TB_1
M_EP_TC_1
M_PS_NA_1
(continued)
220
Practical Industrial Data Communications
Type No.
<21>
<22..29>
Description
Measured value, normalized value without
quality descriptor
Reserved for further compatible definitions
Reference
M_ME_ND_1
Table 10.13a
ASDU types – process information in monitoring direction
Table 10.13b shows types that were added to this category with amendment 2 of IEC
60870-5-101. These provide for a longer time tag format.
Type No.
<30>
<31>
<32>
<33>
<34>
<35>
<36>
<37>
<38>
<39>
<40>
<30>
<31>
<32>
<33>
<34>
<35>
<36>
Description
Single-point information with time tag
CP56Time2a
Double-point information with time tag
CP56Time2a
Step position information with time tag
CP56Time2a
Bitstring of 32 bits with time tag
CP56Time2a
Measured value, normalized value with
time tag CP56Time2a
Measured value, scaled value with time
tag CP56Time2a
Measured value, short floating point
number with time tag CP56Time2a
Integrated totals with time tag
CP56Time2a
Event of protection equipment with
time tag CP56Time2a
Packed start events of protection
equipment with time tag CP56Time2a
Packed output circuit information of
protection equipment with time tag
CP56Time2a
Single-point information with time tag
CP56Time2a
Double-point information with time tag
CP56Time2a
Step position information with time tag
CP56Time2a
Bitstring of 32 bits with time tag
CP56Time2a
Measured value, normalized value with
time tag CP56Time2a
Measured value, scaled value with time
tag CP56Time2a
Measured value, short floating point
number with time tag CP56Time2a
Reference
M_SP_TB_1
M_DP_TB_1
M_ST_TB_1
M_BO_TB_1
M_ME_TD_1
M_ME_TE_1
M_ME_TF_1
M_IT_TB_1
M_EP_TD_1
M_EP_TE_1
M_EP_TF_1
M_SP_TB_1
M_DP_TB_1
M_ST_TB_1
M_BO_TB_1
M_ME_TD_1
M_ME_TE_1
M_ME_TF_1
Fundamentals of IEC 60870-5
Type No.
Description
Reference
<37>
<38>
Integrated totals with time tag CP56Time2a
Event of protection equipment with time
tag CP56Time2a
Packed start events of protection equipment
with time tag CP56Time2a
Packed output circuit information of
protection equipment with time tag
CP56Time2a
Reserved for further compatible definitions
M_IT_TB_1
M_EP_TD_1
<39>
<40>
<41..44>
M_EP_TE_1
M_EP_TF_1
Table 10.13b
ASDU types – process information in monitoring direction cont’d
Type No.
<45>
<46>
<47>
<48>
<49>
<50>
<51>
Description
Single command
Double command
Regulating step command
Set point command, normalized value
Set point command, scaled value
Set point command, short floating point number
Bitstring of 32 bits
Reference
C_SC_NA_1
C_DC_NA_1
C_RC_NA_1
C_SE_NA_1
C_SE_NB_1
C_SE_NC_1
C_BO_NA_1
Table 10.13c
ASDU types – process information in control direction
Type No.
<70>
<70..99>
Description
End of initialization
Reserved for further compatible definitions
Reference
M_EI_NA_1
Table 10.13d
ASDU types – system information in monitor direction
Type No.
<100>
<101>
<102>
<103>
<104>
<105>
<106>
<107..109>
Description
Interrogation command
Counter interrogation command
Read command
Clock synchronization command
Test command
Reset process command
Delay acquisition command
Reserved for further compatible definitions
Table 10.13e
ASDU types – system information in control direction
Reference
C_IC_NA_1
C_CI_NA_1
C_RD_NA_1
C_CS_NA_1
C_TS_NA_1
C_RP_NA_1
C_CD_NA_1
221
222
Practical Industrial Data Communications
Type No.
<110>
<111>
<112>
<113>
<114..119>
Description
Parameter of measured value, normalized value
Parameter of measured value, scaled value
Parameter of measured value, short floating point
number
Parameter activation
Reserved for further compatible definitions
Reference
P_ME_NA_1
P_ME_NB_1
P_ME_NC_1
P_AC_NA_1
Table 10.13f
ASDU types – parameter in control direction
Type No.
<120>
<121>
<122>
<123>
<124>
<125>
<126>
<127>
Description
File ready
Section ready
Call directory, select File, call File, call section
Last section, last segment
Ack File, ack section
Segment
Directory
Reserved for further compatible definitions
Reference
F_FR_NA_1
F_SR_NA_1
F_SC_NA_1
F_LS_NA_1
F_AF_NA_1
F_SG_NA_1
F_DR_TA_1
Table 10.13g
ASDU types – file transfer
Variable structure qualifier
The variable structure qualifier is a single-octet that specifies the number of information
objects or information elements, and how they are addressed. It contains a seven-bit binary
number, and a one-bit field that indicates which of two different possible information
structures is used. Figure 10.17 shows the variable structure qualifier field, followed by a
detail of the two information structures.
It may be seen from Figure 10.17 that there are two different structures, depending on
the state of the most significant bit of the variable structure qualifier. This is termed the
SQ bit, which may be thought of as the structure qualifier bit.
When SQ = 0, the structure is a sequence of information objects. Each information
object carries its own address, and therefore the information elements contained do not
need to have sequential addresses. The number of information objects is given by the
seven-bit value N. Therefore there can be up to 127 information objects in this ASDU.
When SQ = 1, the structure contains just one information object, but this may contain
multiple information elements, all of the same format, such as a measured value. In this
case there is only one information object address, and only one time tag (if used).
The effect of the SQ bit is that for each type identification number, there may
effectively be two ASDUs. This is seen in the sub-section presenting the set of ASDUs,
where it will be seen that some types have both variations (SQ = 0 and SQ = 1), and
others have only one of these.
Cause of transmission
The cause of transmission (COT) field is used to control the routing of messages both
on the communications network, and within a station, by directing the ASDU to the
Fundamentals of IEC 60870-5
223
correct program or task for processing. This sub-section will initially look at the structure
and meaning of the sub-fields within the COT, and then look at how these are used
(Figure 10.18).
Figure 10.17
Variable structure qualifier and structures
Figure 10.18
Cause of transmission field
The cause of transmission or COT is a six-bit code that is used in interpreting the
information at the destination station. The codes are shown in Table 10.14. Each defined
ASDU type has a defined sub-set of the codes that are meaningful with it, and these are
given in the section presenting the set of ASDUs.
The PN bit is the positive/negative confirmation bit. This is meaningful when used with
control commands. This bit is used when the control command is mirrored in the monitor
direction, and it provides indication of whether the command was executed or not. When
the PN bit is not relevant it is cleared to zero.
The T or test bit is set when ASDUs are generated for test purposes and are not
intended to control the process or change the system state. It is used for testing of
transmission and equipment.
224
Practical Industrial Data Communications
COT Code
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14–19
20
21–36
37
38–41
42–43
44
45
46
47
Cause of Transmission
Not used
Periodic, cyclic
Background scan
Spontaneous
Initialized
Request or requested
Activation
Activation confirmation
Deactivation
Deactivation confirmation
Activation termination
Return information caused by a remote
command
Return information caused by a local
command
File transfer
Reserved for future definitions
Interrogated by station interrogation
Interrogated by group (1–16) interrogation
Requested by general counter request
Requested by group (1–4) counter request
Reserved for future definitions
Unknown type identification
Unknown cause of transmission
Unknown common address of ASDU
Unknown information object address
Table 10.14
Cause of transmission codes (COT)
The originator address is optional on a system basis. It provides a means for a
controlling station to explicitly identify itself. This is not necessary when there is only
one controlling station in a system, but is required when there is more than one
controlling station, or some stations are dual-mode stations. These are stations that act
both as controlled and controlling stations. In these circumstances the originator address
can be used to direct command confirmations back to the particular controlling station
that issued the command, rather than to the whole system. The one exception to this is the
originator address of zero <0>. This has the same effect as if there was no originator
address. It is termed the default address and its effect is that the message is transmitted to
all stations. The use of the originator address is discussed in greater detail under message
addressing and routing.
Common address of ASDU
The common address of the ASDU (Figure 10.19) is either one or two octets in length,
fixed on a per-system basis. The address is called a common address because it is in
common to all of the data contained within the ASDU. This is normally interpreted as a
station address, however it can be structured to form a station/sector address where
individual stations are broken up into multiple logical units. The address <0> is not used.
Fundamentals of IEC 60870-5
225
Figure 10.19
Common address of ASDU
The highest address 0xFF or 0xFFFF is global. This means that an ASDU with this
address will be interpreted by all stations. Use of the global address is restricted to the
ASDUs listed below. These are used when the same application function must be initiated
simultaneously.
Type
100
101
103
105
Description
Interrogation command
Counter interrogation command
Clock synchronization command
Reset process command
Purpose
Reply with particular system data
snapshot at common time
Freeze totals at common time
Synchronize clocks to common time
Simultaneous reset
Information object address
The information object address is the first field of the information object. It identifies the
particular data within a defined station. The information object address may be one, two,
or three octets in length. However, the case of three octets is provided only to allow for
structured address systems, and one station is allowed only 65 536 different information
object addresses, as for two-octet addressing. The information object address of zero is
reserved for the case when the address is not relevant. The information object address is
shown in Figure 10.20.
On a system basis, specific data is uniquely identified by the combination of the
common address and the information object address.
Figure 10.20
Information object address
226
Practical Industrial Data Communications
An example of how this might work in practice is where there are a number of identical
intelligent electronic devices IEDs connected to a sub-master RTU, which is in turn
connected to a master station. These could be re-closers on a distribution system. Each
IED will have an identical data structure internally, as determined by the device
manufacturer. Most likely this will be some tens of information elements or data points,
which could be addressed by a one-octet information object address. However, as the
system includes RTUs that have many more data points, two-octet addressing is used
across the system. At a system configuration level, there will be a single model for that
type of re-closer, and configuration of each device into the system database will be a
matter of using the standard point-mapping for that device type and adding the station
number to form unique point references.
10.5.3
Message addressing and routing
Control and monitor directions
An important concept in understanding addressing under IEC 60870-5 is the difference
between control and monitor directions. It is an assumption that the overall system has a
hierarchical structure involving centralized control. Under the protocol, every station is
either a controlling station or a controlled station. The communications network structure
will normally be aligned with this, and for unbalanced communications links the
controlling stations will be primaries, and the controlled stations will be secondaries at
the link level.
This follows naturally from the fact that a hierarchical structure involves multiple
controlled stations, controlled by one or at least few controlling stations.
In such a system, control messages such as commands or interrogations are transmitted
by the controlling station, and these results in actions and return information transmitted
by the controlled station.
Addressing of ASDUs
Messages are addressed in the control direction by the common address field of the
ASDU. This address field is one or two octets, and defines the station (or logical
station) to which the ASDU is being addressed. In the monitor direction, however, the
common address field contains the address of the station returning the data. This is
required so that the data can be uniquely identified and mapped to the right points in
system data images.
In some cases a station that is generally a controlled station may itself act as a
controlling station, perhaps to interrogate the master station for data, or to initiate an
action in another controlled station. This is called reverse direction operation. A station
that can act in both the forward and the reverse direction is called a dual-mode or
combined station. When a dual-mode station issues a control ASDU to another station, it
must set the controlled station’s address as the common address of the ASDU. This is
necessary, as for any control direction ASDU, so that the intended station can recognize
the message as being directed at it (Figure 10.21).
When the action is carried out, further communication will be necessary with the
controlling station to send an action confirmation message, and possibly an execute
message if two-phase operation is being used. But as monitor direction messages carry
the address of the controlled station, this cannot be used to route the communications
back to the controlling station. Instead, the originator address octet of the cause of
transmission field is used for this purpose. Its operation is described in the following
paragraphs and in Figures 10.22 and 10.23.
Fundamentals of IEC 60870-5
227
Figure 10.21
Control command issued from dual-mode RTU
Figure 10.22
Activation confirmation ASDU returned to dual-mode RTU
When a control direction ASDU is transmitted by a dual-mode station that is not the
system master station, that station must include a non-zero value in the originator
address octet of the cause of transmission field. This has no effect in the control
direction of the ASDU, but is used in the monitoring direction to route action
confirmation and action termination messages back to the originator. When the
controlled station returns an action confirmation or other message arising from this
control ASDU, it includes the originator address from the control direction ASDU in
the monitoring direction response. It is the responsibility of any intermediate routing
devices to recognize a non-zero originator address in a monitor direction ASDU, and to
route it back to that originator.
228
Practical Industrial Data Communications
Figure 10.23
Monitoring information returned to SCADA master
As the originator address sub-field is only one octet in length, and common addresses
may be two octets, it is clear that either any dual-mode stations on a system must either
be numbered within the range <1..256>, or a mapping must be used between originator
addresses and common addresses if these are not in that range.
It should be noted that also arising from the control action may be some changes within
the controlled station that need to be conveyed to the master station rather than the dualmode station that initiated the control action. These would typically be to convey the
changed system state, and might also include time-tagged events. The monitor direction
ASDUs that carry this information need to be directed to the master station, and possibly
to other areas of the network if required. These ASDUs are given an originator address
value of zero. Thus, in a system which contains dual-mode stations, it is necessary to use
the originator address sub-field, and all monitor direction messages going back to the
master station will have this field set to zero.
These concepts are illustrated in Figure 10.21. This shows a system with intermediate
RTUs acting as controllers and data concentrators, and multiple controlled stations linked
to these. One of these issues a command to a peer station. Both action confirmation and
change data messages are generated, and are routed to their correct destinations by the use
of the originator address field.
10.6
Information elements
As has been shown in the preceding sections on ASDU structure, application data is
carried within the ASDU within one or more information objects. Depending on the
variable structure flag there may be multiple information objects each containing a
defined set of one or more information elements, or there may be just one information
object containing a number of identical information elements. In either case, the
information element is the fundamental component used to convey information under the
protocol. The information elements are used as building blocks in the definition of the set
of ASDUs under the protocol.
In this section this set of information building blocks is presented. These are referred to
in the following section when the set of ASDUs are defined.
Fundamentals of IEC 60870-5
229
In the following definitions, these interpretation rules should be noted:
• Key descriptions give the logical state for a set bit, i.e. bit = 1.
• Blank bit positions are reserved and must be cleared, i.e. bit = 0.
• Bit positions have been numbered <0..7> for consistency within this text and
the powers of 2 the positions represent. Note that the IEC documents actually
use <1..8>. This is a matter of definition only and does not change the meaning.
The set of information elements is listed in Table 10.15. This is followed by
representations of each of the data elements, grouped by general type.
General Type Symbol
Process
SIQ
DIQ
BSI
SCD
QDS
VTI
NVA
SVA
R32-IEEE STD 754
BCR
Protection
SEP
SPE
OCI
QDP
Commands
SCO
DCO
RCO
Time
CP56Time2a
CP24Time 2a
CP16Time2a
Qualifiers
QOI
QCC
QPM
QPA
QRP
QOC
QOS
File Transfer
FRQ
SRQ
SCQ
Description
Single-point information with quality descriptor
Double-point information with quality descriptor
Binary state information
Status and change detection
Quality descriptor
Value with transient state indication
Normalized value
Scaled value
Short floating point number
Binary counter reading
Single event of protection equipment
Start events of protection equipment
Output circuit information of protection equipment
Quality descriptor for events of protection equipment
Single command
Double command
Regulating step command
Seven octet binary time
Three octet binary time
Two octet binary time
Qualifier of interrogation
Qualifier of counter interrogation command
Qualifier of parameter of measured values
Qualifier of parameter activation
Qualifier of rest process command
Qualifier of command
Qualifier of set-point command
File ready qualifier
Section ready qualifier
Select and call qualifier
(continued)
230
Practical Industrial Data Communications
General Type Symbol
LSQ
AFQ
NOF
NOS
LOF
LOS
CHS
SOF
Miscellaneous
COI
FBP
Description
Last section or segment qualifier
Acknowledge file or section qualifier
Name of file
Name of section
Length of file or section
Length of segment
Checksum
Status of file
Cause of initialization
Fixed test bit pattern, two octets
Table 10.15
Information elements
10.6.1
Quality bits
Quality bits are not information elements in themselves, but appear as individual bits
within information elements. These are defined in this section.
Note that quality bits are set or cleared independently of each other. Examination of
these shows that these may be used to differentiate between different types of situation or
problem that may be affecting the data. Whether all are used will depend on the system.
A simple approach would be to interpret any of the quality bits being set as ‘bad value’,
whereas more sophisticated approaches may differentiate based on the actual bit(s) set.
Their individual meanings are explained further below.
Blocked (BL)
This means that the value of the point is as it was prior to being blocked. Blocking
prevents updating of the value of the point.
Substituted (SB)
This is where a value has been substituted or forced by manual entry or otherwise.
It means that the value is not derived from the normal measurement.
Not topical (NT)
This is means that the value was not updated successfully at the last time it was due to be
updated.
Invalid (IV)
This indicates that the value cannot be used because it may be incorrect due to a fault or
other abnormal condition.
Industrial Ethernet overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe how industrial Ethernet systems operate
• Identify, troubleshoot, and fix problems such as:
–
–
–
–
–
–
–
–
–
–
–
–
–
11a.1
Thin and thick coax cable and connectors
UTP cabling
Incorrect media selection
Jabber
Too many nodes
Excessive broadcasting
Bad frames
Faulty auto-negotiation
10/100 Mbps mismatch
Full-/half-duplex mismatch
Faulty hubs
Switched networks
Loading.
Introduction
During the mid-seventies, Xerox Corporation (Palo Alto) developed the Ethernet network
concept, based on work done by researchers at the University of Hawaii. The University’s
ALOHA network was set up using radio broadcasts to connect sites on the islands. This
was colloquially known as their ‘Ethernet’ since it used the ‘ether’ as the transmission
medium and created a network between the sites. The philosophy was straightforward.
Any station wanting to broadcast would do so immediately. The receiving station then
had a responsibility to acknowledge the message, advising the original transmitting
station of a successful reception of the original message. This primitive system did not
rely on any detection of collisions (two radio stations transmitting at the same time) but
depended on an acknowledgment within a predefined time.
The initial Xerox system was so successful that it was soon applied to other sites,
typically connecting office equipment to shared resources such as printers and large
computers acting as repositories of large databases.
232
Practical Industrial Data Communications
In 1980, the Ethernet Consortium consisting of Xerox, Digital Equipment Corporation
and Intel (a.k.a. the DIX consortium) issued a joint specification, based on the Ethernet
concepts, known as the Ethernet Blue Book 1 specification. This was later superseded by
the Ethernet Blue Book 2 (Ethernet V2) specification, which was offered to the IEEE for
ratification as a standard. In 1983, the IEEE issued the IEEE 802.3 standard for carrier
sense multiple access/collision detect (CSMA/CD) LANs based on the DIX Ethernet
standard.
As a result of this, there are two standards in existence viz. Ethernet V2 (Bluebook) and
IEEE 802.3. The differences between these two later standards are minor, yet they are
nonetheless significant. Despite the generic term ‘Ethernet’ being applied to all
CSMA/CD networks, it should, technically speaking, be reserved for the original DIX
standard. This chapter will continue with popular use and refer to all the LANs of this
type as Ethernet, unless it is important to distinguish between them.
Early Ethernet (of the 10 Mbps variety) uses the CSMA/CD access method. This
results in a system that can operate with little delay, if lightly loaded, but access to the
medium can become very slow if the network is heavily loaded. Ethernet network
interface cards are relatively cheap and produced in vast quantities. Ethernet has, in
fact, become the most widely used networking standard. However, because of its
probabilistic access mechanism, there is no guarantee of message transfer and messages
cannot be prioritized.
Modern Ethernet systems are a far cry from the original design. From 100BASE-T
onwards they are capable of full-duplex (sending and receiving at the same time via
switches, without collisions) and the Ethernet frame has been modified to make provision
for prioritization and virtual LANs.
Ten Gigabit Ethernet became commercially available in 2002. Ethernet has also been
modified for industrial use and as such, has made vast inroads into the process control
environment.
As an introduction to Ethernet, the next section will deal with systems based on the
original IEEE 802.3 (CSMA/CD) standard.
11a.2
10 Mbps Ethernet
11a.2.1
Media systems
The IEEE 802.3 standard (also known as ISO 8802.3) defines a range of media types that
can be used for a network based on this standard such as coaxial cable, twisted-pair cable
and fiber-optic cable. It supports various cable media and transmission rates at 10 Mbps,
such as:
• 10BASE-2: Thin wire coaxial cable (6.3 mm/0.25 in. diameter), 10 Mbps
baseband operation, bus topology
• 10BASE-5: Thick wire coaxial cable (13 mm/0.5 in. diameter), 10 Mbps
baseband operation, bus topology
• 10BASE-T: Unscreened twisted-pair cable (0.4–0.6 mm conductor diameter),
10 Mbps baseband operation, hub topology
• 10BASE-F: Optical fiber cables, 10 Mbps baseband operation, point-to-point
topology.
Other variations include 1BASE-5, 10BASE-FB, 10BASE-FP, and 10Broad36 but
these versions never became commercially viable.
Industrial Ethernet overview 233
10BASE-5
This is the original coaxial cable system and is also called ‘thicknet’. The coaxial cable
(50 Ω characteristic impedance) is yellow or orange in color. The naming convention
10BASE-5 means 10 Mbps, baseband signaling on a cable that will support 500 m
(1640 ft) segment lengths. The cable is difficult to work with, and cannot normally be
taken to the node directly. Instead, it is laid in a cabling tray and the transceiver
electronics (medium attachment unit or MAU) is installed directly on the cable. From
there, an intermediate cable, known as an attachment unit interface (AUI) cable, is used to
connect to the NIC. This cable can be a maximum of 50 m (164 ft) long, compensating
for the lack of flexibility of placement of the coaxial cable. The AUI cable consists of five
individually shielded pairs – two each (control and data) for both transmit and receive,
plus one for power.
Cutting the cable and inserting N-connectors and a coaxial T or more commonly by
using a ‘bee sting’ or ‘vampire’ tap can make the MAU connection to the cable. The
vampire tap is a mechanical connection that clamps directly over the cable. Electrical
connection is made via a probe that connects to the center conductor, and sharp teeth that
physically puncture the cable sheath to connect to the braid. These hardware components
are shown in Figure 11a.1.
Figure 11a.1
10BASE-5 components
The location of the connection is important to avoid multiple electrical reflections on the
cable, and the cable is marked every 2.5 m (8 ft) with a black or brown ring to indicate
where a tap should be placed. Fan-out boxes can be used if there are a number of nodes for
connection, allowing a single tap to feed each node as though it was individually connected.
The connection at either end of the AUI cable is made through a 25 pin-D connector, with a
slide latch, often called a DIX connector after the original consortium.
There are certain requirements if this cable architecture is used in a network. These
include:
• Segments must be less than 500 m (1640 ft) in length to avoid signal attenuation problems
• Not more than 100 taps on each segment
• Taps must be placed at integer multiples of 2.5 m (8 ft)
• The cable must be terminated with a 50 Ω terminator at each end
234
Practical Industrial Data Communications
• It must not be bent at a radius exceeding 25.4 cm or 10 in.
• One end of the cable shield must be earthed.
The physical layout of a 10BASE-5 Ethernet segment is shown in Figure 11a.2.
Figure 11a.2
10BASE-5 Ethernet segment
Note that when an MAU and AUI cable is used, the on-board transceiver on the NIC is
not used. Rather, there is a transceiver in the MAU and this is fed with power from the
NIC via the AUI cable. Since the transceiver is remote from the NIC, the node needs to
be aware that the termination can detect collisions if they occur. A signal quality error
(SQE), or heartbeat test function in the MAU performs this confirmation. The SQE signal
is sent from the MAU to the node on detecting a collision on the bus. However, on
completion of every frame transmission by the MAU, the SQE signal is asserted to ensure
that the circuitry remains active, and that collisions can be detected. The SQE pulses
occur during the 96-bit inter-frame gap between packets, but is recognized by the NIC
and is not confused with a collision. Not all components support SQE test, and mixing
those that do with those that do not could cause problems. If, for example, an MAU is
connected to a repeater, the SQE function on that MAU needs to be turned off. If not, the
repeater will mistakenly see the SQE signal as a collision, and respond with a jam signal.
This is not easily detectable, but can radically slow down a network.
10BASE-2
The other type of coax-based Ethernet network is 10BASE-2, often referred to as ‘thinnet’
or ‘thinwire Ethernet’. It uses type RG-58 A/U or C/U cable with a 50 Ω characteristic
impedance and 5 mm diameter. The cable is normally connected to the NICs in the nodes
by means of a BNC T-piece connector. Connectivity requirements include:
•
•
•
•
•
It must be terminated at each end with a 50 Ω terminator.
The maximum length of a cable segment is 185 m (600 ft) and not 200 m (650 ft).
Not more than 30 transceivers can be connected to any one segment.
There must be a minimum spacing of 0.5 m (1.6 ft) between nodes.
It may not be used as a link segment between two ‘thicknet’ segments.
Industrial Ethernet overview 235
• The minimum bend radius is 5 cm (2 in.).
• The maximum distance between the medium and the transceiver is 4 in.; this is
taken up by the dimensions of the T-connector and the PCB tracks, which
means that no drop cable may be used – the BNC T-piece has to be located on
the front panel of the NIC.
The physical layout of a 10BASE-2 Ethernet segment is shown in Figure 11a.3.
Figure 11a.3
10BASE-2 Ethernet segment
At one stage the use of thinnet cable was very popular as a cheap and relatively easy
way to set up a network. However, there are disadvantages with this approach as a cable
fault can bring the whole system down.
10BASE-T
10BASE-T uses AWG24 unshielded twisted-pair (UTP) cable for connection to the node.
The physical topology of the standard is a star, with nodes connected to a hub. The cable
can be Category 3 (Cat3) or Cat5 UTP, although Cat3 does not support the faster versions
of Ethernet.
The node cable (hub to node):
• Has a maximum length of 100 m (328 ft)
• Consists of four pairs of which only two pairs are used (one for receiving and
one for transmitting)
• Is connected via RJ-45 plugs.
The hub can be considered as a bus internally (any signal input on a given port are
reflected as outputs on all other ports), and so the topology is a logical bus topology
236
Practical Industrial Data Communications
although it physically resembles a star (hub) topology. Figure11a.4 shows schematically
how the hub interconnects the 10BASE-T nodes.
Collisions are detected by the NIC and so the hub must retransmit an input signal on all
output pairs, so that each NIC can also receive its own transmitted signal. The electronics
in the hub must ensure that the stronger retransmitted signal does not interfere with the
weaker input signal. The effect is known as far-end crosstalk (FEXT), and is handled by
special adaptive crosstalk echo cancellation circuits.
Figure 11a.4
10BASE-T hub concept
The standard has become very popular for new networks, although there are some
disadvantages that should be recognized:
• The unshielded cable is not very resistant to electrostatic electrical noise, and
may not be suitable for some industrial environments.
• Whilst the cable is inexpensive, there is the additional cost of the associated
wiring hubs to be considered.
• The cable length is limited to 100 m (328 ft).
Advantages of the system include:
• The twisted-pair cable provides good electromagnetic noise immunity.
• Intelligent hubs can monitor traffic on each port. This improves on the security
of the network – a feature that has often been lacking in a broadcast, common
media network such as Ethernet.
• Because of the cheap wire used, flood wiring can be installed in a new
building, providing many more wiring points than are initially needed, but
giving great flexibility for future expansion. When this is done, patch panels –
or punch down blocks – are often installed for even greater flexibility.
• The star-wiring configuration improves physical reliability. A cut cable only
affects one node.
10BASE-F
This standard makes provision for three architectures viz. 10BASE-FL, 10BASE-FP, and
10BASE-FB. The latter two have never gained commercial acceptance and are not
currently manufactured by any vendor.
Industrial Ethernet overview 237
10BASE-FL
The fiber link segment standard is basically a 2 km (1.2 miles) upgrade to the existing
fiber-optic inter repeater link (FOIRL) standard. The original FOIRL as specified in the
IEEE 802.3 standard was limited to a 1 km (0.6 mile) fiber link between two repeaters,
with a maximum length of 2.5 km (1.5 miles) if there are five segments in the link. Note
that this is a link between two repeaters in a network, and cannot have any nodes
connected to it (Figure 11a.5).
Figure 11a.5
10BASE-FL segment
11a.2.2
Signaling methods
10 Mbps Ethernet signals are encoded using the Manchester encoding scheme. This method
allows a clock to be extracted at the receiver end to synchronize the transmission/reception
process. The encoding is performed by an exclusive or between a 20 MHz clock signal and
the data stream. In the resulting signal, a 0 is represented by a high to low change at the
center of the bit cell, whilst a 1 is represented by a low to high change at the center of the
bit cell. There may or may not be transitions at the beginning of a cell as well, but these are
ignored at the receiver. The transitions in every cell allow the clock to be extracted and
synchronized with the transmitter. This method is very wasteful of bandwidth (it requires a
cable bandwidth of well above twice the bit rate) and is therefore not used on the faster
Ethernet versions.
In the IEEE 802.3 standard, voltages swing between 0 and –2.05 V on coax, or between
–2.5 and +2.5 V on twisted pair.
11a.2.3
Medium access control
10 Mbps Ethernet operates in half-duplex mode, so that a station can either receive or
transmit, but not both at the same time. The method used to achieve this is called
CSMA/CD, or carrier sense multiple access with collision detection.
Essentially, the method used is one of contention. Each node has a connection via a
transceiver to the common bus. As a transceiver, it can both transmit and receive at the
same time. Each node can be in any one of three states at any time. These states are:
• Idle, or listen
• Transmit
• Contention.
In the idle state, the node merely listens to the bus, monitoring all traffic that passes. If
a node then wishes to transmit information, it will defer whilst there is any activity on the
bus, since this is the ‘carrier sense’ component of the architecture. At some stage, the bus
238
Practical Industrial Data Communications
will become silent, and the node, sensing this, will then commence its transmission. It is
now in the transmit mode, and will both transmit and listen at the same time. This is
because there is no guarantee that another node at some other point on the bus has not
also started transmitting, having recognized the absence of traffic.
After a short delay as the two signals propagate towards each other on the cable, there
will be a collision of signals. Obviously the two transmissions cannot coexist on the
common bus, since there is no mechanism for the mixed analog signals to be
‘unscrambled’. The transceiver quickly detects this collision, since it is monitoring both
its input and output and recognizes the difference. The node now goes into the third state
viz. contention. The node will continue to transmit for a short time – the jam signal – to
ensure the other transmitting node detects the contention, and then performs a back-off
algorithm to determine when it should again attempt to transmit its waiting frames.
11a.2.4
Frame transmission
When a frame is to be transmitted, the medium access control monitors the bus and defers
to any passing traffic. After a period of 96 bit times, known as the inter-frame gap (to
allow the passing frame to be received and processed by the destination node) the
transmission process commences. Since there is a finite time for this transmission to
propagate to the ends of the bus cable, and thus ensure that all nodes recognize that the
medium is busy, the transceiver turns on a collision detect circuit whilst the transmission
takes place. Once a certain number of bits (512) have been transmitted, provided that the
network cable segment specifications have been complied with, the collision detection
circuitry can be disabled. If a collision should take place after this, it will be the
responsibility of higher protocols to request retransmission – a far slower process than the
hardware collision detection process.
This is a good reason to comply with cable segment specifications! This initial ‘danger’
period is known as the collision window, and is effectively twice the time interval for the
first bit of a transmission to propagate to all parts of the network. The slot time for the
network is then defined as the worst case time delay that a node must wait before it can
reliably know that a collision has occurred. It is defined as:
Slot time = 2 ´ (transmission path delay) + safety margin
For a 10 Mbps system the slot time is fixed at 512 bits or 51.2 µs.
11a.2.5
Frame reception
The transceiver of each node is constantly monitoring the bus for a transmission signal.
As soon as one is recognized, the NIC activates a carrier sense signal to indicate that
transmissions cannot be made. The first bits of the MAC frame are a preamble and consist
of alternating bits of 1010, etc. On recognizing these, the receiver synchronizes its clock,
and converts the Manchester-encoded signal back into binary form. The eighth octet is a
start of frame delimiter, and this is used to indicate to the receiver that it should strip off the
first eight octets and commence frame reception into a frame buffer within the NIC.
Further processing then takes place, including the calculation and comparison of the
frame CRC with the transmitted CRC. In valid frames, the destination hardware address
is checked. If this matches the MAC address in the card firmware, or a broadcast address,
then the message is passed up the protocol stack, otherwise the frame is discarded. It also
checks that the frame contains an integral number of octets and is either too short or too
long. Provided all is correct, the frame is passed to the data-link layer for further
processing.
Industrial Ethernet overview 239
11a.2.6
MAC frame format
The basic frame format for an IEEE 802.3 network is shown in Figure 11a.6. There are
eight fields in each frame, and they are described in detail.
Figure 11a.6
Ethernet V2/IEEE 802.3 MAC frame formats
Preamble
This field consists of seven octets of the data pattern 10101010. The receiver uses this to
synchronize its clock to the transmitter.
Start frame delimiter
This single octet field consists of the data 10101011. It enables the receiver to recognize
where the address fields commence.
Source and destination address
These are the physical addresses of both the source and the destination nodes. The fields
are six octets long. Assignment of addresses is controlled by the IEEE standards
association (IEEE-SA), which administers the allocation of the first three bytes. When
assigning blocks of addresses to manufacturers, the IEEE-SA provides a 24 bit
organizationally unique identifier (OUI). The OUI (the first three bytes of the hardware
address) identifies the manufacturer of the card and is the same for all cards made by it.
The second half is the device identifier, and these numbers are assigned sequentially
during the manufacturing process so that each card will have a unique address.
The addressing modes according to IEEE 802.3 include:
• Broadcast: The destination address is set to all 1s or FF-FF-FF-FF-FF-FF,
and all nodes on the network will respond to the message.
• Unicast (individual, or point-to-point): If the first bit of the address is 0,
the frame is sent to one specific node. An example of a unicast address is
06-C1-B2-12-56-23.
240
Practical Industrial Data Communications
The addresses allocated by a factory are referred to as globally administered addresses.
The standard also makes provision for a locally administered address (in which the
second bit is set to one), but in practice this is rarely used as cards are invariably bought
with preset MAC addresses.
Length
For IEEE 802.3, this two-octet field contains a number representing the length of the data
field. For Bluebook Ethernet this field represents ‘type’ rather than ‘length’. ‘Type’ is a
number bigger than 1500 decimal representing the protocol that has submitted the data for
delivery.
Data
The information that has been handed down from the layer above, between 0 and
1500 bytes.
Pad
Since there is a minimum frame length of 64 octets (512 bits) that must be transmitted to
ensure that the collision mechanism works, the pad field will pad out any frame that does
not meet this minimum specification by adding up to 46 bytes of ‘padding’. This pad, if
incorporated, is normally random data. The CRC is calculated over the data in the pad
field. Once the CRC checks OK, the receiving node discards the pad data, which it
recognizes by the value in the length field.
FCS
A 32 bit CRC value is computed in hardware at the transmitter and appended to the
frame. It is the same algorithm used in the IEEE 802.4 and IEEE 802.5 standards.
Difference between IEEE 802.3 and Ethernet V2
As mentioned before, there is a difference between IEEE 802.3 and Blue Book Ethernet.
These differences are primarily in the frame structure and are shown in Table 11a.1.
802.3 Network
Ethernet Network
Star topology supported using
UTP, fiber etc., or bus
Only supports bus topology
Baseband and broadband
signaling
Baseband only
Data-link layer divided into LLC
and MAC
No subdivision of DLL
Seven octets of preamble plus
SFD
8 bytes of preamble with no separate
SFD
Length field in data frame
Field used to indicate the higher
level protocol using the data-link
service
Table 11a.1
Differences between IEEE 802.3 and Blue Book Ethernet (V2)
Industrial Ethernet overview 241
11a.2.7
IEEE 802.2 LLC
Ethernet V2 implements both the physical and the data-link layer. The frame then contains
the detail of the higher-layer protocol being carried in the frame. This information
is required for correctly multiplexing and de-multiplexing the packets carried by the
network, i.e. ensuring that they get delivered to the same type of protocol that sent them
(Figure 11a.7).
In the case of IEEE 802.3, however, it was decided to split the data-link layer into two,
namely, a lower half that controls access to the medium (medium access control, or
MAC) and an upper half that performs the link control between the two NICs involved
(the logical link control or LLC). The MAC part is included together with the physical
layer in the IEEE 802.3 specifications, but the LLC is implemented separately in the
IEEE 802.2 specification. This was done since the LLC functionality is not unique to
IEEE 802.3, but is used by many other IEEE 802 network standards as well.
Since not all LANs necessarily have a ‘type’ field in their frames, the protocol
information required for multiplexing is now carried in the SSAP (source service access
point) and DSAP (destination service access point) within the IEEE 802.2 LLC header.
The entire LLC header is only 3–4 bytes and is carried within the ‘Data’ field of the IEEE
802.3 frame, just after the header.
Figure 11a.7
LLC PDU carried in an Ethernet frame
11a.2.8
Reducing collisions
The main contributors to collisions on an Ethernet network are:
•
•
•
•
The number of packets per second
The signal propagation delay between transmitting nodes
The number of stations initiating packets
The bandwidth utilization.
A few suggestions on reducing collisions in an Ethernet network are:
• Keep all cables as short as possible
• Keep all high activity sources and their destinations as close as possible. Possibly,
isolate these nodes from the main network backbone with bridges/routers to reduce
backbone traffic
242
Practical Industrial Data Communications
• Separate segments with bridges and switches rather than with repeaters, which
do not truncate the collision domain
• Check for unnecessary broadcast packets
• Remember that the monitoring equipment to check out network traffic can
contribute to the traffic (and the collision rate).
11a.2.9
Design rules
The following design rules on length of cable segment, node placement, and hardware
usage should be strictly observed.
Length of the cable segments
It is important to maintain the overall Ethernet requirements as far as length of the cable
is concerned. Each segment has a particular maximum length allowable (Table 11a.2).
For example, 10BASE-2 allows 185 m maximum length. The recommended maximum
length is 80% of this figure. Some manufacturers advise that you can disregard this limit
with their equipment. This can be a risky strategy and should be carefully considered.
System
Maximum
Recommended
10BASE-5
500 m (1640 ft)
400 m (1310 ft)
10BASE-2
185 m (600 ft)
150 m (500 ft)
10BASE-T
100 m (330 ft)
80 m (260 ft)
Table 11a.2
Suggested maximum lengths
Cable segments need not be made from a single homogenous length of cable and may
comprise multiple lengths joined by coaxial connectors (two male plugs and a connector
barrel).
To achieve maximum performance on 10BASE-5 cable segments, it is preferable that the
total segment be made from one length of cable or from sections of the same drum of cable.
If multiple sections of cable from different manufacturers are used, then these should be
standard lengths of 23.4 m (76.8 ft), 70.2 m (230.3 ft), or 117 m (383.9 ft) (±0.5 m/1 ft),
which are odd multiples of 23.4 m/76.8 ft (half wavelength in the cable at 5 MHz). These
lengths ensure that reflections from the cable-to-cable impedance discontinuities are
unlikely to add in phase. Using these lengths exclusively a mix of cable sections should be
able to be made up to the full 500 m (1640 ft) segment length.
If the cable is from different manufacturers and you suspect potential mismatch
problems, you should check that signal reflections, as impedance mismatches do not
exceed 7% of the incident wave.
Maximum transceiver cable length
In 10BASE-5 systems, the maximum length of the transceiver cables is 50 m (164 ft) but
it should be noted that this only applies to specified IEEE 802.3 compliant cables. Other
AUI cables using ribbon or office grade cables can only be used for short distances (less
than 12.5 m/41 ft), so check the manufacturer specifications for these.
Industrial Ethernet overview 243
Node placement rules
Connection of the transceiver media access units (MAU) to the cable causes signal
reflections due to their bridging impedance. Placement of the MAUs must therefore be
controlled to ensure that reflections from them do not significantly add in phase.
In 10BASE-5 systems, the MAUs are spaced at multiples of 2.5 m (8.2 ft), coinciding
with the cable markings.
In 10BASE-2 systems, the minimum MAU spacing is 0.5 m (1.6 ft).
Maximum transmission path
The maximum transmission path is made of five segments connected by four repeaters.
The total number of segments can be made up of a maximum of three coax segments
containing station nodes and two link segments. The link segments are defined as pointto-point full-duplex links that connect 2 (and only 2) MAUs. This is summarized as the
5–4–3–2 rule (Table 11a.3).
5 segments
4 repeaters
3 coax segments
2 link segments
OR
5 segments
4 repeaters
3 link segments
2 coax segments
Table 11a.3
The 5–4–3–2 rule
It is important to verify that the above transmission rules are met by all paths between
any two nodes on the network.
Note that the maximum sized network of four repeaters supported by IEEE 802.3 can
be susceptible to timing problems. The maximum configuration is limited by propagation
delay.
Note that 10BASE-2 segments should not be used to link 10BASE-5 segments.
Repeater rules
Repeaters are connected to transceivers that count as one node on the segments.
Special transceivers are used to connect repeaters and these do not implement the signal
quality error test (SQE).
Fiber-optic repeaters are available giving up to 3000 m links at 10 MB/s. Check the
vendor’s specifications for adherence with IEEE 802.3 repeater performance and compliance
with the fiber-optic inter repeater link (FOIRL) standard.
Cable system grounding
Grounding has safety and noise connotations. IEEE 802.3 states that the shield conductor
of each coaxial cable shall make electrical contact with an effective earth reference at one
point only.
The single point earth reference for an Ethernet system is usually located at one of the
terminators. Most terminators for Ethernet have a screw terminal to which a ground lug
can be attached using a braided cable, preferably to ensure good earthing.
Ensure that all other splices taps or terminators are jacketed so that no contact can be
made with any metal objects. Insulating boots or sleeves should be used on all in-line
coaxial connectors to avoid unintended earth contacts.
244
Practical Industrial Data Communications
Round trip delay time
The maximum round trip propagation delay is 51.2 µs because of the minimum frame
size of 64 bytes (512 bits). This time is determined by adding the propagation delays in
all of the electronic components and cables that make up the longest signal path and then
doubling this figure to obtain the round trip delay time.
Note that this calculation needs to be done for all components within a collision domain,
store-and-forward devices such as bridges, routers, or switches terminate collision domains.
Tables 11a.4 and 11a.5 give typical maximum one-way delay times for various components
and cables. Repeater and NIC delays for specific components can be obtained from the
manufacturers.
Component
Maximum Delay (µs)
Standard transceiver
0.86
Twisted-pair transceiver
0.27
Fiber-optic transceiver
0.20
Multiport transceiver
0.10
10BASE-T concentrator
1.90
Local repeater
0.65
Fiber-optic repeater
1.55
Multiport repeater
1.55
Table 11a.4
Maximum one-way Ethernet component delays
Cable Medium
Maximum Delay
per meter (µs)
UTP
0.0057
Coax (10BASE-5)
0.00433
Coax (10BASE-2)
0.00514
Fiber optic
0.005
AUI
0.00514
Table 11a.5
Maximum one-way Ethernet cable delays
11a.3
100 Mbps Ethernet
11a.3.1
Introduction
100BASE-T is the shorthand identifier for all 100 Mbps Ethernet systems, including
twisted-pair copper and fiber versions. These include 100BASE-TX, 100BASE-FX,
100BASE-T4, and 100BASE-T2.
100BASE-X is the designator for the 100BASE-TX (copper) and 100BASE-FX (fiber)
systems based on the same 4B/5B block encoding system used for FDDI (ANSI X3T9.5)
Industrial Ethernet overview 245
fiber distributed interface system. 100BASE-TX, the most widely used version, is a
100 Mbps baseband system operating over two pairs of Cat5 UTP (Figure 11a.8) while
100BASE-FX operates at 100 Mbps in baseband mode over two multi-mode optic
fibers.
Figure 11a.8
100BASE-TX connection
100BASE-T4 was designed to operate at 100 Mbps over four pairs of Cat3 cable, but
this option never gained widespread acceptance. Yet another version, 100BASE-T2, was
supposed to operate over just two pairs of Cat3 cable but was never implemented by any
vendor.
One of the limitations of the 100BASE-T systems is the size of the collision domain,
which is 250 m or 5.12 µs. This is the maximum size of a network segment in which
collisions can be detected, being one-tenth of the size of the maximum-size 10 Mbps
network. This effectively limits the distance between a workstation and a hub to 100 m,
the same as for 10BASE-T. This means that networks larger than 200 m must be logically
connected together by store-and-forward type devices such as bridges, routers, or
switches. This is not a bad thing, since it segregates the traffic within each collision
domain, reducing the number of collisions on the network. The use of bridges and routers
for traffic segregation, in this manner, is often done on industrial Ethernet networks.
The format of the frame has been left unchanged. The only difference is that it is
transmitted 10 times faster than in 10 Mbps Ethernet, hence its length (in time) is 10
times less.
11a.3.2
Media access: full-duplex
The original Ethernet could only operate in half-duplex mode using CSMA/CD. Later
versions of Ethernet (100BASE-T and up) can also operate in full-duplex mode, in which
case the CSMA/CD mechanism is switched off. Full-duplex means a node can transmit
246
Practical Industrial Data Communications
and receive simultaneously and whenever it wants, since there is no possibility of
contention. Full-duplex operation is specified in IEEE 802.3x.
The IEEE 802.3x supplement also describes an optional set of mechanisms used for flow
control over full-duplex links, called MAC control and PAUSE. These are unfortunately
beyond the scope of this document.
The following requirements must be met for full-duplex operation.
• The media must have separate transmit and receive paths. This is typically true
for twisted-pair and fiber-optic links.
• There must only two stations on a segment. As a result of this, there can never
be any collisions as the two stations can talk and listen simultaneously, with
their transmit and receive ports cross connected (as in a null modem). Each port
of a switching hub exists in its own segment, so if only one station is connected
to each switch port, this condition is satisfied.
• Both stations must be capable of, and must be set up for, full-duplex operation.
11a.3.3
Auto-negotiation
The specifications for auto-negotiation were published in 1995 as part of the 802.3u Fast
Ethernet supplement to the IEEE standard, based on the nWay system developed by
National Semiconductor.
Auto-negotiation can be supported on all Ethernet systems using twisted-pair cabling,
as well as the Gigabit Ethernet fiber-optic system. It enables devices to set to a mutually
acceptable speed and operating mode (full-duplex/half-duplex). Auto-negotiation
includes the following concepts.
11a.3.4
Fiber-optic cable distances 100BASE-FX
The following maximum cable distances apply:
• Node to hub: Maximum distance of multi-mode cable (62.5/125) is 160 m
(for connections using a single Class II hub).
• Node to switch: Maximum multi-mode cable distance is 210 m.
• Switch to switch: Maximum distance of multi-mode cable for a backbone
connection between two 100BASE-FX switch ports is 412 m.
• Switch to switch full-duplex: Maximum distance of multi-mode cable for a
full-duplex connection between two 100BASE-FX switch ports is 2000 m.
The IEEE has not included the use of single-mode fiber in the 802.3u standard.
However numerous vendors have products available enabling switch-to-switch distances
of up to 10–20 km (6–12 miles) using single-mode fiber.
11a.3.5
100BASE-T repeater rules
The cable distance and the number of repeaters (hubs), which can be used in a 100BASE-T
collision domain, depends on the delay in the cable and the time delay in the repeaters and
NIC delays. The maximum round-trip delay for 100BASE-T systems is the time to transmit
64 bytes or 512 bits, and equals 5.12 µs. A frame has to go from the transmitter to the most
remote node then back to the transmitter for collision detection within this round trip time.
Therefore the one-way time delay will be half of this.
Industrial Ethernet overview 247
The maximum sized collision domain can then be determined by the following
calculation:
Repeater delays + Cable delays + NIC delays + Safety factor (5 bits minimum) <2.048 µs
Table 11a.6 gives typical maximum one-way delays for various components. Repeater
and NIC delays for specific components can be obtained from the manufacturer.
Component
Fast Ethernet NIC
Fast Ethernet switch port
Class I hub
Class II hub
UTP cable (per 100 m/330 ft)
Multi-mode fiber (per 100 m/330 ft)
Maximum Delay (µs)
0.25
0.25
0.7 max.
0.46 max.
0.55
0.50
Table 11a.6
Maximum one-way fast Ethernet component delays
Notes
• If the desired distance is too great it is possible to create a new collision domain
by using a switch instead of a repeater.
• Most 100BASE-T repeaters are stackable, which means multiple units can be
placed on top of one another and connected together by means of a fast
backplane bus, such connections do not count as a repeater hop and make the
ensemble function as a single repeater.
Sample calculation
The following calculation is made to confirm whether it is possible to connect two fast
Ethernet nodes together using two Class II hubs connected by 50 m fiber. One node is
connected to the first repeater with 50 m UTP while the other has a 100 m fiber connection.
Calculation: Using the time delays in Table 11a.6.
The total one-way delay of 2.445 µs is within the required interval (2.56 µs) and allows
at least 5 bits safety factor, so this connection is permissible.
11a.4
Gigabit Ethernet
11a.4.1
Introduction
1000BASE-X is the shorthand identifier for the Gigabit Ethernet system based on the 8B/10B
block encoding scheme adapted from the Fiber Channel networking standard, developed by
ANSI. 1000BASE-X includes 1000BASE-SX, 1000BASE-LX, and 1000BASE-CX.
• 1000BASE-SX is the short wavelength fiber version.
• 1000BASE-LX is the long wavelength fiber version.
• 1000BASE-CX is a short copper cable version, based on the Fiber Channel
standard.
1000BASE-T, on the other hand, is a 1000 Mbps version capable of operating over Cat5
(or better, such as Cat5e) UTP. This system is based on a different encoding scheme.
248
Practical Industrial Data Communications
As with fast Ethernet, Gigabit Ethernet supports full-duplex and auto-negotiation. It
uses the same IEEE 802.3 frame format as 10 and 100 Mbps Ethernet systems. This
operates at ten times the clock speed of Fast Ethernet at 1 Gbps. By retaining the same
frame format as the earlier versions of Ethernet, backward compatibility is assured.
Despite the similar frame format, the system had to undergo a small change to enable it to
function effectively at 1 Gbps. The slot time of 64 bytes used with both 10 and 100 Mbps
systems has been increased to 512 bytes. Without this increased slot time the network
would have been impracticably small at one-tenth of the size of fast Ethernet – only 20
m! The slot time defines the time during which the transmitting node retains control of
the medium, and in particular is responsible for collision detection. With Gigabit Ethernet
it was necessary to increase this time by a factor of eight to 4.096 µs to compensate for
the tenfold speed increase. This then gives a collision domain of about 200 m (660 ft).
If the transmitted frame is less than 512 bytes the transmitter continues transmitting to
fill the 512 byte window. A carrier extension symbol is used to mark frames that are
shorter than 512 bytes and to fill the remainder of the frame.
11a.4.2
Gigabit Ethernet full-duplex repeaters
Gigabit Ethernet nodes are connected to full-duplex repeaters, also known as nonbuffered switches or buffered distributors. These devices have a basic MAC function in
each port, which enables them to verify that a complete frame is received and compute its
frame check sequence (CRC) to verify the frame validity. Then the frame is buffered in
the internal memory of the port before being forwarded to the other ports of the repeater.
It is therefore combining the functions of a repeater with some features of a switch.
All ports on the repeater operate at the same speed of 1 Gbps, and operate in full-duplex
so it can simultaneously send and receive from any port. The repeater uses IEEE 802.3x
flow control to ensure the small internal buffers associated with each port do not
overflow. When the buffers are filled to a critical level, the repeater tells the transmitting
node to stop sending until the buffers have been sufficiently emptied. The repeater does
not analyze the packet address fields to determine where to send the packet, like a switch
does, but simply sends out all valid packets to all the other ports on the repeater.
The IEEE does allow for half-duplex Gigabit repeaters – however none exist at this
time.
11a.4.3
Gigabit Ethernet design considerations
Fiber-optic cable distances
The maximum cable distances that can be used between the node and a full-duplex
1000BASE-SX and LX repeater depend mainly on the chosen wavelength, the type of
cable, and its bandwidth. The maximum transmission distances on multi-mode cable are
limited by the differential mode delay (DMD). The very narrow beam of laser light
injected into the multi-mode fiber results in a relatively small number of rays going
through the fiber core. These rays each have different propagation times because they are
going through differing lengths of glass by zigzagging through the core to a greater or
lesser extent. These pulses of light can cause jitter and interference at the receiver. This is
overcome by using a conditioned launch of the laser into the multi-mode fiber. This
spreads the laser light evenly over the core of the multimedia fiber so that the laser source
looks more like a light emitting diode (LED) source. This spreads the light in a large
number of rays across the fiber resulting in smoother spreading of the pulses, so less
interference. This conditioned launch is done in the 1000BASE-SX transceivers.
Table 11a.7 gives the maximum distances for full-duplex 1000BASE-X repeaters.
Industrial Ethernet overview 249
NIC
0.25 µs
50 m (165 ft) UTP
0.275 µs
Repeater class II
0.46 µs
50 m (165 ft) fiber
0.25 µs
Repeater class II
0.46 µs
100 m (330 ft) fiber
0.50 µs
NIC
0.25 µs
Total delay
2.445 µs
Table 11a.7
Maximum fiber distances for 1000BASE-X (full-duplex)
Gigabit repeater rules
The cable distance and the number of repeaters, which can be used in a half-duplex
1000BASE-T collision domain depends on the delay in the cable and the time delay in the
repeaters and NIC delays. The maximum round-trip delay for 1000BASE-T systems is
the time to transmit 512 bytes or 4096 bits and equals 4.096 µs. A frame has to go from
the transmitter to the most remote node then back to the transmitter for collision detection
within this round trip time. Therefore the one-way time delay will be half this.
The maximum sized collision domain can then be determined by the following
calculation:
Repeater delays + Cable delays + NIC delays
+ Safety factor (5 bits minimum) <2.048 µs
Table 11a.8 gives typical maximum one-way delays for various components. Repeater
and NIC delays for your specific components can be obtained from the manufacturer.
Wavelength
(nm)
850
850
850
850
1300
1300
1300
Cable Type
50/125 multi-mode
50/125 multi-mode
62.5/125 multi-mode
62.5/125 multi-mode
50/125 multi-mode
62.5/125 multi-mode
9/125 single-mode
Bandwidth
(MHz/km)
Attenuation
(dB/km)
Maximum
Distance
(m)
400
500
160
200
500
500
Infinite
3.25
3.43
160
200
2.32
1.0
0.4
500
550
220
275
550
550
5000
Table 11a.8
Maximum one-way Gigabit Ethernet component delays
These calculations give the maximum collision diameter for IEEE 802.3z half-duplex
Gigabit Ethernet systems. The maximum Gigabit Ethernet network diameters specified by
the IEEE are shown in Table 11a.9.
250
Practical Industrial Data Communications
Component
Gigabit NIC
Gigabit repeater
UTP cable (per 100 m)
Multi-mode fiber (per 100 m)
Maximum Delay (µs)
0.432
0.488
0.55
0.50
Table 11a.9
Maximum half-duplex Gigabit Ethernet network diameters
Note half-duplex Gigabit Ethernet repeaters are not available for sale. Use full-duplex
repeaters with the point-to-point cable distances between node and repeater or node and
switch as listed in Table 11a.7.
11a.5
Industrial Ethernet
11a.5.1
Introduction
Early Ethernet was not entirely suitable for control functions as it was primarily developed
for office-type environments. The Ethernet technology has, however, made rapid advances
over the past few years. It has gained such widespread acceptance in industry that it is
becoming the de facto field bus technology. An indication of this trend is the inclusion of
Ethernet as the Levels 1 and 2 infrastructure for Modbus/TCP (Schneider), Ethernet/IP
(Rockwell Automation and ODVA), ProfiNet (PNO), and Foundation Fieldbus HSE
(Fieldbus Foundation).
The following sections will deal with problems related to early Ethernet, and how they
have been addressed in subsequent upgrades.
11a.5.2
Connectors and cabling
Earlier industrial Ethernet systems such as the first-generation Siemens SimaticNet
(Sinec-H1) are based on the 10BASE-5 configuration, and thus the connectors involved
include the screw-type N-connectors and the D-type connectors, which are both fairly
rugged. The heavy-gage twin-screen (braided) RG-8 coaxial cable is also quite
impervious to electrostatic interference.
Most modern industrial Ethernet systems are, however, based on a 10BASE-T/100BASE-X
configuration and thus have to contend with RJ-45 connectors and (typically) Cat5
twisted-pair cable. Despite its inherent resistance to electromagnetically induced interference,
the UTP cable is not really suited to industrial applications and therefore the trend is towards
screened or shielded twisted pair such as Cat5i, or optic fiber. The connectors are problematic
as well. The RJ-45 connectors are everything but rugged and are suspect when subjected to
great temperature extremes, contact with oils and other fluids, dirt, UV radiation, EMI as well
as shock, vibration, and mechanical loading.
As in interim measure, some manufacturers started using D-type (known also as DB or
D-Submin) connectors, but currently there is a strong movement towards the M12-based
series of connectors. The M12 form factor, governed by IEC 61076, is widely accepted in
field bus and automation applications, and is supplied by vendors such as Lumberg, Turck,
Escha, InterlinkBT, and Hirschmann. Typical M12 connectors for Ethernet are of the 8-pole
variety, with threaded connectors. They can accept various types of Cat5/5e twisted-pair
wiring such as braided or shielded wire (solid or stranded), and offer excellent protection
against moisture, dust, corrosion, EMI, RFI, mechanical vibration and shock, UV radiation,
and extreme temperatures (–40 to 75 °C).
Industrial Ethernet overview 251
As far as the media is concerned, several manufacturers are producing Cat5 and Cat5e
wiring systems using braided or shielded twisted pairs. An example of an integrated
approach to Industrial Ethernet cabling is Lumberg’s EtherMATE system, which also
includes both the cabling and an M12 connector system.
11a.5.3
Deterministic vs stochastic operation
One of the most common complaints with early Ethernet was that it uses CSMA/CD
(a probabilistic or stochastic method) as opposed to other automation technologies such
as Fieldbus that uses deterministic access methods such as token passing or the publisher–
subscriber model. CSMA/CD essentially means that it is impossible to guarantee delivery
of a possibly critical message within a certain time. This could be due either to congestion
on the network (often due to other less critical traffic) or to collisions with other frames.
In office applications there is not much difference between 5 s and 500 ms, but in
industrial applications a millisecond counts. Industrial processes often require scans in a
5–20 ms range, and some demanding processes could even require 2–5 ms. On 10BASET Ethernet, for example, the access time on a moderately loaded 100-station network
could range from 10 to 100 ms, which is acceptable for office applications but not for
processes.
With Ethernet, the loading or traffic needs to be carefully analyzed to ensure that the
network is not overwhelmed at critical or peak operational times. While a typical
utilization factor on a commercial Ethernet LAN of 25–30% is acceptable, figures of less
than 10% utilization on an industrial Ethernet LAN are required. Most industrial networks
run at 3 or 4% utilization with a fairly large number of I/O points being transferred across
the system.
The advent of fast and Gigabit Ethernet, switching hubs, VLAN technology, full-duplex
operation, and deterministic Ethernet has effectively put this concern to rest for most
applications.
11a.5.4
Size and overhead of Ethernet frame
Data-link encoding efficiency is another problem, with the Ethernet frames taking up far
more space than for an equivalent Fieldbus frame. If the TCP/IP protocol is used in addition
to the Ethernet frame, the overhead increases dramatically. The efficiency of the overall
system is, however, more complex than simply the number of bytes on the transmitting
cable and issues such as raw speed on the cable and the overall traffic need to be examined
carefully. For example, if 2 bytes of data from an instrument had to be packaged in a
60 byte message (because of TCP/IP and Ethernet protocols being used) this would result in
an enormous overhead compared to a Fieldbus protocol. However if the communications
link was running at 100 Mbps or 1 Gbps with full-duplex communications this would put a
different light on the problem and make the overhead issue almost irrelevant.
11a.5.5
Noise and interference
Due to higher electrical noise near to the industrial LANs some form of electrical
shielding and protection is useful to minimize errors in the communication. A good
choice of cable is fiber optic (or sometimes coaxial cable). Twisted pair can be used but
care should be taken to route the cables far away from any potential sources of noise.
If twisted-pair cable is selected, a good decision is to use screened twisted-pair cable
(ScTP) rather than the standard UTP.
252
Practical Industrial Data Communications
In saying the above, it should be noted that Ethernet-based networks are installed in a
wide variety of systems and rarely have problems reported due to reliability issues. The
use of fiber ensures that there are minimal problems due to earth loops or electrical noise
and interference.
11a.5.6
Partitioning of the network
It is very important that the industrial network operates separately from that of the
commercial network, as speed of response and real-time operation are often critical
attributes of an industrial network. An office type network may not have the same
response requirements. In addition, security is another concern where the industrial
network is split off from the commercial networks so any problems in the commercial
network will not affect the industrial side.
Industrial networks are also often partitioned into individual sub-networks for reasons
of security and speed of response, by means of bridges and switches.
In order to reduce network traffic, some PLC manufacturers use exception reporting.
This requires only changes in the various digital and analog parameters to be transmitted
on the network. For example, if a digital point changes state (from on to off or off to on),
this change would be reported. Similarly, an analog value could have associated with it a
specified change of span before reporting the new analog value to the master station.
11a.5.7
Switching technology
Both the repeating hub and bridge technologies are being superseded by switching
technology. This allows traffic between two nodes on the network to be directly
connected in a full-duplex fashion. The nodes are connected through a switch with
extremely low levels of latency. Furthermore, the switch is capable of handling all the
ports communicating simultaneously with one another without any collisions. This means
that the overall speed of the switch backplane is considerably greater than the sum of the
speeds of the individual Ethernet ports.
Most switches operate at the data-link layer and are also referred to as switching hubs
and Layer 2 switches. Some switches can interpret the network layer addresses (e.g. the
IP address) and make routing decisions on that. These are known as Layer 3 switches.
Advanced switches can be configured to support virtual LANs. This allows the user to
configure a switch so that all the ports on the switch are subdivided into predefined
groups. These groups of ports are referred to as virtual LANs (VLANs) – a concept that is
very useful for industrial networks. Only switch ports allocated to the same VLAN can
communicate with each other.
Switches do have performance limitations that could affect a critical industrial
application. If there is traffic on a switch from multiple input ports aimed at one particular
output port, the switch may drop some of the packets. Depending on the vendor
implementation it may force a collision back to the transmitting device so that the
transmitting node backs off long enough for the congestion to clear. This means that the
transmitting node does not have a guarantee on the transmission between two nodes –
something that could impact a critical industrial application.
In addition, although switches do not create separate broadcast domains, each virtual
LAN effectively forms one (if this is enabled on the switch). An Ethernet broadcast
message received on one port is retransmitted onto all ports in the VLAN. Hence a switch
will not eliminate the problem of excessive broadcast traffic that can cause severe
performance degradation in the operation of the network. TCP/IP uses the Ethernet
broadcast frame to obtain MAC addresses and hence broadcasts are fairly prevalent here.
Industrial Ethernet overview 253
A problem with a switched network is that duplicate paths between two given nodes
could cause a frame to be passed around and around the ‘ring’ caused by the two
alternative paths. This possibility is eliminated by the ‘Spanning Tree’ algorithm, the
IEEE 802.1d standard for Layer 2 recovery. However, this method is quite slow and
could take from 2 to 5 s to detect and bypass a path failure and could leave all networked
devices isolated during the process. This is obviously unacceptable for industrial
applications.
A solution to the problem is to connect the switches in a dual redundant ring
topology, using copper or fiber. This poses a new problem since an Ethernet broadcast
message will be sent around the loop indefinitely. One vendor, Hirschmann, has solved
this problem by developing a switch with added redundancy management capabilities.
The redundancy manager allows a physical 200 Mbps ring to be created, by terminating
both ends of the traditional Ethernet bus in itself. Although the bus is now looped back
to itself, the redundancy manager logically breaks the loop, preventing broadcast
messages from wreaking havoc. Logically the redundancy manager behaves like two
nodes, sitting back to back, transmitting and receiving messages to the other around the
ring using IEEE 802.1p/Q frames. This creates a deterministic path through any IEEE
802.1p/Q compliant switches (up to 50) in the ring, which results in a real-time
‘awareness’ of the state of the ring.
When a network failure is detected (i.e. the loop is broken), the redundancy manager
interconnects the two segments attached to it, thereby restoring the loop. This takes place
in between 20 and 500 ms, depending on the size of the ring.
11a.5.8
Active electronics
With the hub-based Ethernet topologies (e.g. 10BASE-T, 100BASE-TX) the hub (be that
a repeating hub or a switching hub) is an active electronic device that requires power to
operate. If it fails this will terminate any communications occurring between nodes
connected.
To address this problem, some vendors have introduced the concept of dual-redundant
switch rings with the ability to reconfigure itself in a very short time (30 ms for small
networks to 600 ms for large networks) after a switch failure.
In addition, the power requirements for an Ethernet card (and other hardware
components) are often significantly more than an equivalent Fieldbus card (such as from
Foundation Fieldbus). There is limited provision within Ethernet for loop powering the
instrument through the Ethernet cable. This is called power over Ethernet (PoE) and is
often used to power wireless hubs on a network.
11a.5.9
Fast and Gigabit Ethernet
The recent developments in Ethernet technology are making it even more relevant in the
industrial market. Fast Ethernet as defined in the IEEE specification 802.3u is essentially
Ethernet running at 100 Mbps. The same frame structure, addressing scheme and
CSMA/CD access method are used as with the 10 Mbps standard. In addition, Fast Ethernet
can also operate in full-duplex mode as opposed to CSMA/CD, which means that there are
no collisions. Fast Ethernet operates at ten times the speed of that of standard IEEE 802.3
Ethernet. Video and audio applications can enjoy substantial improvements in performance
using Fast Ethernet. Smart instruments that require far smaller frame sizes will not see such
an improvement in performance. One area, however, where there may be significant
throughput improvements is in the area of collision recovery. The back-off times for
100 Mbps Ethernet will be a tenth that of standard Ethernet. Hence a heavily loaded
254
Practical Industrial Data Communications
network with considerable individual messages and nodes would see performance
improvements. If loading and collisions are not really an issue on the slower 10 Mbps
network then there will not be many tangible improvements in the higher LAN speed of
100 Mbps.
Note that with the auto-negotiation feature built into standard switches and many
Ethernet cards, the device can operate at either the 10 or 100 Mbps speeds. In addition the
category 5 wiring installed for 10BASE-T Ethernet would be adequate for the 100 Mbps
standard as well.
Gigabit Ethernet is another technology that could be used to connect instruments and
PLCs. However its speed would probably not be fully exploited by the instruments for the
reasons indicated above.
11a.5.10 TCP/IP and industrial systems
The TCP/IP suite of protocols provides for a common open protocol. In combination with
Ethernet this can be considered to be a truly open standard available to all users and
vendors. However, there are some problems at the application layer area. Although
TCP/IP implements four layers which are all open (network interface, Internet, host–host
and application layers), most industrial vendors still implement their own specific
application layer. Hence equipment from different vendors can coexists on the factory
shop floor but cannot inter-operate. Protocols such as MMS (manufacturing messaging
service) have been promoted as truly ‘open’ automation application layer protocols but
with limited acceptance to date.
11a.5.11 Industrial Ethernet architectures for high availability
There are several key technology areas involved in the design of Ethernet-based industrial
automation architecture. These include available switching technologies, quality of
service (QOS) issues, the integration of existing (legacy) field buses, sensor bus
integration, high availability and resiliency, security issues, long-distance communication
and network management – to name but a few.
For high availability systems a single network interface represents a single point of
Failure (SPOF) that can bring the system down. There are several approaches that can be
used on their own or in combination, depending on the amount of resilience required (and
hence the cost). The cost of the additional investment in the system has to be weighed
against the costs of any downtime.
For a start, the network topology could be changed to represent a switched ring. This
necessitates the use of a special controlling switch (redundancy manager) and protects the
system against a single failure on the network. It does not, however, guard against a
failure of the network interface on one of the network devices.
The next level of resiliency would necessitate two network interfaces on the controller
(i.e. changing it to a dual-homed system), each one connected to a different switch. This
setup would be able to tolerate both a single network failure AND a network interface
failure.
Ultimately one could protect the system against a total failure by duplicating the
switched ring, connecting each port of the dual-homed system to a different ring.
Other factors supporting a high degree of resilience would include hot swappable
switches and NICs, dual redundant power supplies, and on-line diagnostic software.
Industrial Ethernet troubleshooting
11b.1
Introduction
This section deals with addressing common faults on Ethernet networks. Ethernet
encompasses layers 1 and 2, namely the physical and data-link layers of the OSI model.
This is equivalent to the bottom layer (the network interface layer) in the ARPA (DoD)
model. This section will focus on those layers only, as well as on the actual medium over
which the communication takes place.
11b.2
Common problems and faults
Ethernet hardware is fairly simple and robust, and once a network has been commissioned,
providing the cabling has been done professionally and certified, the network should be
fairly trouble-free.
Most problems will be experienced at the commissioning phase, and could theoretically
be attributed to either the cabling, the LAN devices (such as hubs and switches), the
network interface cards (NICs) or the protocol stack configuration on the hosts.
The wiring system should be installed and commissioned by a certified installer (the
suppliers of high-speed Ethernet cabling systems, such as ITT, will not guarantee their
wiring if not installed by a certified installer). This effectively rules out wiring problems
for new installations.
If the LAN devices such as hubs and switches are from reputable vendors, it is highly
unlikely that they will malfunction in the beginning. Care should nevertheless be taken to
ensure that intelligent (managed) hubs and switches are correctly set up.
The same applies to NICs. NICs rarely fail and nine times out of ten the problem lies
with a faulty setup or incorrect driver installation, or an incorrect configuration of the
higher level protocols such as IP.
11b.3
Tools of the trade
Apart from a fundamental understanding of the technologies involved the following tools
are helpful in isolating Ethernet-related problems, sufficient time, a pair of eyes, patience,
and many cups of coffee/tea.
11b.3.1
Multimeters
A simple multimeter can be used to check for continuity and cable resistance, as will be
explained in this section.
256
Practical Industrial Data Communications
11b.3.2
Handheld cable testers
There are many versions on the market, ranging from simple devices that basically check
for wiring continuity to sophisticated devices that comply with all the prerequisites for
1000BASE-T wiring infrastructure tests (see Figure 11b.1). Testers are available from
several vendors such as MicroTest, Fluke, and Scope.
Figure 11b.1
OMNIscanner2 cable tester
11b.3.3
Fiber-optic cable testers
Fiber-optic testers are simpler than UDP testers, since they basically only have to
measure continuity and attenuation loss. Some UDP testers can be turned into fiber-optic
testers by purchasing an attachment that fits onto the existing tester. For more complex
problems such as finding the location of a damaged section on a fiber-optic cable, an
alternative is to use a proper optical time domain reflectometer (OTDR) but these are
expensive instruments and it is often cheaper to employ the services of a professional
wire installer (with his own OTDR) if this is required.
11b.3.4
Traffic generators
A traffic generator is a device that can generate a pre-programed data pattern on the
network. Although they are not strictly speaking used for fault finding, they can be used
to predict network behavior due to increased traffic, for example, when planning network
changes or upgrades. Traffic generators can be stand-alone devices or they can be
integrated into hardware LAN analyzers such as the Hewlett Packard 3217.
11b.3.5
RMON probes
An RMON (Remote MONitoring) probe is a device that can examine a network at a
given point and keep track of captured information at a detailed level. The advantage of a
RMON probe is that it can monitor a network at a remote location. The data captured by
the RMON probe can then be uploaded and remotely displayed by the appropriate RMON
management software. RMON probes and the associated management software are
available from several vendors such as 3COM, Bay Networks and NetScout. It is also
possible to create an RMON probe by running commercially available RMON software
on a normal PC, although the data collection capability will not be as good as that of a
dedicated RMON probe.
Industrial Ethernet troubleshooting
11b.3.6
257
Handheld frame analyzers
Handheld frame analyzers are manufactured by several vendors such as Fluke, Scope,
Finisar and PsiberNet, for up to Gigabit Ethernet speeds. These little devices can perform
link testing, traffic statistics gathering, etc. and can even break down frames by protocol
type. The drawback of these testers is the small display and the lack of memory, which
results in a lack of historical or logging functions on these devices.
An interesting feature of some probes is that they are non-intrusive, that is, they simply
clamp on to the wire and do not have to be attached to a hub port (see Figure 11b.2).
Figure 11b.2
Clamp-on Gigabit Ethernet probe
11b.3.7
Software protocol analyzers
Software protocol analyzers are software packages running on PCs and using either a
general purpose or a specialized NIC to capture frames from the network. The NIC is
controlled by a so-called promiscuous driver, which enables the NIC to capture all
packets on the medium and not only those addressed to it in broadcast or unicast mode.
There are excellent freeware protocol analyzers such as Ethereal or Analyzer, the latter
being developed at the Polytechnic of Torino in Italy. Top range software products such
as Network Associates’ ‘Sniffer’ or WaveTek Wandel Goltemann’s ‘Domino’ suite have
sophisticated expert systems that can aid in the analysis of the captured software, but
unfortunately these products come at a higher price.
11b.3.8
Hardware BASE-d protocol analyzers
Several manufacturers such as Hewlett Packard, Network Associates and WaveTek
Wandel Goltemann also supply hardware BASE-d protocol analyzers using their protocol
analysis software running on a proprietary hardware infrastructure. This makes them very
expensive but dramatically increases the power of the analyzer. For fast and Gigabit
Ethernet, this is probably the better approach.
11b.4
Problems and solutions
11b.4.1
Noise
If excessive noise is suspected on a coax or UTP cable, an oscilloscope can be connected
between the signal conductor(s) and ground. This method will show up noise on the
conductor, but will not necessarily give a true indication of the amount of power in the
noise. A simple and cheap method to pick up noise on the wire is to connect a small
258
Practical Industrial Data Communications
loudspeaker between the conductor and ground. A small operational amplifier can be
used as an input buffer, so as not to ‘load’ the wire under observation. The noise will be
heard as an audible signal.
The quickest way to get rid of a noise problem, apart from using screened UTP (ScTP),
is to change to a fiber-BASE-d instead of a copper-BASE-d network, for example, by
using 100BASE-FX instead of 100BASE-TX.
Noise can to some extent be counteracted on a coax-BASE-d network by earthing the
screen AT ONE END ONLY. Earthing it on both sides will create an earth loop. This is
normally accomplished by means of an earthing chain or an earthing screw on one of the
terminators. Care should also be taken not to allow contact between any of the other
connectors on the segment and ground.
11b.4.2
Thin coax problems
Incorrect cable type
The correct cable for thin Ethernet is RG58-A/U or RG-58C/U. This is a 5 mm diameter
coaxial cable with 50
characteristic impedance and a stranded center conductor.
Incorrect cable used in a thin Ethernet system can cause reflections, resulting in CRC
errors, and hence many retransmitted frames.
The characteristic impedance of coaxial cable is a function of the ratio between the
center conductor diameter and the screen diameter. Hence other types of coax may
closely resemble RG-58, but may have different characteristic impedance.
Loose connectors
The BNC coaxial connectors used on RG-58 should be of the correct diameter, and
should be properly crimped onto the cable. An incorrect size connector or a poor crimp
could lead to intermittent contact problems, which are very hard to locate. Even worse is
the ‘Radio Shack’ hobbyist type screw-on BNC connector that can be used to quickly
make up a cable without the use of a crimping tool. These more often not lead to very
poor connections. A good test is to grip the cable in one hand, and the connector in
another, and pull very hard. If the connector comes off, the connector mounting
procedures need to be seriously reviewed.
Excessive number of connectors
The total length of a thin Ethernet segment is 185 m and the total number of stations on
the segment should not exceed 30. However, each station involves a BNC T-piece plus
two coax connectors and there could be additional BNC barrel connectors joining the
cable. Although the resistance of each BNC connector is small, they are still finite and
can add up. The total resistance of the segment (cable plus connectors) should not exceed
10 otherwise problems can surface.
An easy method of checking the loop resistance (the resistance to the other end of the
cable and back) is to remove the terminator on one end of the cable and measure the
resistance between the connector body and the center contact. The total resistance equals
the resistance of the cable plus connectors plus the terminator on the far side. This should
be between 50 and 60 . Anything more than this is indicative of a problem.
Overlong cable segments
The maximum length of a thin net segment is 185 m. This constraint is not imposed by
collision domain considerations, but rather by the attenuation characteristics of the cable.
Industrial Ethernet troubleshooting
259
If it is suspected that the cable is too long, its length should be confirmed. Usually, the
cable is within a cable trench and hence it cannot be visually measured. In this case, a
time domain reflectometer (TDR) can be used to confirm its length.
Stub cables
For thin Ethernet (10BASE-2), the maximum distance between the bus and the
transceiver electronics is 4 cm. In practice, this is taken up by the physical connector plus
the PC board tracks leading to the transceiver, which means that there is no scope for a
drop cable or ‘stub’ between the NIC and the bus. The BNC T-piece has to be mounted
directly on to the NIC.
Users might occasionally get away with putting a short stub between the T-piece and
the NIC, but this invariably leads to problems in the long run.
Incorrect terminations
10BASE-2 is designed around 50 coax and hence requires a 50 terminator at each end.
Without the terminators in place, there would be so many reflections from each end that the
network would collapse. A slightly incorrect terminator is better than no terminator, yet
may still create reflections of such magnitude that it affects the operation of the network.
A 93 terminator looks no different than a 50 terminator; therefore it should not be
automatically assumed that a terminator is of the correct value.
If two 10BASE-2 segments are joined with a repeater, the internal termination on the
repeater can be mistakenly left enabled. This leads to three terminators on the segment,
creating reflections and hence affecting the network performance.
The easiest way to check for proper termination is by alternatively removing the
terminators at each end, and measuring the resistance between connector body and center
pin. In each case, the result should be 50–60 . Alternatively, one of the T-pieces in the
middle of the segment can be removed from its NIC and the resistance between the
connector body and the center pin measured. The result should be the value of the two
half cable segments (including terminators) in parallel, that is, 25–30 .
Invisible insulation damage
If the internal insulation of coax is inadvertently damaged, for example, by placing a
heavy point load on the cable, the outer cover could return to its original shape whilst
leaving the internal dielectric deformed. This leads to a change of characteristic
impedance at the damaged point resulting, in reflections. This, in turn, could lead to
standing waves being formed on the cable.
An indication of this problem is when a work station experiences problems when attached
to a specific point on a cable, yet functions normally when moved a few meters to either
side. The only solution is to remove the offending section of the cable. Because of the
nature of the damage, it cannot be seen by the naked eye and the position of the damage has
to be located with a TDR. Alternatively, the whole cable segment has to be replaced.
Invisible cable break
This problem is similar to the previous one, with the difference that the conductor has
been completely severed at a specific point. Despite the terminators at both ends of the
cable, the cable break effectively creates two half segments, each with an un-terminated
end, and hence nothing will work.
The only way to discover the location of the break is by using a TDR.
260
Practical Industrial Data Communications
11b.4.3
Thick coax problems
Thick coax (RG-8), as used for 10BASE-5 or thick Ethernet, will basically exhibit the
same problems as thin coax yet there are a few additional complications.
Loose connectors
10BASE-5 use N-type male screw on connectors on the cable. As with BNC connectors,
incorrect procedures or a wrong sized crimping tool can cause sloppy joints. This can lead
to intermittent problems that are difficult to locate.
Again, a good test is to grab hold of the connector and to try and rip it off the cable with
brute force. If the connector comes off, then it was not properly installed in the first place.
Dirty taps
The MAU transceiver is often installed on a thick coax by using a vampire tap, which
necessitates pre-drilling into the cable in order to allow the center pin of the tap to contact
the center conductor of the coax. The hole has to go through two layers of braided screen
and two layers of foil. If the hole is not properly cleaned pieces of the foil and braid can
remain and cause short circuits between the signal conductor and ground.
Open tap holes
When a transceiver is removed from a location on the cable, the abandoned hole should
be sealed. If not, dirt or water could enter the hole and create problems in the long run.
Tight cable bends
The bend radius on a thick coax cable may not exceed 10 in. If it does, the insulation can
deform to such an extent that reflections are created leading to CRC errors. Excessive
cable bends can be detected with a TDR.
Excessive loop resistance
The resistance of a cable segment may not exceed 5 . As in the case of thin coax, the
easiest way to do this is to remove a terminator at one end and measure the loop
resistance. It should be in a range of 50–55 .
11b.4.4
UTP problems
The most commonly used tool for UTP troubleshooting is a cable meter or pair scanner.
At the bottom end of the scale a cable tester can be an inexpensive tool, only able to
check for the presence of wire on the appropriate pins of a RJ-45 connector. High-end
cable testers can also test for noise on the cable, cable length, and crosstalk (such as near
end signal crosstalk or NEXT) at various frequencies. It can check the cable against
CAT5/5e specifications and can download cable test reports to a PC for subsequent
evaluation.
The following is a description of some wiring practices that can lead to problems.
Incorrect wire type (solid/stranded)
Patch cords must be made with stranded wire. Solid wire will eventually suffer from
metal fatigue and crack right at the RJ-45 connector, leading to permanent or intermittent
open connection/s. Some RJ-45 plugs, designed for stranded wire, will actually cut
Industrial Ethernet troubleshooting
261
through the solid conductor during installation, leading to an immediate open connection.
This can lead to CRC errors resulting in slow network performance, or can even disable a
workstation permanently. The length of the patchcords or flyleads on either end of the
link may not exceed 5 m, and these cables must be made from stranded wire.
The permanently installed cable between hub and workstation, on the other hand,
should not exceed 90 m and must be of the solid variety. Not only is stranded wire more
expensive for this application, but the capacitance is higher, which may lead to a
degradation of performance.
Incorrect wire system components
The performance of the wire link between a hub and a workstation is not only dependent
on the grade of wire used, but also on the associated components such as patch panels,
surface mount units (SMUs) and RJ-45 type connectors. A single substandard connector
on a wire link is sufficient to degrade the performance of the entire link.
High quality fast and Gigabit Ethernet wiring systems use high-grade RJ-45 connectors
that are visibly different from standard RJ-45 type connectors.
Incorrect cable type
Care must be taken to ensure that the existing UTP wiring is of the correct category for
the type of Ethernet being used. For 10BASE-T, Cat3 UTP is sufficient, while fast
Ethernet (100BASE-T) requires Cat5 and Gigabit Ethernet requires Cat5e or better. This
applies to patch cords as well as the permanently installed (‘infrastructure’) wiring.
Most industrial Ethernet systems nowadays are 100BASE-X based and hence use Cat5
wiring. For such applications, it might be prudent to install screened Cat5 wiring (ScTP)
for better noise immunity. ScTP is available with a common foil screen around the 4 pairs
or with an individual foil screen around each pair.
A common mistake is to use telephone grade patch (‘silver satin’) cable for the
connection between an RJ-45 wall socket (SMU) and the network interface card in a
computer. Telephone patch cables use very thin wires that are untwisted, leading to high
signal loss and large amounts of crosstalk. This will lead to signal errors causing
retransmission of lost packets, which will eventually slow the network down.
Straight vs crossover cable
A 10BASE-T or 100BASE-TX patch cable consists of 4 wires (two pairs) with an RJ-45
connector at each end. The pins used for the TX and RX signals are typically 1, 2 and
3, 6. Although a typical patch cord has 8 wires (4 pairs), the 4 unused wires are
nevertheless crimped into the connector for mechanical strength. In order to facilitate
communication between computer and hub, the TX and RX ports on the hub are reversed,
so that the TX on the computer and the RX on the hub are interconnected whilst the TX
on the hub is connected to the RX on the hub. This requires a ‘straight’ interconnection
cable with pin 1 wired to pin 1, pin 2 wired to pin 2, etc.
If the NICs on two computers are to be interconnected without the benefit of a hub, a
normal straight cable cannot be used since it will connect TX to TX and RX to RX. For
this purpose, a crossover cable has to be used, in the same way as a ‘null’ modem cable.
Crossover cables are normally color coded (for example, green or black) in order to
differentiate them from straight cables.
A crossover cable can create problems when it looks like a normal straight cable, an
unsuspecting person may use it to connect a NIC to a hub or a wall outlet. A quick way to
262
Practical Industrial Data Communications
identify a crossover cable is to hold the two RJ-45 connectors side by side and observe
the colors of the 8 wires in the cable through the clear plastic of the connector body. The
sequence of the colors should be the same for both connectors.
Hydra cables
Some 10BASE-T hubs feature 50 pin connectors to conserve space on the hub.
Alternatively, some building wire systems use 50 pin connectors on the wiring panels but
the hub equipment has RJ-45 connectors. In both cases, hydra or octopus cable has to be
used. This consists of a 50 pin connector connected to a length of 25 pair cable, which is
then broken out as a set of 12 small cables, each with a RJ-45 connector. Depending on the
vendor the 50 pin connector can be attached through locking clips, Velcro strips or screws.
It does not always lock down properly, although at a glance it may seem so. This can cause
a permanent or intermittent break of contact on some ports.
For 10BASE-T systems, near end crosstalk (NEXT), which occurs when a signal is
coupled from a transmitting wire pair to a receiving wire pair close to the transmitter,
(where the signal is strongest) causes most problems. On a single 4 pair cable, this is not a
serious problem, as only two pairs are used but on the 25 pair cable, with many signals in
close proximity, this can create problems. This can be very difficult to troubleshoot since
it will require test equipment that can transmit on all pairs simultaneously.
Excessive untwists
On Cat5 cable, crosstalk is minimized by twisting each cable pair. However, in order to
attach a connector at the end the cable has to be untwisted slightly. Great care has to be
taken since excessive untwists (more than 1 cm) is enough to create excessive crosstalk,
which can lead to signal errors. This problem can be detected with a high quality cable
tester.
Stubs
A stub cable is an abandoned telephone cable leading from a punch-down block to some
other point. This does not create a problem for telephone systems, but if the same Cat3
telephone cabling is used to support 10BASE-T, then the stub cables may cause signal
reflections that result in bit errors. Again, a high quality cable tester only will be able to
detect this problem.
Damaged RJ-45 connectors
On RJ-45 connectors without protective boots, the retaining clip can easily break off
especially on cheaper connectors made of brittle plastic. The connector will still mate
with the receptacle but will retract with the least amount of pull on the cable, thereby
breaking contact. This problem can be checked by alternatively pushing and pulling on
the connector and observing the LED on the hub, media coupler or NIC – wherever the
suspect connector is inserted. Because of the mechanical deficiencies of ‘standard’ RJ-45
connectors they are not commonly used on Industrial Ethernet systems.
T4 on 2 pairs
100BASE-TX is a direct replacement for 10BASE-T in that it uses the same 2 wire pairs
and the same pin allocations. The only prerequisite is that the wiring must be Cat5.
100BASE-T4, however, was developed for installations where all the wiring is
Cat3, and cannot be replaced. It achieves its high speed over the inferior wire by using
Industrial Ethernet troubleshooting
263
all 4 pairs instead of just 2. In the event of deploying 100BASE-T4 on a Cat3 wiring
infrastructure, a cable tester has to be used to ensure that in fact, all 4 pairs are available
for each link and have acceptable crosstalk.
100BASE-T4 required the development of a new physical layer technology, as opposed to
100BASE-TX/FX that used existing FDDI technology. It therefore only became
commercially available a year after 100BASE-X, and never gained real market acceptance.
As a result, very few users will actually be faced with this problem.
11b.4.5
Fiber-optic problems
Since fiber does not suffer from noise, interference and crosstalk problems there are
basically only two issues to contend with namely attenuation and continuity.
The simplest way of checking a link is to plug each end of the cable into a fiber hub,
NIC or fiber-optic transceiver. If the cable is OK, the LEDs at each end will light up.
Another way of checking continuity is by using an inexpensive fiber-optic cable tester
consisting of a light source and a light meter to test the segment.
More sophisticated tests can be done with an optical time domain reflectometer
(OTDR). OTDRs can not only measure losses across a fiber link, but can also determine
the nature and location of the losses. Unfortunately, they are very expensive but most
professional cable installers will own one.
10BASE-FX and 100BASE-FX use LED transmitters that are not harmful to the eyes,
but Gigabit Ethernet use laser devices that can damage the retina of the eye. It is therefore
dangerous to try and stare into the fiber (all systems are infrared and therefore invisible
anyway!).
Incorrect connector installation
Fiber-optic connectors can propagate light even if the two connector ends are not
touching each other. Eventually, the gap between the fiber ends may be so far apart that
the link stops working. It is therefore imperative to ensure that the connectors are
properly latched.
Dirty cable ends
Because of the small fiber diameter (8–62 microns) and the low light intensity, a speck of
dust or some finger oil deposited by touching the connector end is sufficient to affect
communication. For this reason, dust caps must be left in place when the cable is not in
use and a fiber-optic cleaning pad must be used to remove dirt and oils from the
connector point before installation.
Component aging
The amount of power that a fiber-optic transmitter can radiate diminishes during the
working life of the transmitter. This is taken into account during the design of the link but
in the case of a marginal design, the link could start failing intermittently towards the end
of the design life of the equipment. A fiber-optic power meter can be used to confirm the
actual amount of loss across the link but an easy way to trouble shoot the link is to
replace the transceivers at both ends of the link with new ones.
264
Practical Industrial Data Communications
11b.4.6
AUI problems
Excessive cable length
The maximum length of the AUI cable is 50 m but this assumes that the cable is a proper
IEEE 802.3 cable. Some installations use lightweight office grade cables that are limited
to 12 m in length. If these cables are too long, the excessive attenuation can lead to
intermittent problems.
DIX latches
The DIX version of the 15 pin D-connector uses a sliding latch. Unfortunately, not all
vendors adhere to the IEEE 802 specifications and some use lightweight latch hardware
which results in a connector that can very easily become unstuck. There are basically two
solutions to the problem. The first solution is to use a lightweight (office grade) AUI
cable, providing that distance is not a problem. This places less stress on the connector.
The second solution is to use a special plastic retainer such as the ‘ET Lock’ made
specifically for this purpose.
SQE test
The signal quality error (SQE) test signal is used on all AUI BASE-d equipment to test
the collision circuitry. This method is only used on the old 15 pin AUI BASE-d external
transceivers (MAUs) and sends a short signal burst (about 10 bit times in length) to the
NIC just after each frame transmission. This tests both the collision detection circuitry
and the signal paths. The SQE operation can be observed by means of an LED on the
MAU.
The SQE signal is only sent from the transceiver to the NIC and not on to the network
itself. It does not delay frame transmissions but occurs during the interframe gap and is
not interpreted as a collision.
The SQE test signal must, however, be disabled if an external transceiver (MAU) is
attached to a repeater hub. If this is not done the hub will detect the SQE signal as a
collision and will issue a jam signal. As this happens after each packet, it can seriously
delay transmissions over the network. The problem is that it is not possible to detect this
with a protocol analyzer.
11b.4.7
NIC problems
Basic card diagnostics
The easiest way to check if a particular NIC is faulty is to replace it with another
(working) NIC. Modern NICs for desktop PC’s usually have auto-diagnostics included
and these can be accessed, for example, from the device manager in Windows Some
cards can even participate in a card-to-card diagnostic. Provided there are two identical
cards, one can be set up as an initiator and one as a responder. Since the two cards will
communicate at the data-link level, the packets exchanged will, to some extent, contribute
to the network traffic but will not affect any other devices or protocols present on the
network.
The drivers used for card auto-diagnostics will usually conflict with the NDIS and ODI
drivers present on the host, and a message is usually generated, advising the user that the
Windows drivers will be shut down, or that the user should re-boot in DOS.
With PCMCIA cards, there is an additional complication in that the card diagnostics
will only run under DOS, but under DOS the IRQ (interrupt address) of the NIC typically
Industrial Ethernet troubleshooting
265
defaults to 5, which happens to be the IRQ for the sound card! Therefore the diagnostics
will usually pass every test, but fail on the IRQ test. This result can then be ignored safely
if the card passes the other diagnostics. If the card works, it works!
Incorrect media selection
Some cards support more than one medium, for example, 10BASE-2/10BASE-5, or
10BASE-5/10BASE-T, or even all three. It may then happen that the card fails to operate
since it fails to ‘see’ the attached medium.
It is imperative to know how the selection is done. Modern cards usually have an autodetect function but this only takes place when the machine is booted up. It does NOT
re-detect the medium if it is changed afterwards. If the connection to a machine is
therefore changed from 10BASE-T to 10BASE-2, for example, the machine has to be rebooted.
Some older cards need to have the medium set via a setup program, whilst even older
cards have DIP switches on which the medium has to be selected.
Wire hogging
Older interface cards find it difficult to maintain the minimum 9.6 µs inter frame spacing
(IFS) and as a result of this, nodes tend to return to and compete for access to the bus
in a random fashion. Modern interface cards are so fast that they can sustain the
minimum 9.6 µs IFS rate. As a result of this, it becomes possible for a single card to gain
repetitive sequential access to the bus in the face of slower competition and hence
‘hogging’ the bus.
With a protocol analyzer, this can be detected by displaying a chart of network
utilization vs time and looking for broad spikes above 50%. The solution to this problem
is to replace shared hubs with switched hubs and increase the bandwidth of the system by
migrating from 10 to 100 Mbps, for example.
Jabbers
A jabber is a faulty NIC that transmits continuously. NICs have a built-in jabber control
that is supposed to detect a situation whereby the card transmits frames longer than the
allowed 1518 bytes, and shut the card down. However, if this does not happen, the
defective card can bring the network down. This situation is indicated by a very high
collision rate coupled with a very low or nonexistent data transfer rate. A protocol
analyzer might not show any packets, since the jabbering card is not transmitting any
sensible data. The easiest way to detect the offending card is by removing the cables from
the NICs or the hub one by one until the problem disappears in which case the offending
card has been located.
Faulty CSMA/CD mechanism
A card with a faulty CSMA/CD mechanism will create a large number of collisions since
it transmits legitimate frames but does not wait for the bus to be quiet before transmitting.
As in the previous case, the easiest way to detect this problem is to isolate the cards one
by one until the culprit is detected.
Too many nodes
A problem with CSMA/CD networks is that the network efficiency decreases as the
network traffic increases. Although Ethernet networks can theoretically utilize well over
266
Practical Industrial Data Communications
90% of the available bandwidth, the access time of individual nodes increase dramatically
as network loading increases. The problem is similar to that encountered on many urban
roads during peak hours. During rush hours, the traffic approaches the design limit of the
road. This does not mean that the road stops functioning. In fact, it carries a very large
number of vehicles, but to get into the main traffic from a side road becomes problematic.
For office type applications, an average loading of around 30% is deemed acceptable
while for industrial applications 3% is considered the maximum. Should the loading of
the network be a problem, the network can be segmented using switches instead of shared
hubs. In many applications, it will be found that the improvement created by changing
from shared to switched hubs, is larger than the improvement to be gained by upgrading
from 10 Mbps to fast Ethernet.
Improper packet distribution
Improper packet distribution takes place when one or more nodes dominate most of the
bandwidth. This can be monitored by using a protocol analyzer and checking the source
address of individual packets. Another way of easily checking this is by using the NDG
software Web Boy facility and checking the contribution of the top 10 transmitters.
Nodes like this are typically performing tasks such as video-conferencing or database
access, which require a large bandwidth. The solution to the problem is to give these
nodes separate switch connections or to group them together on a faster 100BASE-T or
1000BASE-T segment.
Excessive broadcasting
A broadcast packet is intended to reach all the nodes in the network and is sent to a MAC
address of ff-ff-ff-ff-ff-ff. Unlike routers, bridges and switches forward the broadcast
packets throughout the network and therefore cannot contain the broadcast traffic. Too
many simultaneous broadcast packets can degrade network performance.
In general, it is considered that if broadcast packets exceed 5% of the total traffic on the
network, it would indicate a broadcast overload problem. Broadcasting is a particular
problem with Netware servers and networks using NetBIOS/NetBEUI. Again, it is fairly
easy to observe the amount of broadcast traffic using the WebBoy utility.
A broadcast overload problem can be addressed by adding routers, layer 3 switches or
VLAN switches with broadcast filtering capabilities.
Bad packets
Bad packets can be caused by poor cabling infrastructure, defective NICs, external noise,
or faulty devices such as hubs, devices or repeaters. The problem with bad packets is that
they cannot be analyzed by software protocol analyzers.
Software protocol analyzers obtain packets that have already been successfully received
by the NIC. That means they are one level removed from the actual medium on which the
frames exist and hence cannot capture frames that are rejected by the NIC. The only
solution to this problem is to use a software protocol analyzer that has a special custom
NIC, capable of capturing information regarding packet deformities or by using a more
expensive hardware protocol analyzer.
Faulty packets include:
Runts Runt packets are shorter than the minimum 64 bytes and are typically created by a
collision taking place during the slot time.
Industrial Ethernet troubleshooting
267
As a solution, try to determine whether the frames are collisions or under-runs. If they
are collisions, the problem can be addressed by segmentation through bridges and
switches. If the frames are genuine under-runs, the packet has to be traced back to the
generating node that is obviously faulty.
CRC errors CRC errors occur when the CRC check at the receiving end does not match
the CRC check sum calculated by the transmitter.
As a solution, trace the frame back to the transmitting node. The problem is either
caused by excessive noise induced into the wire, corrupting some of the bits in the
frames, or by a faulty CRC generator in the transmitting node.
Late collisions Late collisions are typically caused when the network diameter exceeds
the maximum permissible size. This problem can be eliminated by ensuring that the
collision domains are within specified values, that is, 2500 m for 10 Mbps Ethernet,
250 m for fast Ethernet and 200 m for Gigabit Ethernet.
Check the network diameter as outlined above by physical inspection or by using a
TDR. If that is found to be a problem, segment the network by using bridges or switches.
Misaligned frames Misaligned frames are frames that get out of sync by a bit or two,
due to excessive delays somewhere along the path or frames that have several bits
appended after the CRC check sum.
As a solution, try and trace the signal back to its source. The problem could have been
introduced anywhere along the path.
Faulty auto-negotiation
Auto-negotiation is specified for
•
•
•
•
•
10BASE-T
100BASE-TX
100BASE-T2
100BASE-T4 and
1000BASE-T.
It allows two stations on a link segment (a segment with only two devices on it) e.g. an
NIC in a computer and a port on a switching hub to negotiate a speed (10/100/1000 Mbps)
and an operating mode (full-/half-duplex). If auto-negotiation is faulty or switched off on
one device, the two devices might be set for different operating modes and as a result, they
will not be able to communicate.
On the NIC side the solution might be to run the card diagnostics and to confirm that
auto-negotiation is, in fact, enabled.
On the switch side, this depends on the diagnostics available for that particular switch.
It might also be an idea to select another port, or to plug the cable into another switch.
10/100 Mbps mismatch
This issue is related to the previous one since auto-negotiation normally takes care of the
speed issue.
Some system managers prefer to set the speeds on all NICs manually, for example, to
10 Mbps. If such an NIC is connected to a dual-speed switch port, the switch port will
automatically sense the NIC speed and revert to 10 Mbps. If, however, the switch port is
only capable of 100 Mbps, then the two devices will not be able to communicate.
268
Practical Industrial Data Communications
This problem can only be resolved by knowing the speed(s) at which the devices are
supposed to operate, and then by checking the settings via the setup software.
Full-/half-duplex mismatch
This problem is related to the previous two.
A 10BASE-T device can only operate in half-duplex (CSMA/CD) whilst a 100BASETX can operate in full-duplex OR half-duplex.
If, for example, a 100BASE-TX device is connected to a 10BASE-T hub, its autonegotiation circuitry will detect the absence of a similar facility on the hub. It will therefore
know, by default, that it is ‘talking’ to 10BASE-T and it will set its mode to half-duplex. If,
however, the NIC has been set to operate in full-duplex only, communications will be
impossible.
11b.4.8
Host-related problems
Incorrect host setup
Ethernet V2 (or IEEE 802.3 plus IEEE 802.2) only supplies the bottom layer of the DoD
model. It is therefore able to convey data from one node to another by placing it in the data
field of an Ethernet frame, but nothing more. The additional protocols to implement the
protocol stack have to be installed above it, in order to make networked communications
possible.
In industrial Ethernet networks this will typically be the TCP/IP suite, implementing the
remaining layers of the ARPA model as follows.
The second layer of the DoD model (the Internet layer) is implemented with IP (as well
as its associated protocols such as ARP and ICMP).
The next layer (the host-to-host layer) is implemented with TCP and UDP.
The upper layer (the application layer) is implemented with the various application
layer protocols such as FTP, Telnet, etc. The host might also require a suitable application
layer protocol to support its operating system in communicating with the operating
system on other hosts, on Windows, that is NetBIOS by default.
As if this is not enough, each host needs a network ‘client’ in order to access resources
on other hosts, and a network ‘service’ to allow other hosts to access its own resources in
turn. The network client and network service on each host do not form part of the
communications stack, but reside above it and communicate with each other across the
stack.
Finally, the driver software for the specific NIC needs to be installed, in order to create
a binding (‘link’) between the lower layer software (firmware) on the NIC and the next
layer software (for example, IP) on the host. The presence of the bindings can be
observed, for example, on a Windows 95/98 host by clicking ‘settings’ –> ‘control panel’
–> ‘networks’ –> ‘configuration’, then selecting the appropriate NIC and clicking
‘Properties’ –> ‘Bindings’.
Without these, regardless of the Ethernet NIC installed, networking is not possible.
Failure to log in
When booting a PC, the Windows dialogue will prompt the user to log on to the server, or
to log on to his/her own machine. Failure to log in will not prevent Windows from
completing its boot-up sequence but the network card will not be enabled. This is clearly
visible as the LEDs on the NIC and hub will not light up.
Industrial Ethernet troubleshooting
11b.4.9
269
Hub - related problems
Faulty individual port
A port on a hub may simply be ‘dead’. Everybody else on the hub can ‘see’ each other,
except the user on the suspect port. Closer inspection will show that the LED for that
particular channel does not light up. The quickest way to verify this is to remove the UTP
cable from the suspect hub port and plugging it into another port. If the LEDs light up
on the alternative port, it means that the original port is not operational.
On managed hubs the configuration of the hub has to be checked by using the hub’s
management software to verify that the particular port has not in fact been disabled by the
network supervisor.
Faulty hub
This will be indicated by the fact that none of the LEDs on the hub are illuminated and
that none of the users on that particular hub are able to access the network. The easiest
way to check this is by temporarily replacing the hub with a similar one and checking if
the problem disappears.
Incorrect hub interconnection
If hubs are interconnected in a daisy-chain fashion by means of interconnecting ports with
a UTP cable, care must be taken to ensure that either a crossover cable is used or that the
crossover/uplink port on one hub ONLY is used. Failure to comply with this precaution
will prevent the interconnected hubs from communicating with each other although it will
not damage any electronics.
A symptom of this problem will be that all users on either side of the faulty link will be
able to see each other but that nobody will be able to see anything across the faulty link.
This problem can be rectified by ensuring that a proper crossover cable is being used or,
if a straight cable is being used, that it is plugged into the crossover/uplink port on one
hub only. On the other hub, it must be plugged into a normal port.
11b.5
Troubleshooting switched networks
Troubleshooting in a shared network is fairly easy since all packets are visible every
where in the segment and as a result, the protocol analysis software can run on any host
within that segment. In a switched network, the situation changes radically since each
switch port effectively resides in its own segment and packets transferred through the
switch are not seen by ports for which they are not intended.
In order to address the problem, many vendors have built traffic monitoring modules
into their switches. These modules use either RMON or SNMP to build up statistics on
each port and report switch statistics to switched management software.
Capturing the packets on a particular switched port is also a problem, since packets are
not forwarded to all ports in a switch hence there is no place to plug in a LAN analyzer
and view the packets.
One solution implemented by vendors is port aliasing, also known as port mirroring or
port spanning. The aliasing has to be set up by the user and the switch copies the packets
from the port under observation to a designated spare port. This allows the LAN user to
plug in a LAN analyzer onto the spare port in order to observe the original port.
270
Practical Industrial Data Communications
Another solution is to insert a shared hub in the segment under observation, that is,
between the host and the switch port to which it was originally connected. The LAN
analyzer can then be connected to the hub in order to observe the passing traffic.
11b.6
Troubleshooting fast Ethernet
The most diagnostic software is PC BASE-d and it uses a NIC with a promiscuous mode
driver. This makes it easy to upgrade the system by simply adding a new NIC and driver.
However, most PCs are not powerful enough to receive, store and analyze incoming data
rates. It might therefore be necessary to rather consider the purchase of a dedicated
hardware analyzer.
Most of the typical problems experienced with fast Ethernet, have already been discussed.
These include a physical network diameter that is too large, the presence of Cat3 wiring in
the system, trying to run 100BASE-T4 on 2 pairs, mismatched 10BASE-T/100BASE-TX
ports, and noise.
11b.7
Troubleshooting Gigabit Ethernet
Although Gigabit Ethernet is very similar to its predecessors, the packets arrive so fast
that they cannot be analyzed by normal means. A Gigabit Ethernet link is capable of
transporting around 125 MB of data per second and few analyzers have the memory
capability to handle this. Gigabit Ethernet analyzers such as those made by Hewlett
Packard (LAN Internet Advisor), Network Associates (Gigabit Sniffer Pro) and WaveTek
Wandel Goltemann (Domino Gigabit Analyzer) are highly specialized Gigabit Ethernet
analyzers. They minimize storage requirements by filtering and analyzing capture packets
in real time, looking for a problem. Unfortunately, they come with a rather high price tag.
AS-interface (AS-i) overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe the main features of AS-i
• Fix problems with:
– Cabling
– Connections
– Gateways to other standards.
12a.1
Introduction
The actuator sensor interface is an open system network developed by eleven
manufacturers. These manufacturers created the AS-i association to develop the AS-i
specifications. Some of the more widely known members of the association include
Pepperl-Fuchs, Allen-Bradley, Banner Engineering, Datalogic Products, Siemens,
Telemecanique, Turck, Omron, Eaton and Festo. The governing body is ATO, the AS-i
Trade Organization. The number of ATO members currently exceeds fifty and continues to
grow. The ATO also certifies products under development for the network that meet the
AS-i specifications. This will assure compatibility between products from different vendors.
AS-i is a bit-oriented communication link designed to connect binary sensors and actuators.
Most of these devices do not require multiple bytes to adequately convey the necessary
information about the device status, so the AS-i communication interface is designed for bitoriented messages in order to increase message efficiency for these types of devices.
The AS-i interface is just that, an interface for binary sensors and actuators, designed to
interface binary sensors and actuators to microprocessor based controllers using bit length
‘messages’. It was not developed to connect intelligent controllers together since this
would be far beyond the limited capability of bit length message streams.
Modular components form the central design of AS-i. Connection to the network is
made with unique connecting modules that require minimal, or in some cases no, tools
and provide rapid, positive device attachment to the AS-i flat cable. Provision is made in
the communications system to make ‘live’ connections, permitting the removal or
addition of nodes with minimum network interruption.
Connection to higher level networks (e.g. Profibus) is made possible through plug-in
PC and PLC cards or serial interface convertor modules.
The following sections examine these features of the AS-i network in more detail.
272
Practical Industrial Data Communications
12a.2
Layer 1: the physical layer
AS-i uses a two-wire untwisted, unshielded cable that serves as both communication link
and power supply for up to thirty-one slaves. A single master module controls
communication over the AS-i network, which can be connected in various configurations
such as bus, ring, or tree (see Figure 12a.1). The AS-i flat cable has a unique cross section
that permits only properly polarized connections when making field connections to the
modules. Alternatively, ordinary 2 wire cable (#16 AWG, 1.5 mm) can be used.
A special shielded cable is also available for high noise environments.
Figure 12a.1
Various AS-i configurations
Each slave is permitted to draw a maximum of 65 mA from the 30 V DC-power supply.
If devices require more than this, separate supplies must be provided for each device.
With a total of 31 slaves drawing 65 mA, a total limit of 2 A has been established to
prevent excessive voltage drop over the 100 m permitted network length. A 16 AWG
cable is specified to insure this condition. If this limitation on power drawn from the
(yellow) signal cable is a problem, then a second (black) cable, identical in dimensions to
the yellow cable, can be used in parallel for power distribution only.
The slave (or field) modules are available in four configurations:
•
•
•
•
•
Input modules for 2-and 3-wire DC sensors or contact closure
Output modules for actuators
Input/output (I/O) modules for dual purpose applications
Field connection modules for direct connection to AS-i compatible devices.
12-bit analog to digital converter.
The original AS-i specification (V2) allowed for 31 devices per segment of cable
(see Figure 12a.2), with a total of 124 digital inputs and 124 digital outputs that is, a total
of 248 I/O points. The latest specification, V2.1, allows for 62 devices, resulting in
248 inputs and 186 outputs, a total of 434 I/O points. With the latest specification, even
12-bit A to D converters can be read over 5 cycles.
AS-interface (AS-i) overview 273
Figure 12a.2
Cross section of AS-i cable (mm)
A unique design allows the field modules to be connected directly into the bus while
maintaining network integrity (see Figure 12a.3). The field module is composed of an
upper and lower section, secured together once the cable is inserted. Specially designed
contact points pierce the self-sealing cable providing bus access to the I/O points and/or
continuation of the network. True to the modular design concept, two types of lower
sections and three types of upper sections are available to permit ‘mix-and-match’
combinations to accommodate various connection schemes and device types. Plug
connectors are utilized to interface the I/O devices to the slave (or with the correct choice
of modular section screw terminals) and the entire module is sealed from the environment
with special seals provided where the cable enters the module. The seals conveniently
store away within the module when not in use.
Figure 12a.3
Connection to the cable
The AS-i network is capable of a transfer rate of 167 kbps. Using an access procedure
known as ‘master–slave access with cyclic polling’, the master continually polls all the
slave devices during a given cycle to ensure rapid update times. For example, with
31 slaves and 124 I/O points connected, the AS-i network can ensure a 5 ms-cycle time,
making the AS-i network one of the fastest available.
274
Practical Industrial Data Communications
A modulation technique called ‘alternating pulse modulation’ provides this high
transfer rate capability as well as high data integrity. This technique will be described in
the following section.
12a.3
Layer 2: the data-link layer
The data-link layer of the AS-i network consists of a master call-up and slave response.
The master call-up is exactly fourteen bits in length while the slave response is 7 bits.
A pause between each transmission is used for synchronization. Refer Figure 12a.4, for
example, call-up and response frames.
Figure 12a.4
Example call up and response frames
Various code combinations are possible in the information portion of the call-up frame
and it is precisely these various code combinations that are used to read and write
information to the slave devices. Examples of some of the master call-ups are listed in
Figure 12a.5. A detailed explanation of these call-ups is available from the ATO
literature and is only included here to illustrate the basic means of information transfer on
the AS-i network.
The modulation technique used by AS-i is known as ‘Alternating Pulse Modulation’
(APM). Since the information frame is of a limited size, providing conventional error
checking was not possible and therefore the AS-i developers chose a different technique
to ensure high level of data integrity.
Referring to the following figure, the coding of the information is similar to Manchester
II coding but utilizing a ‘sine-squared’ waveform for each pulse (see Figure 12a.6). This
waveform has several unique electrical properties, which reduce the bandwidth required
of the transmission medium (permitting faster transfer rates) and reduce the end of line
reflections common in networks using square wave pulse techniques. Also, notice that
each bit has an associated pulse during the second half of the bit period. This property is
utilized as a bit level of error checking by all AS-i devices. The similarity to Manchester
II coding is no accident since this technique has been used for many years to pass
synchronizing information to a receiver along with the actual data.
AS-interface (AS-i) overview 275
Figure 12a.5
Some AS-i call-ups
Figure 12a.6
Sine-squared wave form
In addition, AS-i developers also established a set of rules for the APM coded signal
that is used to further enhance data integrity. For example, the start bit or first bit in the
AS-i telegram must be a negative impulse and the stop bit a positive impulse. Two
subsequent impulses must be of opposite polarity and the pause between two consecutive
276
Practical Industrial Data Communications
impulses should be 3 µs. Even parity and a prescribed frame length are also incorporated
at the frame level. As a result the ‘odd’ looking waveform, in combination with the rules
of the frame formatting, the set of APM coding rules and parity checking, work together
to provide timing information and high-level data integrity for the AS-i network.
12a.4
Operating characteristics
AS-i node addresses are stored in nonvolatile memory and can be assigned either by the
master or one of the addressing or service units. Should a node fail, AS-i has the ability to
automatically reassign the replaced node’s address and, in some cases, reprogram the
node itself allowing rapid response and repair times.
Since AS-i was designed to be an interface between lower level devices, connection to
higher-level systems enables the capability to transfer data and diagnostic information.
Plug-in PC cards and PLC cards are currently available. The PLC cards allow direct
connection with various Siemens PLCs. Serial communication converters are also available
to enable AS-i connection to conventional RS-232, 422, and 485 communication links.
Direct connection to a Profibus field network is also possible with the Profibus coupler,
enabling several AS-i networks access to a high-level digital network.
Handheld and PC based configuration tools are available, which allow initial start-up
programing and also serve as diagnostic tools after the network is commissioned. With
these devices, on-line monitoring is possible to aid in determining the health of the
network and locating possible error sources.
AS-i troubleshooting
12b.1
Introduction
The AS-i system has been designed with a high degree of ‘maintenance friendliness’ in
mind and has a high level of built-in auto-diagnosis. The system is continuously
monitoring itself against faults such as:
• Operational slave errors (permanent or intermittent slave failure, faulty
configuration data such as addresses, I/O configuration, and ID codes)
• Operational master errors (permanent or intermittent master failure, faulty
configuration data such as addresses, I/O configuration, and ID codes)
• Operational cable errors (short circuits, cable breakage, corrupted telegrams
due to electrical interference and voltage outside the permissible range)
• Maintenance-related slave errors (false addresses entered, false I/O
configuration and false ID codes)
• Maintenance-related master errors (faulty projected data such as I/O
configuration, ID codes, parameters, etc.)
• Maintenance - related cable errors (counter poling the AS-i cable).
The fault diagnosis is displayed by means of LEDs on the master.
Where possible, the system will protect itself. During a short-circuit, for example, the
power supply to the slaves is interrupted, which causes all actuators to revert to a safe
state. Another example is the jabber control on the AS-i chips, whereby a built-in fuse
blows if too much current is drawn by a chip, disconnecting it from the bus.
The following tools can be used to assist in fault finding.
12b.2
Tools of the trade
12b.2.1
Addressing handheld
Before an AS-i system can operate, all the operating addresses must be assigned to the
connected slaves, which store this on their internal nonvolatile memory (EEPROM).
Although this can theoretically be done on-line, it requires that a master device with this
addressing capability be available.
In the absence of such a master, a specialized battery powered addressing handheld (for
example, one manufactured by Pepperl and Fuchs) can be used. The device is capable of
reading the current slave address (from 0 to 31) as well as reprograming the slave to a
new address entered via the keyboard.
278
Practical Industrial Data Communications
The slaves are attached to the handheld device, one at a time, by means of a special
short cable or plug directly into the handheld device. They are only powered via the
device while the addressing operation takes place (about 1 s) with the result that several
hundred slaves can be configured in this way before a battery change is necessary.
12b.2.2
Monitor
A monitor is essentially a protocol analyzer, which allows a user to capture and analyze
the telegrams on the AS-i bus. A good monitor should have triggering and filtering
capabilities, as well as the ability to store, retrieve and analyze captured data. Monitors
are usually implemented as PC-based systems.
12b.2.3
Service device
An example of such a device is the SIE 93 handheld manufactured by Siemens. It can
perform the following:
• Slave addressing, as described above
• Monitoring, that is, the capturing, analysis and display of telegrams
• Slave simulation, in which case it behaves like a supplementary slave, the user
can select its operating address
• Master simulation, in which case the entire cycle of master requests can be
issued to test the parameters, configuration and address of a specific slave
device (one at a time).
12b.2.4
Service book
A ‘service book’ is a commissioning and servicing tool based on a notebook computer. It
is capable of monitoring an operating network, recording telegrams, detecting errors,
addressing slaves off-line, testing slaves off-line, maintaining a database of sensor/
actuator data and supplying help functions for user support.
Bus data for use by the software on the notepad is captured, preprocessed and
forwarded to the laptop by a specialized network interface, a so-called ‘hardware
checker’. The hardware checker is based on an 80C535 single chip micro-controller and
connects to the notepad via an RS-232 interface.
12b.2.5
Slave simulator
Slave simulators are PC-based systems used by software developers to evaluate the
performance of a slave (under development) in a complete AS-i network. They can
simulate the characteristics of up to 32 slaves concurrently and can introduce errors that
would be difficult to set up in real situations.
DeviceNet overview
Objectives
When you have completed study of this chapter you will be able to:
• List the main features of DeviceNet
• Identify and correct problems with:
–
–
–
–
–
–
–
–
13a.1
Cable topology
Power and earthing
Signal voltage levels
Common mode voltages
Terminations
Cabling
Noise
Node communications problems.
Introduction
DeviceNet, developed by Allen-Bradley, is a low-level device oriented network based on
the controller area network (CAN) developed by Bosch (GmbH) for the automobile
industry. It is designed to interconnect lower level devices (sensors and actuators) with
higher level devices (controllers).
The variable, multi-byte format of the CAN message frame is well suited to this task as
more information can be communicated per message than with bit-type systems. The
Open DeviceNet Vendor Association, Inc. (ODVA) has been formed to issue DeviceNet
specifications, ensure compliance with the specifications and offer technical assistance
for manufacturers wishing to implement DeviceNet. The DeviceNet specification is an
open specification and available through the ODVA.
DeviceNet can support up to 64 nodes, which can be removed individually under power
and without severing the trunk line. A single, four-conductor cable (round or flat)
provides both power and data communications. It supports a bus (trunk line drop line)
topology, with branching allowed on the drops. Reverse wiring protection is built into all
nodes, protecting them against damage in case of inadvertent wiring errors.
The data rates supported are 125, 250 and 500 kbaud, although a specific installation
does not have to support all, as data rates can be traded for distance.
As DeviceNet was designed to interface lower level devices with higher level controllers, a
unique adaptation of the basic CAN protocol was developed. This is similar to the familiar
poll/response or master/slave technique but still utilizes the speed benefits of the original CAN.
280
Practical Industrial Data Communications
Figure 13a.1 illustrates the positioning of DeviceNet and CANBUS within in the OSI
model. Note that ControlNet and Ethernet/IP use the same application layer protocols.
Because of this particular architecture, messages are routable between DeviceNet,
ControlNet and Ethernet/IP systems.
Figure 13a.1
DeviceNet vs the OSI model
[Note: Use Figure 19.11 from FE manual]
13a.2
Physical layer
13a.2.1
Topology
The DeviceNet media consists of a physical bus topology (Figure 13a.2). The bus or
‘trunk’ (white and blue wires) is the backbone of the network and must be terminated at
either end by a 120 Ω 1/4 W resistor.
Drop lines of up to 6 m (20 ft) in length enable the connection of nodes (devices) to the
main trunk line, but care has to be taken not to exceed the total drop line budget for a
specific speed. Branching to multiple nodes is allowed only on drop lines.
Figure 13a.2
DeviceNet topology
DeviceNet overview 281
Three types of cable are available, all of which can be used as the trunk. They are thick,
thin and flat wire.
13a.3
Connectors
DeviceNet has adopted a range of open and closed connectors that are considered suitable
for connecting equipment onto the bus and drop lines. This range of recommended
connectors is described in this section.
DeviceNet users can connect to the system using other proprietary connectors, the only
restrictions placed on the user regarding the types of connectors used are as follows:
• All nodes (devices), whether using sealed or unsealed connections, supplying
or consuming power, must have male connectors.
• Whatever connector is chosen, it must be possible for the related device to
connect or disconnect from the DeviceNet bus without compromising the
system’s operation.
• Connectors must be upgraded to carry high levels (8 A or more at 24 V, or
200 VA) of current.
• A minimum of 5 isolated connector pins are required, with the possible
requirement of a 6th, or metal body shield connection for safety ground use.
There are two basic styles of DeviceNet connectors that are used for bus and drop line
connections in normal, harsh, and hazardous conditions. These are:
1. An open style connector (pluggable or hard-wired)
2. A closed style connector (mini or micro style).
13a.3.1
Pluggable (unsealed) connector
This is a 5 pin, unsealed open connector utilizing direct soldering, crimping, screw
terminals, barrier strips or screw type terminations. This type of connector entails
removing system power for connection (Figure 13a.3).
Figure 13a.3
Unsealed screw connector
282
Practical Industrial Data Communications
13a.3.2
Hard-wired (unsealed) connection
Loose wire connections can be used to make direct attachment to a node or a bus tap
without the presence of a connector, although this is not a preferred method. It is only a
viable option if the node can be removed from the trunk line without severing the trunk.
The ends of the cable are ‘live’ if the cable has been removed from the node in question
and are still connected as part of the bus infrastructure. As such, care MUST be taken to
insulate the exposed ends of the cable (Figure 13a.4).
Figure 13a.4
Open wire connection
13a.3.3
Mini (sealed) connector
This 18 mm round connector is recommended for harsh environments (field connections).
This connection must meet ANSI/B93.55M-1981. The female connector (attached to the
bus cable) must have rotational locking. This connector requires a minimum voltage
rating of 25 V, and for trunk use a current rating of 8 A is required. Additional options
can include oil and water resistance (Figure 13a.5).
Figure 13a.5
Sealed mini-type connector (face views)
13a.3.4
Micro (sealed) connector
This connector is effectively a 12 mm diameter miniature version of the mini style connector,
except its suitability is for thin wire drop connections requiring reduction in both physical and
current carrying capacity (Figure 13a.6).
DeviceNet overview 283
Figure 13a.6
Sealed micro-style connector (face views)
It has 5 pins, 4 in a circular periphery pattern and the fifth pin in the center. This
connector should have a minimum voltage rating of 25 V, and for drop connections a
current rating of 3 A is required. The male component must mate with Lumberg Style
RST5-56/xm or equivalent, the female component part must also conform to Lumberg
Style RST5-56/xm or equivalent. Additional options can include oil and water resistance.
13a.4
Cable budgets
DeviceNet’s transmission media can be constructed of either DeviceNet thick, thin or flat
cable or a combination thereof. Thick or flat cable is used for long distances and is
stronger and more resilient than the thin cable, which is mainly used as a local drop line
connecting nodes to the main trunk line.
The trunk line supports only tap or multiport taps that connect drop lines into the
associated node. Branching structures are allowed only on drop lines and not on the main
trunk line.
The following Figures 13a.7, 13a.8, 13a.9 show the distance vs length trade-off for the
different types of cable.
DATA RATES
Trunk distance
Max. drop length
Cumulative drop
Number of nodes
125 kbaud
500 m (1640 ft)
20 ft
512 ft
64
250 kbaud
250 m (630 ft)
20 ft
256 ft
64
500 kbaud
100 m (328 ft)
20 ft
128 ft
64
Figure 13a.7
Constraints: thick wire
DATA RATES
Trunk distance
Max. drop length
Cumulative drop
Number of nodes
Figure 13a.8
Constraints: thin wire
125 kbaud
100 m (326 ft)
20 ft
512 ft
64
250 kbaud
100 m (326 ft)
20 ft
256 ft
64
500 kbaud
100 m (328 ft)
20 ft
128 ft
64
284
Practical Industrial Data Communications
DATA RATES
Trunk distance
Max. drop length
Cumulative drop
Number of nodes
125 kbaud
420 m (1640 ft)
20 ft
512 ft
64
250 kbaud
200 m (630 ft)
20 ft
256 ft
64
500 kbaud
75 m (328 ft)
20 ft
128 ft
64
Figure 13a.9
Constraints: flat wire
13a.5
Device taps
13a.5.1
Sealed taps
Sealed taps are available in single port (T type) and multiport configurations. Regardless
of whether the connectors are mini or micro style, DeviceNet requires that male
connectors must have external threads while female connectors must have internal
threads. In either case, the direction of rotation is optional (Figure 13a.10).
Figure 13a.10
Sealed taps
13a.5.2
IDC taps
Insulation displacement connectors (IDCs) are used for KwikLink flat cable (Figure
13a.11). They are modular, relatively inexpensive and compact. They are compatible with
existing media and require little installation effort. The enclosure conforms to NEMA 6P
and 13, and IP 67.
Figure 13a.11
Insulation displacement connector
DeviceNet overview 285
13a.5.3
Open style taps
DeviceNet has three basic forms of open taps (Figure 13a.12). They are:
1. Zero length drop line, suitable for daisy-chain applications
2. Open tap, able to connect a 6 m (20 ft) drop line onto the trunk
3. An open style connector, supporting ‘temporary’ attachment of a node to a drop line.
Figure 13a.12
Open and temporary DeviceNet taps
The temporary connector is suitable for connection both to and from the system when
the system is powered. It is of similar construction to a standard telephone wall plug,
being of molded construction and equipped with finger grips to assist removal, and is
286
Practical Industrial Data Communications
styled as a male pin-out. The side cheeks are polarized to prevent reversed insertion into
the drop line open tap connector.
13a.5.4
Multi-port open taps
If a number of nodes or devices are located within a close proximity of each other, for
example within a control cabinet or similar enclosure, an open tap can be used (Figure 13a.13).
Alternatively, devices can be wired into a DeviceBox multi-port tap. The drops from
the individual devices are not attached to the box via sealed connectors but are fed in via
cable grips and connected to a terminal strip.
Figure 13a.13
Multi-port open taps
13a.5.5
Power taps
Power taps differ from device taps in that they have to perform four essential functions
that are not specifically required by the device taps. These include:
• Two protection devices in the V+ supply
• Connection from the positive output of the supply to the V+ bus line via a
Schottky diode
• Provision of a continuous connection for the signaling pair, drain and negative
wires through the tap
• Provision of current limiting in both directions from the tap.
The following Figure 13a.14 illustrates the criteria listed above.
Figure 13a.14
Principle of a DeviceNet power tap
DeviceNet overview 287
13a.6
Cable description
The ‘original’ (round) DeviceNet cable has two shielded twisted pairs. These are twisted
on a common axis with a drain wire in the center, and are equipped with an overall braid.
13a.6.1
Thick cable
This cable is used as the trunk line when length is important (Figure 13a.15). Overall
diameter is 0.480 in. (10.8 mm) and it comprises of:
• A signal pair, consisting of one twisted-pair (3 twists per foot) coded blue/white
with a wire size of #18 (19 × 30 AWG) copper and individually tinned; the
impedance is 120 Ω ±10% at 1 MHz, the capacitance between conductors is
12 pF /ft and the propagation delay is 1.36 nS/ft maximum.
• A power pair, consisting of one twisted-pair (3 twists per foot) coded black/red
with a wire size of #15 (19 × 28 AWG) copper and individually tinned.
This is completed by separate aluminized Mylar shields around each pair and an overall
foil/braided shield with an #18 (19 × 30 AWG) bare drain wire. The power pair has an
8 A power capacity and is PVC/nylon insulated. It is also flame resistant and UL oil
resistant too.
Figure 13a.15
DeviceNet thick cable
13a.6.2
Thin cable specification
This cable is used for both drop lines as well as short trunk lines. Its overall diameter is
0.27 in. (6.13 mm) and it comprises of:
• A signal pair, consisting of a twisted-pair (4.8 twists per foot) coded blue/white
with a wire size of #24 (19 × 36 AWG) copper and individually tinned; the
impedance is 120 Ω ±10% at 1 MHz, the capacitance between conductors is
12 pF/ft and the propagation delay is 1.36 nS/ft maximum.
• A power pair consisting of one twisted-pair (4.8 twists per foot) coded black/red
with a wire size of #22 (19 × 34 AWG) copper and individually tinned.
This is completed by separate aluminized Mylar shields around each pair and an overall
foil/braided shield with an #22 (19 × 34 AWG) bare drain wire. The power pair has a 3 A
power capacity and is PVC insulated (Figure 13a.16).
288
Practical Industrial Data Communications
Figure 13a.16
DeviceNet thin cable
13a.6.3
Flat cable
DeviceNet flat cable is a highly flexible cable that works with existing devices (Figure
13a.17). It has the following specifications:
•
•
•
•
600 V 8 A rating
A physical key
Fitting into 1 in. (25 mm) conduit
The jacket made of TPE/Santoprene.
Figure 13a.17
DeviceNet flat cable
13a.7
Network power
13a.7.1
General approach
One or more 24 V power supplies can be used to power the devices on the DeviceNet
network, provided that the 8 A current limit on thick/flat wire and the 3 A limit on thin
wire is not exceeded. The power supplies used should be dedicated to the DeviceNet
cable power ONLY!
Although, technically speaking, any suitable power supply can be used, supplies such as the
Rockwell automation 1787-DNPS 5.25 A supply are certified specifically for DeviceNet.
The power calculations can be done by hand, but it is easier to use a design spreadsheet
such as the Rockwell automation/Allen Bradley DeviceNet Power Supply Configuration
toolkit running under Microsoft Excel.
DeviceNet overview 289
The network can be constructed of both thick and thin cable as long as only one type of
cable is used per section of network, comprising a section between power taps or between
a power tap and the end of the network.
Using the steps illustrated below, a quick initial evaluation can be achieved as to the
power requirements needed for a particular network.
Sum the total current requirements of all network devices, then evaluate the total
permissible network length (be conservative here) using the following Table 13a.1.
Thick cable Network Current Distribution and Allowable Current Loading
0 25
50
100 150 200 250 300 350 400 450 500
Network length in
meters
0 83.3 167 333 500 666 833 999 1166 1332 1499 1665
Network length in feet
Maximum current in
amperes
8
8
5.42 2.93 2.01 1.53 1.23 1.03 0.89 0.78 0.69 0.63
Thin Cable Network Current Distribution and Allowable Current Loading
10
20
30
40
50
60
70
80
90
Network length in meters 0
99
132
165
198
231
264
297
100
Network length in feet
0
33
66
330
Maximum current in
amperes
8
8
5.42 2.93 2.01 1.53 1.23 1.03 0.89 0.78 0.69
Table 13a.1
Thick and thin cable length and power capacity
Depending on the final power requirement cost and network complexity, either a single
supply end or center connected can be used.
13a.7.2
Single supply – end connected
Total network length = 200 m (656 ft)
Total current = Sum of node 1, 2, 3, 4 and 5 currents = 0.65 A
Referring to Table 14.1 the current limit for 200 m = 1.53 A.
Configuration are correct as long as THICK cable is used (see Figure 13a.18).
13a.7.3
Single supply – center connected
Current in Section 1 = 1.05 A over a length of 90 m (300 ft)
Current in Section 2 = 1.88 A over a length of 120 m (400 ft)
Current limits for a distance of 90 m is 3.3 A and for 120 m is 2.63 A
Power for both sections is correct, and a 3 A (minimum) power supply is required
(see Figure 13a.19).
290
Practical Industrial Data Communications
Figure 13a.18
Single supply – end connected
Figure 13a.19
Single supply – center connected
Tables 13a.2 and 13a.3 indicate parameters that control load limits and allowable
tolerances as related to DeviceNet power.
System Power Load Limits
Maximum voltage drop on both the –ve and +ve
5.0 V on each line
power lines
Maximum thick cable trunk line current
8.0 A in any section
Maximum thin cable trunk line current
3.0 A in any section
Maximum drop line current
0.75 to 3.0 A
Voltage range at each node
11.0 to 25.0 V
Operating current on each product
Specified by the product
manufacturer
Table 13a.2
System power load limits
DeviceNet overview 291
Maximum Drop Line Currents
Current limits are calculated by the following equations, where I = allowable
drop line current and L = distance.
In meters: I = 4.57/L
In feet I = 15/L
Drop Length
Maximum Allowable Current
1.00
3.00
3.00
3.00
5.00
3.00
7.50
2.00
10.00
1.50
15.00
1.00
20.00
0.75
Table 13a.3
Maximum drop line currents
13a.7.4
Suggestions for avoiding errors and power supply options
The following steps can be used to minimize errors when configuring power on a
network.
• Ensure the calculations made for current and distances are correct (be
conservative)
• Conduct a network survey to verify correct voltages, remembering that a
minimum of 11 V at a node is required and that a maximum voltage drop of
10 V across each node is allowed
• Allow for a good margin to have reserves of power to correct problems if
needed
• If using multiple supplies, it is essential that they must all be turned on
simultaneously to prevent both power supply and cable overloading
• Power supplies MUST be capable of supporting linear and switching regulators
• Supply MUST be isolated from both the AC supply and chassis.
13a.8
System grounding
Grounding of the system must be done at one point only, preferably as close to the
physical center of the network as possible. This connection should be done at a power tap
where a terminal exists for this purpose. A main ground connection should be made from
this point to a good earth or building ground via a copper braid or at least a 8 AWG
copper conductor not more than 3 m (10 ft) in length.
At this point of connection, the following conductors and circuits should be connected
together in the form of a ‘star’ configuration.
• The drain wire of the main trunk cable
• The shield of the main trunk cable
292
Practical Industrial Data Communications
• The negative power conductor
• The main ground connection as described above.
If the network is already connected to ground at some other point, do NOT
connect the ground terminal of a power tap to a second ground connection. This can
result in unwanted ground loop currents occurring in the system. It is essential that a
single ground connection is established for the network bus and failure to ground the
bus’s –ve supply at ONE POINT will only result in a low signal to noise ratio appearing
in the system.
Care must be exercised when connecting the drain/shield of the bus or drop line cable at
nodes, which are already grounded. This can happen when the case or enclosure of the
equipment, comprising the node, is connected to ground for electrical safety and/or a
signaling connection to other self-powered equipment. In cases where this condition exists,
the drain/shield should be connected to the node ground through a 0.01 µF/500 V capacitor
wired in parallel with a 1 megohm 1/4 W resistor. If the node has no facility for grounding,
the drain and shield must be left UNCONNECTED.
13a.9
Signaling
DeviceNet is a two wire differential network. Communication is achieved by switching
the CAN_H wire (white) and the CAN_L wire (blue) relative to the V– wire (black).
CAN_H swings between 2.5 V DC (recessive state) and 4.0 V DC (dominant state)
while CAN_L swings between 2.5 V DC (recessive state) and 1.5 V DC (dominant
state).
With no network master connected, the CAN_H and CAN_L lines should be in the
recessive state and should read (with a voltmeter set to DC mode) between 2.5 and 3.0 V
relative to V– at the point where the power supply is connected to the network. With a
network master connected AND polling the network, the CAN_H to V– voltage will be
around 3.2 V DC and the CAN_L to V– voltage will be around 2.4 V DC. This is because
the signals are switching, which affects the DC value read by the meter.
The voltage values given here assume that no common mode voltages are present.
Should they be present, voltages measured closer to the power supply will be higher than
those measured furthest from the power supply. However, the differential voltages
(CAN_H minus CAN_L) will not be affected.
DeviceNet uses a differential signaling system. A logical ‘1’ is represented by CAN_H
being Low (recessive) and CAN_L being High (recessive). Conversely, a logical ‘0’ is
represented by CAN_H being High (dominant) and CAN_L being Low (dominant).
Figure 14.22 depicts this graphically.
The nodes are all attached to the bus in parallel, resulting in a wired–AND
configuration. This means that as long as ANY one node imposes a Low signal (logical 0)
on the bus, the resulting signal on the bus will also be low. Only when ALL nodes output
a high signal (logical 1), the signal on the bus will also be high.
13a.10 Data-link layer
13a.10.1 Frame format
The format of a DeviceNet frame is shown in Figure 13a.20. Note that the data field is
rather small (8 bytes) and that any messages larger than this need to be fragmented.
DeviceNet overview 293
Figure 13a.20
DeviceNet frame
This frame will be placed on the bus as sequential 1s and 0s, by changing the levels of
the CAN_H and CAN_L in a differential fashion.
In Figure 13a.21, A represents the CAN_H signal in its dominant (high) state
(+3.5 to +4.0 V DC), C represents the CAN_L signal in its dominant (low) state (+1.5 to
2.5 V DC) and B represents both the CAN_H signal recessive (low) and the CAN_L
signal recessive (high) states of +2.5 V–3.0 V DC.
Figure 13a.21
DeviceNet transmission
13a.10.2 Medium access
The medium access control method is described as ‘carrier sense multiple access with
non-destructive bit-wise arbitration’ (CSMA/NBA), where the arbitration takes place on a
bit-by-bit basis on the first field in the frame (the 11 bit identifier field). If a node wishes
to transmit, it has to defer to any existing transmission. Once that transmission has ended,
the node wishing to transmit has to wait for 3 bit times before transmitting. This is called
the interframe space.
294
Practical Industrial Data Communications
Despite this precaution, it is possible for two nodes to start transmitting concurrently. In
the following example (Figure 13a.22) nodes 1 and 2 start transmitting concurrently, with
both nodes monitoring their own transmissions. All goes well for the first few bits since
the bit sequences are the same. Then the situation arises where the bits are different. Since
the ‘0’ state is dominant, the output of node 2 overrides that of node 1. Node 1 loses the
arbitration and stops transmitting. It does, however, still acknowledges the message by
means of the ACK field in the frame.
Figure 13a.22
DeviceNet arbitration
Because of this method of arbitration, the node with the lowest number (i.e. the most
significant ‘0’ s in its identifier field) will win the arbitration.
13a.10.3 Fragmentation
Any device that needs more than 8 bytes of data sent in any direction will cause
fragmentation to occur. This happens since a frame can only contain 8 bytes of data.
When fragmentation occurs, only 7 bytes of data can be sent at a time since the first byte
is used to facilitate the reassembly of fragments. It is used as follows:
First byte
00
41–7F
80–FF
Significance
First fragment (number 0)
Intermediate fragment
(lower 6 bits of the byte is the fragment number)
Last fragment
(lower 6 bits of the byte is the fragment number)
Example:
Data packet
00 12 34 56 78 90 12 34
41 56 78 90 12 34 56 78
42 90 12 34 56 78 90 12
83 34 56
Description
First fragment, number 0
Intermediate fragment number 1
Intermediate fragment number 2
Last fragment number 3
DeviceNet overview 295
13a.11 The application layer
The CAN specification does not dictate how information within the CAN message frame
fields are to be interpreted – that was left up to the developers of the DeviceNet
application software.
Through the use of special identifier codes (bit patterns) in the identifier field, master is
differentiated from slave. Also, sections of this field tell the slaves how to respond to the
master’s message. For example, slaves can be requested to respond with information
simultaneously in which case the CAN bus arbitration scheme assures the timeliest
consecutive response from all slaves in decreasing order of priority. Or, slaves can be
polled individually, all through the selection of different identifier field codes. This
technique allows the system implementers more flexibility when establishing node
priorities and device addresses.
The main protocol at the TCP application level (layers 5, 6 and 7 in the OSI model) is
CIP the ‘control and information protocol’. As shown in Figure 13a.1, CIP covers not
only layers 5, 6 and 7 of the OSI model, but also the ‘user layer’ (layer 8), which includes
the user device profiles. Apart from the common device profiles, CIP also includes the
common object library, the common control services and the common routing services.
The OSI model has no layer 8, but items such as device profiles do not fit within the
conceptual structure of the OSI model hence vendors add a ‘layer 8’ above layer 7 for this
purpose.
CIP gives generic DeviceNet an ‘industrial functionality’ and allows the control,
collection and configuration of data. It also allows a producer-consumer model to run on
CANBUS, or whatever technology being used at layers 1 and 2.
CIP provides the following common features for users of DeviceNet, ControlNet and
Ethernet/IP:
• It provides the user with a standard set of messaging services for all three
networks.
• It lets the user connect to any network and configure and collect data from any
network.
• It saves time and effort during system configuration because no routing tables
or added logic are necessary to move data between networks.
• It reduces the amount of training needed when moving personal between
different networks by providing similar configuration tools and features.
DeviceNet troubleshooting
13b.1
Introduction
Networks, in general, exhibit the following types of problems from time to time.
The first type of problem is of an electronic nature, where a specific node (e.g. a
network interface card) malfunctions. This can be due to a component failure or to an
incorrect configuration of the device.
The second type is related to the medium that interconnects the nodes. Here, the
problems are more often of an electromechanical nature and include open and short
circuits, electrical noise, signal distortion and attenuation. Open and short circuits in the
signal path are caused by faulty connectors or cables. Electrical interference (noise) is
caused by incorrect grounding, broken shields or external sources of electromagnetic or
radio frequency interference. Signal distortion and attenuation can be caused by incorrect
termination, failure to adhere to topology guidelines (e.g. drop cables too long), or faulty
connectors.
Whereas these are general network-related problems, the following ones are very
specific to DeviceNet:
• Missing terminators
• Excessive common mode voltage, caused by faulty connectors or excessive
cable length
• Low power supply voltage caused by faulty connectors or excessive cable length
• Excessive signal propagation delays caused by excessive cable length.
These problems will be discussed in more detail.
13b.2
Tools of the trade
The following list is by no means complete, but is intended to give an overview of the
types of tools available for commissioning and troubleshooting DeviceNet networks.
Whereas some tools are sophisticated and expensive, many DeviceNet problems can be
sorted out with common sense and a multimeter.
13b.2.1
Multimeter
A multimeter capable of measuring DC volts, resistance, and current is an indispensable
troubleshooting tool. On the current scale, it should be capable of measuring several
amperes.
DeviceNet troubleshooting 297
13b.2.2
Oscilloscope
An inexpensive 20 MHz dual-trace oscilloscope comes in quite handy. It can be used for
all the voltage tests as well as observing noise on the lines, but caution should be
exercised when interpreting traces.
Firstly, signal lines must be observed in differential mode (with probes connected to
CAN_H and CAN_L). If they are observed one at a time with reference to ground, they
may seem unacceptable due to the common mode noise (which is not a problem since it is
rejected by the differential mode receivers on the nodes).
13b.2.3
Handheld analyzers
Handheld DeviceNet analyzers such as the NetAlert NetMeter (Figure 13b.1) or DeviceNet
Detective can be used for several purposes. Depending on the capabilities of the specific
device, they can configure node addresses and baud rates, monitor power and signal levels,
log errors and network events of periods ranging from few minutes to several days, indicate
short circuits and poorly wired connections, and obtain configuration states as well as
firmware versions and serial numbers from devices.
Figure 13b.1
NetAlert NetMeter
13b.2.4
Intelligent wiring components
Examples of these are the NetAlert traffic monitor and NetAlert power monitor. These
are ‘intelligent’ tee-pieces that are wired into the system (Figure 13b.2). The first device
monitors and displays network traffic by means of LEDs and gives a visual warning if
traffic levels exceed 90%. The second device monitors voltages and visually indicates
whether they are OK, too high, too low, or totally out of range.
The NetMeter can be attached to the above-mentioned tees for more detailed diagnostics.
Figure 13b.2
Power monitor tee
298
Practical Industrial Data Communications
13b.2.5
Controller software
Many DeviceNet controllers have associated software, running under various operating
systems such as Windows 2000 and NT4 that can display sophisticated views of the
network for diagnostic purposes. The example given here is one of many generated by the
ApplicomIO software (Figure 13b.3) and displays data obtained from a device, down to
bit level (in hexadecimal).
Figure 13b.3
ApplicomIO display
13b.2.6
CAN bus monitor
Since DeviceNet is based on the controller area network, CAN protocol analysis software
can be used on DeviceNet networks to capture and analyze packets (frames). An example
is Synergetic’s CAN Explorer for Windows, running on a PC. The same vendor also
supplies the ISA, PC/104, PCI and parallel port network interfaces for connecting the PC
to the DeviceNet network.
A PC with this software can not only function as a protocol analyzer, but also as a data
logger.
13b.3
Fault finding procedures
In general, the system should not be operating that is, there should be no communication
on the bus, and all devices should be installed.
A low-tech approach to troubleshooting could involve disconnecting parts of the
network, and observing the effect on the problem. This does, unfortunately, not work
well for problems such as excessive common mode voltage and ground loops since
disconnecting part of the network often solves the problem.
13b.3.1
Incorrect cable lengths
If the network exhibits problems during the commissioning phase or after
modifications/additions have been made to the network, the cable lengths should be
DeviceNet troubleshooting 299
double-checked against the DeviceNet topology restrictions. The maximum cable lengths
are as follows:
Thick trunk length
Thin trunk length
Single drop
Cumulative all drops
125 kbaud
500 m
100 m
6m
156 m
250 kbaud
250 m
100 m
6m
78 m
50 kbaud
100 m
100 m
6m
39 m
For simplicity, only the metric sizes are given here.
The following symptoms are indicative of a topology problem.
• If drop lines too long, that is, the total amount of drops exceeds the permitted
length, variations on CAN signal amplitude will occur throughout the network.
• If a trunk line is too long it will cause ‘transmission line’ effects in which
reflections in the network cause faulty reception of messages; this will result in
CAN frame errors.
13b.3.2
Power and earthing problems
Shielding
Shielding is checked in the following way.
Connect a 16 A DC ammeter from DC common to shield at the end of the network
furthest from the power supply. If the power supply is in the middle, then this test must
be performed at both ends. In either case, there should be significant current flow. If
practical, this test can also be performed at the end of each drop.
If there is no current flowing, then the shield is broken or the network is improperly
grounded.
Grounding
In general, the following rules should be observed:
• Physically connect the DC power supply ground wire and the shield together to
earth ground at the location of the power supply.
• In the case of multiple power supplies, connect this ground only at the power
supply closest to the middle of the network.
• Ensure that all nodes on the network connect to the shield, the signal and power
lines.
Note: CAN frame errors are a symptom of grounding problems. CAN error messages
can be monitored with a handheld DeviceNet analyzer or CAN bus analyzer.
Break the shield at a few points in the network, and insert a DC ammeter in the shield.
If there is a current flow, then the shield is connected to DC common or ground in more
than one place, and a ground loop exists.
Power
This can be measured with a voltmeter or handheld DeviceNet analyzer and is measured
between V+ (red) and V– (black).
300
Practical Industrial Data Communications
Measure the network voltage at various points across the network, especially at the ends
and at each device. The measured voltage should ideally be 24 V, but no more than 25 V
and not less than 11 V DC.
If devices draw a lot of current, then voltages on the bus can fluctuate hence bus
voltages should be monitored over time.
If the voltages are not within specification, then:
• Check for faulty or loose connectors
• Check the power system design calculations by measuring the current flow in
each section of cable.
On some DeviceNet analyzers, one can set a supply alarm voltage below which a
warning should be generated. Plug the analyzer at locations far from the power supply
and leave it running over time. If the network voltage falls below this level at any time,
this low voltage event will be logged by the analyzer.
Note that ‘thin’ cable, which has a higher DC resistance, will have greater voltage drop
across distance.
13b.3.3
Incorrect signal voltage levels
The following signal levels should be observed with a voltmeter, oscilloscope or DeviceNet
analyzer. Readings that differ by more than 0.5 V with the following values are most likely
indicative of a problem.
CAN_H can NEVER be lower than CAN_L and if this is observed, it means that the
two wires have probably been transposed.
• If bus communications are OFF (idle) the following values can be observed
with any measuring device
– CAN_H (white) 2.5 V DC
– CAN_L (blue) 2.5 V DC.
• If bus communications are ON, the following can be observed with a voltmeter:
– CAN_H (white) 3.0 V DC
– CAN_L (blue) 2.0 V DC.
Alternatively, the voltages can be observed with an oscilloscope or DeviceNet analyzer,
in which case both minimum and maximum values can be observed.
These are:
• CAN_H (white) 2.5 V min., 4.0 V max.
• CAN_L (blue) 1.0 V min., 2.5 V max.
13b.3.4
Common mode voltage problems
This test assumes that the shield had already been checked for continuity and current
flow and can be done with a voltmeter or an oscilloscope:
• Turn all network power supplies on
• Configure all nodes to draw the maximum amount of power from the network
• Turn on all outputs that draw current.
Now measure the DC voltage between V– and the shield. The difference should,
technically speaking, be less than 4.65 V. For a reasonable safety margin, this value
should be kept below 3 V.
DeviceNet troubleshooting 301
These measurements should be taken at the two furthest ends (terminator position), at
the DeviceNet master(s) and at each power supply. Should a problem be observed here, a
solution could be to relocate the power supply to the middle of the network or to add
additional power supplies.
In general, one can design a network using any number of power supplies, providing
that:
• The voltage drop in the cable between a power supply and each station it
supplies does not exceed 5 V DC.
• The current does not exceed the cable/connector ratings.
• The power supply common ground voltage level does not vary by more than
5 V between any two points in the network.
13b.3.5
Incorrect termination
These tests can be performed with a MultiMate. They must be done with all bus
communications off (bus off) and the meter set to measure resistance.
Check the resistance from CAN_H to CAN_L at each device. If the values are larger than
60 Ω (120 Ω in parallel with 120 Ω) there could be a break in one of the signal wires or
there could be a missing terminator or terminators somewhere. If, on the other hand, the
measured values are less than 50 Ω, this could indicate a short between the network wires,
(an) extra terminating resistor(s), one or more faulty transceivers or unpowered nodes.
13b.3.6
Noise
Noise can be observed with a loudspeaker or with an oscilloscope. However, more
important than the noise itself, is the way in which the noise affects the actual
transmissions taking place on the medium. The most common effect of EMI/RFI
problems are CAN frame errors, which can be monitored with a CAN analyzer or
DeviceNet analyzer.
The occurrence of frame errors must be related to specific nodes and to the occurrence
of specific events, for example, a state change on a nearby variable frequency drive.
13b.3.7
Node communication problems
Node presence
One method to isolate defective nodes is to use the master configuration software or a
DeviceNet analyzer to create a ‘live list’ to see which nodes are active on the network,
and to compare this with the list of nodes that are supposed to be on the network.
Excessive traffic
The master configuration software or a DeviceNet analyzer can measure the percentage
traffic on the network. A figure of 30–70% is normal and anything over 90% is excessive.
Loads over 90% indicate problems. High bus loads can indicate any of the following:
• Some nodes could be having difficulty making connections with other nodes
and have to retransmit repeatedly to get messages through. Check termination,
bus length, topology, physical connections and grounding.
• Defective nodes can ‘chatter’ and put garbage on the network.
• Nodes supplied with corrupt or noisy power may chatter.
• Change of state (COS) devices may be excessively busy with rapidly changing
data and cause high percentage bus load.
302
Practical Industrial Data Communications
• Large quantities of explicit messages (configuration and diagnostic data) being
sent can cause high percentage bus load.
• Diagnostic instruments such as DeviceNet analyzers add traffic of their own; if
this appears to be excessive, the settings on the device can be altered to reduce
the additional traffic.
MACID/baud rate settings
A network status LED is built into many devices. This LED should always be flashing
GREEN. A solid RED indicates a communication fault, possibly an incorrect baud rate or
a duplicate station address (MACID).
Network configuration software can be used to perform a ‘network who’ to verify that
all stations are connected and communicating correctly.
In the absence of indicator LEDs, a DeviceNet analyzer will only be able to indicate
that one or more devices have wrong baud rates. The devices will have to be found by
inspection, and the baud rate settings corrected. First disconnect the device with the
wrong baud rate, correct the setting, and then reconnect the device.
In the absence of an indicator LED, there is no explicit way of checking duplicate
MACIDs either. If two nodes have the same address, one will just passively remain offline. One solution is to look for nodes that should appear in the live list, but do not.
Profibus PA/DP/FMS overview
Objectives
When you have completed study of this chapter, you will be able to:
• List the main features of Profibus PA/DP/FMS
• Fix problems with:
–
–
–
–
–
–
–
–
–
–
–
–
–
14a.1
Cabling
Fiber
Shielding
Grounding/earthing
Segmentation
Color coding
Addressing
Token bus operation
Unsolicited messages
Fine tuning of impedance terminations
Drop-line lengths
GSD files usage
Intrinsic safety concerns.
Introduction
Profibus (PROcess FIeld BUS) is a widely accepted international networking standard,
commonly found in process control and in large assembly and material handling
machines. It supports single-cable wiring of multi-input sensor blocks, pneumatic valves,
complex intelligent devices, smaller sub-networks (such as AS-i), and operator interfaces.
Profibus is nearly universal in Europe and also popular in North America, South
America, and parts of Africa and Asia. It is an open, vendor-independent standard. It
adheres to the OSI model and ensures that devices from a variety of different vendors can
communicate together easily and effectively. It has been standardized under the German
National standard as DIN 19 245 Parts 1 and 2 and, in addition, has also been ratified
under the European national standard EN 50170 Volume 2.
The development of Profibus was initiated by the BMFT (German Federal Ministry of
Research and Technology) in cooperation with several automation manufacturers in 1989.
The bus interfacing hardware is implemented on ASIC (application specific integrated
circuit) chips produced by multiple vendors, and is based on the RS-485 standard as well
304
Practical Industrial Data Communications
as the European EN50170 Electrical specification. The standard is supported by the
Profibus Trade Organization, whose website can be found at www.profibus.com.
Profibus uses nine-pin D-type connectors (impedance terminated) or 12 mm quickdisconnect connectors. The number of nodes is limited to 127. The distance supported
is up to 24 km (with repeaters and fiber-optic transmission), with speeds varying from
9600 bps to 12 Mbps. The message size can be up to 244 bytes of data per node per
message (12 bytes of overhead for a maximum message length of 256 bytes), while the
medium access control mechanisms are polling and token passing.
Profibus supports two main types of devices, namely, masters and slaves:
1. Master devices control the bus and when they have the right to access the bus,
they may transfer messages without any remote request. These are referred to
as active stations.
2. Slave devices are typically peripheral devices i.e. transmitters/sensors and
actuators. They may only acknowledge received messages or, at the request of a
master, transmit messages to that master. These are also referred to as passive
stations.
There are several versions of the standard, namely, Profibus DP (master/slave),
Profibus FMS (multi-master/peer-to-peer), and Profibus PA (intrinsically safe).
• Profibus DP (distributed peripheral) allows the use of multiple master devices,
in which case each slave device is assigned to one master. This means that
multiple masters can read inputs from the device but only one master can write
outputs to that device. Profibus DP is designed for high-speed data transfer at
the sensor/actuator level (as opposed to Profibus FMS which tends to focus on
the higher automation levels) and is based around DIN 19 245 parts 1 and 2
since 1993. It is suitable as a replacement for the costly wiring of 24 V and
4–20 mA measurement signals. The data exchange for Profibus DP is generally
cyclic in nature. The central controller, which acts as the master, reads the input
data from the slave and sends the output data back to the slave. The bus cycle
time is much shorter than the program cycle time of the controller (less than
10 ms).
• Profibus FMS (fieldbus message specification) is a peer-to-peer messaging
format, which allows masters to communicate with one another. Just as in
Profibus DP, up to 126 nodes are available and all can be masters if desired.
FMS messages consume more overhead than DP messages.
• ‘COMBI mode’ is when FMS and DP are used simultaneously in the same
network, and some devices (such as Synergetic’s DP/FMS masters) support
this. This is most commonly used in situations where a PLC is being used in
conjunction with a PC, and the primary master communicates with the
secondary master via FMS. DP messages are sent via the same network to I/O
devices.
• The Profibus PA protocol is the same as the latest Profibus DP with V1
diagnostic extensions, except that voltage and current levels are reduced to
meet the requirements of intrinsic safety (class I division II) for the process
industry. Many DP/FMS master cards support Profibus PA, but barriers are
required to convert between DP and PA. PA devices are normally powered by
the network at intrinsically safe voltage and current levels, utilizing the
transmission technique specified in IEC 61158-2 (which Foundation Fieldbus
H1 uses as well).
Profibus PA/DP/FMS overview 305
14a.2
Profibus protocol stack
The architecture of the Profibus protocol stack is summarized in Figure 14a.1. Note the
addition of an eighth layer, the so-called ‘user’ layer, on top of the seven-layer OSI
model.
Figure 14a.1
Profibus protocol stack
All three Profibus variations namely FMS, DP, and PA use the same data-link layer
protocol (layer 2). The DP and PA versions us the same physical layer (layer 1)
implementation, namely RS-485, while PA uses a variation thereof (as per IEC 61158-2)
in order to accommodate intrinsic safety requirements.
14a.2.1
Physical layer (layer 1)
The physical layer of the Profibus DP standard is based on RS-485 and has the following
features:
• The network topology is a linear bus, terminated at both ends.
• Stubs are possible.
• The medium is a twisted-pair cable, with shielding conditionally omitted
depending on the application. Type-A cable is preferred for transmission speeds
greater than 500 kbaud. Type B should only be used for low baud rates and
short distances. These are very specific cable types of which the details are
given below.
• The data rate can vary between 9.6 kbps and 12 Mbps, depending on the cable
length. The values are:
9.6 kbps
19.2 kbps
93.75 kbps
187.5 kbps
500 kbps
1.5 Mbps
12 Mbps
1200 m
1200 m
1200 m
600 m
200 m
200 m
100 m.
306
Practical Industrial Data Communications
The specifications for the two types of cable are as follows:
Type A cable
Impedance:
Cable capacity:
Core diameter:
Cable type:
Resistance:
Signal attenuation:
Shielding:
135 up to 165 Ω (for frequency of 3–20 MHz)
<30 pF per meter
>0.34 mm2 (AWG 22)
twisted-pair cable. 1 × 2 or 2 × 2 or 1 × 4
<110 Ω per km
max. 9 dB over total length of line section
Cu shielding braid or shielding braid and shielding foil
Type B cable
Impedance:
Cable capacity:
Core diameter:
Cable type:
Resistance:
Signal attenuation:
Shielding:
135 up to 165 Ω (for frequency >100 kHz)
<60 pF per meter
>0.22 mm2 (AWG 24)
twisted-pair cable. 1 × 2 or 2 × 2 or 1 × 4
<110 Ω per km
Max. 9 dB over total length of line section
Cu shielding braid or shielding braid and shielding foil
For a more detailed discussion of RS-485, refer to the chapter on RS-485 installation
and troubleshooting.
14a.2.2
Data-link layer (layer 2)
The second layer of the OSI model implements the functions of medium access control as
well as that of the logical link control i.e. the transmission and reception of the actual
frames. The latter includes the data integrity function i.e. the generation and checking of
checksums.
The medium access control determines when a station may transmit on the bus and
Profibus supports two mechanisms, namely, token passing and polling.
Token passing is used for communication between multiple masters on the bus. It
involves the passing of software token between masters, in a sequence of ascending
addresses. Thus, a logical ring is formed (despite the physical topology being a bus). The
polling method (or master–slave method), on the other hand, is used by a master that
currently has the token to communicate with its associated slave devices (passive
stations).
Profibus can be set up either as a pure master–master system (token passing), or as a
polling system (master–slave), or as a hybrid system using both techniques (Figure 14a.2).
The following is a more detailed description of the token-passing mechanism:
• The token is passed from master station to master station in ascending order.
• When a master station receives the token from a previous station, it may then
transfer messages to slave devices as well as to other masters.
• If the token transmitter does not recognize any bus activity within the slot time,
it repeats the token and waits for another slot time. It retires if it recognizes bus
activity. If there is no bus activity, it will repeat the token frame for the last
time. If there is still no activity, it will try to pass the token to the next but one
master station. It continues repeating the procedure until it identifies a station
that is alive.
Profibus PA/DP/FMS overview 307
Figure 14a.2
Hybrid medium access control
• Each master station is responsible for the addition or removal of stations in the
address range from its own station address to the next station. Whenever a
station receives the token, it examines one address in the address range between
itself and its current successor. It does this maintenance whenever its currently
queued message cycles have been completed. Whenever a station replies saying
that it is ready to enter the token ring it is then passed the token. The current
token holder also updates its new successor.
• After a power up and after a master station has waited a predefined period, it
claims the token if it does not see any bus activity. The master station with the
lowest station address commences initialization. It transmits two token frames
addressed to itself. This then informs the other master stations that it is now the
only station on the logical token ring. It then transmits a ‘request field data-link
status’ to each station in an increasing address order. The first master station
that responds is then passed the token. The slave stations and ‘master not ready’
stations are recorded in an address list called the GAP list.
• When the token is lost, it is not necessary to re-initialize the system. The lowest
address master station creates a new token after its token timer has timed out.
It then proceeds with its own messages and then passes the token onto its
successor.
• The real token rotation time is calculated by each master station on each cycle
of the token. The system reaction time is the maximum time interval between
two consecutive high-priority message cycles of a master station at maximum
bus load. From this, a target token rotation time is defined. The real token
rotation time must be less than the target token rotation time for low-priority
messages to be sent out.
• There are two priorities that can be selected by the application layer, namely ‘low’
and ‘high’. The high-priority messages are always dispatched first. Independent
of the token rotation time, a master station can always transmit one high-priority
message. The system’s target token rotation time depends on the number of
stations, the number of high-priority messages and the duration of each of these
messages. Hence it is important only to set very important and critical messages
to high-priority. The predefined target token rotation time should contain
308
Practical Industrial Data Communications
sufficient time for low-priority message cycles with some safety margin built in
for retries and loss of messages.
Basically the Profibus layer 2 operates in a connectionless fashion, i.e. it transmits
frames without prior checking as to whether the intended recipient is able or willing to
receive the frame. In most cases, the frames are ‘unicast’, i.e. they are intended for a
specific device, but broadcast and multicast communication is also possible. Broadcast
communication means that an active station sends an unconfirmed message to all other
stations (masters and slaves). Multicast communication means that a device sends out an
unconfirmed message to a group of stations (masters or slaves).
Layer 2 provides data transmission services to layer 7. These services are as defined in
DIN 19241-2, IEC 955, ISO 8802-2 and ISO/IEC JTC 1/SC 6N 4960 (LLC Type 1 and
LLC Type 3) and comprise three acyclic data services as well as one cyclic data service.
The following data transmission services are defined:
•
•
•
•
Send-data-with-acknowledge (SDA) – acyclic.
Send-data-with-no-acknowledge (SDN) – acyclic.
Send-and-request-data-with-reply (SRD) – acyclic.
Cyclic-send-and-request-data-with-reply (CSRD) – cyclic.
All layer 2 services are accessed by layer 7 in software through so-called service access
points or SAPs. On both active and passive stations, multiple SAPs (service access
points) are allowed simultaneously:
• 32 Stations are allowed without repeaters, but with repeaters this number may
be increased to 127.
• The maximum bus length is 1200 m. This may be increased to 4800 m with
repeaters.
• Transmission is half-duplex, using NRZ (non-return to zero) coding.
• The data rate can vary between 9.6 kbps and 12 Mbps, with values of 9.6, 19.2,
93.75, 187.5, 500, 1500 kbps, or 12 Mbps.
• The frame formats are according to IEC-870-5-1, and are constructed with a
hamming distance of 4. This means that despite up to four consecutive faulty
bits in a frame (and despite a correct checksum), a corrupted message will still
be detected.
• There are two levels of message priority.
14a.2.3
Application layer
Layer 7 of the OSI model provides the application services to the user. These services
make an efficient and open (as well as vendor independent) data transfer possible
between the application programs and layer 2.
The Profibus application layer is specified in DIN 19 245 part 2 and consists of:
• The Fieldbus message specification (FMS)
• The lower layer interface (LLI)
• The Fieldbus management services layer 7 (FMA 7).
14a.2.4
Fieldbus message specification (FMS)
From the viewpoint of an application process (at layer 8), the communication system is a
service provider offering communication services, known as the FMS services. These are
basically classified as either confirmed or unconfirmed services (Figure 14a.3).
Profibus PA/DP/FMS overview 309
Figure 14a.3
Execution of confirmed and unconfirmed services
Confirmed services are only permitted on connection-oriented communication relationships
while unconfirmed services may also be used on connectionless relationships. Unconfirmed
services may be transferred with either a high or a low priority.
In the Profibus standard, the interaction between requester and responder, as implemented
by the appropriate service, is described by a service primitive.
The Profibus FMS services can be divided into the following groups:
• Context management services allow establishment and release of logical
connections, as well as the rejection of inadmissible services.
• Variable access services permit access (read and write) to simple variables,
records, arrays, and variable lists.
• The domain management services enable the transmission (upload or download)
of contiguous memory blocks. The application process splits the data into
smaller segments (fragments) for transmission purposes.
• The program invocation services allow the control (start, stop, etc.) of program
execution.
• The event management services are unconfirmed services, which make the
transmission of alarm messages possible. They may be used with high or low
priority, and messages may be transmitted on broadcast or multicast communication relationships.
• The VFD support messages permit device identification and status reports.
These reports may be initiated at the discretion of individual devices, and
transmitted on broadcast or multicast communication relationships.
• The (OD) management services permit object dictionaries to be read and
written. Process objects must be listed as communication objects in an object
dictionery OD. The application process on the device must make its objects
visible and available before these can be addressed and processed by the
communication services.
310
Practical Industrial Data Communications
As can be seen, there are large amounts of Profibus FMS application services to satisfy
the various requirements of field devices. Only a few of these (5, in fact) are mandatory
for implementation in all Profibus devices. The selection of further services depends on
the specific application and is specified in the so-called profiles.
14a.2.5
Lower layer interface (LLI)
Layer 7 needs a special adaptation to layer 2. This is implemented by the LLI in the
Profibus protocol. The LLI conducts the data flow control and connection monitoring as
well as the mapping of the FMS services onto layer 2, with due consideration of the
various types of devices (master or slave).
Communications relationships between application processes with the specific purpose
of transferring data must be defined before a data transfer is started. These definitions are
listed in layer 7 in the communications relationship list (CRL).
The main tasks of the LLI are:
•
•
•
•
Mapping of FMS services onto the data-link layer services
Connection establishment and release
Supervision of the connection
Flow control.
The following types of communication relationships are supported by:
•
•
•
•
Connectionless communication, which can be either
Broadcast, or
Multicast, and
Connection-oriented communication, which can be either
– Master/master (cyclic or acyclic), or
– Master/slave – with or without slave initiative – (cyclic or acyclic).
Connection-oriented communication relationships represent a logical peer-to-peer
connection between two application processes. Before any data can be sent over this
connection, it has to be established with an initiate service, one of the context management services. This comprises the connection establishment phase. After successful
establishment, the connection is protected against third party access and can then be used
for data communication between the two parties involved. This comprises the data transfer
phase. In this phase, both confirmed and unconfirmed services can be used. When the
connection is no longer needed, it can be released with yet another context management
service, the Abort service. This comprises the connection release phase (Figure 14a.4).
Figure 14a.4
Supported communication relationships
Profibus PA/DP/FMS overview 311
14a.2.6
Fieldbus management layer (FMA 7)
This describes object and management services. The objects are manipulated locally or
remotely using management services. There are three groups here:
1. Context management: This provides a service for opening and closing a
management connection.
2. Configuration management: This provides services for the identification of
communication components of a station, for loading and reading the communi
cation relationship list (CRL), and for accessing variables, counters, and the
parameters of the lower layers.
3. Fault management: This provides services for recognizing and eliminating
errors.
14a.3
The Profibus communication model
From a communication point of view, an application process includes all programs,
resources, and tasks that are not assigned to one of the communication layers. The
Profibus communication model permits the combination of distributed application
processes into a common process, using communications relationships. This acts to unify
distributed application processes to a common process. That part of an application
process in a field device that is reachable for communication is called a virtual field
device (VFD).
All objects of a real device that can be communicated with (such as variables, programs,
data ranges) are called communication objects. The VFD contains the communication
objects that may be manipulated by the services of the application layer via Profibus.
14a.4
Relationship between application process
and communication
Between two application processes, one or more communication relationships may exist;
each one having a unique communication end point as shown in Figure 14a.5:
Figure 14a.5
Assignment of communication relationships to application process
312
Practical Industrial Data Communications
Mapping of the functions of the VFD onto the real device is provided by the application
layer interface. Figure 14a.6 shows the relationship between the real field device and the
virtual field device.
Figure 14a.6
Virtual field device (VFD) with object dictionary (OD)
In this example, only the variables pressure, fill level, and temperature may be read or
written via two communication relationships.
14a.5
Communication objects
All communication objects of a Profibus station are entered into a local object dictionary.
This object dictionary may be predefined at simple devices; however on more complex
devices it is configured and locally or remotely downloaded into the device.
The object dictionary (OD) structure contains:
• A header, which contains information about the structure of the OD
• A static list of types, containing the list of the supported data types and data
structures
• A static object dictionary, containing a list of static communication objects
• A dynamic list of variable lists, containing the actual list of the known variable
lists, and a dynamic list of program invocations, which contains a list of the
known programs.
Defined static communication objects include simple variable, array (a sequence of
simple variables of the same type), record (a list of simple variables not necessarily of the
same type), domain (a data range), and event.
Dynamic communication objects are entered into the dynamic part of the OD. They
include program invocation and variable list (a sequence of simple variables, arrays, or
records). These can be predefined at configuration time, dynamically defined, deleted, or
changed with the application services in the operational phase.
Profibus PA/DP/FMS overview 313
Logical addressing is the preferred way of addressing of communication objects. They
are normally accessed with a short address called an index (unsigned 16 bit). This makes
for efficient messaging and keeps the protocol overhead down.
There are, however, two other optional addressing methods:
1. Addressing by name, where the symbolic name of the communication objects
is transferred via the bus.
2. Physical addressing. Any physical memory location in the field device may be
accessed with the services PhysRead and PhysWrite.
It is possible to implement password protection on certain objects and also to make
them read-access only, for example.
14a.6
Performance
A short reaction time is one of the main advantages of Profibus DP. The figures are
typical.
512 Inputs and outputs distributed over 32 stations can be accessed:
• In 6 ms at 1.5 Mbps and
• In 2 ms at 12 Mbps.
Figure 14a.7 gives a visual indication of Profibus performance.
Figure 14a.7
Bus cycle time of a Profibus DP mono-master system
The main service used to achieve these results is the send and receive data service of
layer 2. This allows for the transmission of the input and output data in a single message
cycle. Obviously, the other reason for increased performance is the higher transmission
speed of 12 Mbps.
14a.7
System operation
14a.7.1
Configuration
The choice is up to user as to whether the system should be a mono-master or multimaster system. Up to 126 stations (masters or slaves) can be accommodated.
314
Practical Industrial Data Communications
There are different device types:
• DP-master class 1 (DPM1): This is typically a PLC (programmable logical
controller).
• DP-master class 2 (DPM2): These devices are used for programing,
configuration or diagnostics.
• DP-slave A: This is typically a sensor or actuator. The amount of I/O data is
limited to 246 bytes.
The two configurations possible are shown in Figures 14a.8 and 14a.9.
Figure 14a.8
Profibus DP mono-master system
Figure 14a.9
Profibus DP multi-master system
The following states can occur with DPM1:
• Stop: In this state, no data transfer occurs between the DPM1 and DP-slaves.
• Clear: The DPM1 puts the outputs into a fail-safe mode and reads the input
data from the DP-slaves.
• Operate: The DPM1 is in the data transfer state with a cyclic message
sequence where input data is read and output data is written down to the slave.
Profibus PA/DP/FMS overview 315
14a.7.2
Data transfer between DPM1 and the DP-slaves
During configuration of the system, the user defines the assignment of a DP slave to a
DPM1 and which of the DP-slaves are included in the message cycle. In the so-called
parameterization and configuration phases, each slave device compares its real configuration with that received from the DPM1. This configuration information has to be
identical. This safeguards the user from any configuration faults. Once this has been
successfully checked, the slave device will enter into the data transfer phase as indicated
in Figure 14a.10.
Figure 14a.10
User data exchange for Profibus DP
14a.7.3
Synchronization and freeze modes
In addition to the standard cyclic data transfer mechanisms automatically executed by
the DPM1, it is possible to send control commands from a master to an individual
or group of slaves.
If the ‘sync’ command is transmitted to the appropriate slaves, they enter this state and
freeze the outputs. They then store the output data during the next cyclic data exchange.
When they receive the next ‘sync’ command, the stored output data is issued to the field.
If a ‘freeze’ command is transmitted to the appropriate slaves, the inputs are frozen in
the present state. The input data is only updated on receiving the next ‘freeze’ command.
14a.7.4
Safety and protection of stations
At the DPM1 station, the user data transfer to each slave is monitored with a watchdog
timer. If this timer expires indicating that no successful transfer has taken place, the user
is informed and the DPM1 leaves the OPERATE state and switches the outputs of all the
assigned slave devices to the fail-safe state. The master changes to the CLEAR state.
Note that the master ignores the timer if the automatic error reaction has been enabled
(Auto_Clear = True).
316
Practical Industrial Data Communications
At the slave devices, the watchdog timer is again used to monitor any failures of the
master device or the bus. The slave switches its outputs autonomously to the fail-safe
state if it detects a failure.
14a.7.5
Mixed operation of FMS and DP stations
Where lower reaction times are acceptable, it is possible to operate FMS and DP devices
together on the same bus. It is also possible to use a composite device, which supports
both FMS and DP protocols simultaneously. This can make sense if the configuration is
done using FMS and the higher speed cyclic operations are done for user data transfer.
The only difference between the FMS and DP protocols are of course the application
layers (Figure 14a.11).
Figure 14a.11
Mixed operation of Profibus FMS and DP
Profibus troubleshooting
14b.1
Introduction
Profibus DP and FMS use RS-485 at the physical layer (layer 1) and therefore all the
RS-485 installation and troubleshooting guidelines apply. Refer to Chapter 14a. Profibus
PA uses the same physical layers as the 61158-2 standard (which is the same as the
Foundation Fieldbus H1 standard). This section will discuss some additional specialized
tools.
14b.2
Troubleshooting tools
14b.2.1
Handheld testing device
These are similar to the ones available for DeviceNet, and can be used to check the
copper infrastructure before connecting any devices to the cable. A typical example is the
unit made by Synergetic.
They can indicate:
•
•
•
•
A switch (i.e. reversal) of the A and B lines
Wire breaks in the A and B lines as well as in the shield
Short circuits between the A and B lines and the shield
Incorrect or missing terminations.
The error is indicated via text shown in the display of the device.
These devices can also be used to check the RS-485 interfaces of Profibus devices, after
they have been connected to the network. Typical functions include:
• Creating a list with the addresses of all stations connected to the bus (useful for
identifying missing devices)
• Testing individual stations (e.g. identifying duplicate addresses)
• Measuring distance (checking whether the installed segment lengths comply
with the Profibus requirements)
• Measuring reflections (e.g. locating an interruption of the bus line).
14b.2.2
D-type connectors with built-in terminators
For further location of cable break errors reported by a handheld tester, nine-pin D
connectors with integrated terminations are very helpful. When the termination is switched
318
Practical Industrial Data Communications
to ‘on’ at the connector, the cable leading out of the connector is disconnected. This feature
can be used to identify the location of the error, as follows.
If, for example, the handheld is connected at the beginning of the network and a wire
break of the A line is reported, plug the D connector somewhere in the middle of the
network and switch the termination to ‘on’. If the problem is still reported by the tester, it
means that the introduced termination is still not ‘seen’ by the tester and thus the cable
break must be between the beginning of the network and the D connector.
14b.2.3
Configuration utilities
Each Profibus network must be configured and various products are commercially available
to perform this task. Examples include the Profibus DP configuration tool by SST, the
Allen Bradley plug & play software, and the Siemens COM package (Figure 14b.1). In
many cases, the decision on the tool to be used for configuration is made automatically by
choosing the controlling device for the bus. The choice of configuration tool should not be
treated lightly because the easier the tool is to use, the less likely a configuration error will
be made.
Figure 14b.1
Applicom configuration tool
With Profibus, all parameters of a device (including text to provide a good explanation
of the parameters and of the possible choices, values and ranges) are specified in a
so-called GSD file, which is the electronics data sheet of the device. Therefore the
configuration software has all the information it needs to create a friendly user interface
with no need for the interpretation of hexadecimal values.
14b.2.4
Built-in diagnostics
Several diagnostic functions were designed into Profibus. The standard defines
different timers and counters used by the physical layer to maintain operation and
monitor the quality of the network. One counter, for example, counts the number of
Profibus troubleshooting
319
invalid start delimiters received as an indication of installation problems or an
interface not working properly. These timers and counters can be used by the
Profibus device or by its configuration tool to identify a problem and to indicate it
to the user.
For Profibus DP, a special diagnostic message is defined, which can be indicated by a
Profibus DP slave or requested by a Profibus DP master. The first 6 bytes are implemented for all Profibus DP devices.
This information is used to indicate various problems to the user and could include:
• Configuration of the specific device incorrect
• Required features not supported on the device or
• Device does not answer.
The user normally gets access to all this information through the configuration tool. The
user selects a device, the tool reads the diagnostic information from the device, and
provides high-level text information.
During operation, a DP device automatically reports problems to the Profibus DP
master. The master stores the diagnostic information and provides it to the user. This can,
for example, be done by a PC-based system that utilizes diagnostic flow charts to evaluate
the information and then make it available to the operator.
The definition of additional diagnostics enables each manufacturer to simplify matters
for end-users. The additional information differentiates between a device-related, an
identifier-related, and a channel-related part. The device-related part provides the
opportunity to encode manufacturer-specific details. This can be used to report that the
module place in slot #4 is not the same as the one configured. Module-related diagnostics
provides an overview of the status of all modules; it identifies whether a module supports
diagnostics or not. The channel-related part offers the possibility to report problems down
to the bit level. This means a DP slave can indicate that channel #3 of the module in slot
#5 has a short-circuit to ground.
With the additional diagnostics, a Profibus DP device can send very detailed error
reports to the controlling device. As a result, the master device is able to provide details
to the user such as ‘ERROR oven control: lower temperature limit exceeded’ or ‘station
address 23 (conveyor control): wire break at module 2, channel 5’. This feature provides
not only the flexibility to report any kind of error at a device but also often how to correct
it. Because the protocol for Profibus PA is identical to that for DP, the diagnostic
mechanism is the same.
14b.2.5
Bus monitors
A bus monitor (protocol analyzer) is an additional tool for troubleshooting a Profibus
network, enabling the user to perform packet content and timing verification. It is capable
of monitoring and capturing all Profibus network activity (messages, ACKs, etc.) on the
bus, and then saving the captured data to disk. Each captured message is time-stamped
with sub-millisecond resolution. A monitor does not have a Profibus station address nor
does it affect the speed or efficiency of the network.
A monitor provides a wide range of trigger and filter functions that allow capturing
messages between two stations only or triggering on a special event like diagnostic
requests. Such a tool can be used for the indication of problems with individual devices
(e.g. wrong configuration) and also to visualize physical problems.
320
Practical Industrial Data Communications
Bus monitors are typically PCs with a special Profibus interface card and the
appropriate data capturing software. An example is the Profibus DP capturing utility by
SST. Bus monitors are sophisticated tools and are recommending only for people with a
reasonable knowledge of Profibus and its protocol.
14b.3
Tips
Profibus (especially DP) is straightforward from a user’s point of view, as the messaging
format is fairly simple.
However, the following notes will be helpful in identifying common problems:
• Profibus has a relatively high (12 Mbps) maximum data rate but it can also be
operated at speeds as low as 9600 baud. If Profibus is to be used at high speeds,
it might be necessary to use a scope or analyzer to check and fine-tune
impedance terminations and drop-line lengths. Such problems are magnified
at higher speeds. Users who initially intend to run their network at maximum
speed often find that a lower speed setting performs just as well and is easier to
get working.
• One of the most common problems encountered in configuring a Profibus
network is selecting the wrong GSD (device description) file for a particular
node. As GSD files reside in a separate disk and are not embedded in the
product itself, files are sometimes paired with the wrong devices.
• When installing a new network, follow the Profibus installation guidelines:
– Use connectors suitable for an industrial environment and according to
the defined standard
– Use only the specified (blue) cable
– Make sure the cable has no wire break and none of the wires causes a
short circuit condition
– Do not crisscross the wires; always use the green wire for A and the red
wire as B throughout the whole network
– Make sure the segment length is according to the chosen transmission rate
(use repeaters to extend the network)
– Make sure the number of devices/RS-485 drivers per segment does not
exceed 32 (use repeaters where necessary)
– Check proper termination of all copper segments (an RS-485 segment
must be terminated at both ends)
– If so-called ‘activated terminations’ are used, they must be powered at all
times
– Avoid drop lines or make sure the overall length does not exceed the
specified maximum. In case T-drops are needed, use repeaters or active
bus terminals
– In case the network connects buildings or runs in a hazardous
environment, consider the use of fiber optics
– Check whether the station addresses are set to the correct value
– Check if the network configurations match the physical setup
– For RS-485 implementations (Profibus FMS and Profibus DP), type A
cable is preferred for transmission speeds greater than 500 kbaud; type B
should only be used for low baud rates and short distances.
Profibus troubleshooting
321
The specifications for the two types of cable are as follows:
• The connection between shield and protective ground is made via the metal
cases and screw tops of the D-type connectors. Should this not be possible, then
the connection should be made via pin 1 of the connectors. This is not an
optimum solution and it is probably better to bare the cable shield at the
appropriate point and to ground it with a cable, as short as possible, to the
metallic structure of the cabinet.
Foundation Fieldbus overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe how Foundation Fieldbus operates
• Remedy problems with:
–
–
–
–
–
–
–
–
–
–
–
15a.1
Wiring
Earths/grounds
Shielding
Wiring polarity
Power
Terminations
Intrinsic safety
Voltage drop
Power conditioning
Surge protection
Configuration.
Introduction to Foundation Fieldbus
Foundation Fieldbus (FF) takes full advantage of the emerging ‘smart’ field devices and
modern digital communications technology allowing end-user benefits such as:
•
•
•
•
•
•
•
Reduced wiring
Communications of multiple process variables from a single instrument
Advanced diagnostics
Interoperability between devices of different manufacturers
Enhanced field level control
Reduced start-up time
Simpler integration.
The concept behind Foundation Fieldbus is to preserve the desirable features of the
present 4–20 mA standard (such as a standardized interface to the communications link,
bus power derived from the link and intrinsic safety options) while taking advantage of
the new digital technologies.
Foundation Fieldbus overview 323
This will provide the features noted above because of:
• Reduced wiring due to the multi-drop capability
• Flexibility of supplier choices due to interoperability
• Reduced control room equipment due to distribution of control functions to the
device level
• Increased data integrity and reliability due to the application of digital
communications.
Foundation Fieldbus consists of four layers. Three of them correspond to OSI layers 1,
2, and 7. The fourth is the so-called ‘user layer’ that sits on top of layer 7 and is often said
to represent OSI ‘layer 8’, although the OSI model does not include such a layer. The
user layer provides a standardized interface between the application software and the
actual field devices.
15a.2
The physical layer and wiring rules
The physical layer standard has been approved and is detailed in the IEC 61158-2 and the
ISA standard S50.02-1992. It supports communication rates of 31.25 kbps and uses
the Manchester Bi-phase L encoding scheme with four encoding states as shown in
Figure 15a.1. Devices can be optionally powered from the bus under certain conditions. The
31.25 kbps (or H1, or low-speed bus) can support 2–32 devices that are not bus-powered,
2–12 devices that are bus-powered, or 2–6 devices that are bus-powered in an intrinsically
safe area. Repeaters are allowed and will increase the length and number of devices that can
be put on the bus. The H2 or high-speed bus options was not implemented as originally
planned, but was superseded by the high-speed Ethernet (HSE) standard. This is discussed
later in this section.
Figure 15a.1
Use of N+ and N– encoding states
The low-speed (H1) bus is intended to utilize existing plant wiring and uses #22 AWG
type-B wiring (shielded twisted pair) for segments up to 1200 m (3936 ft) and #18 AWG
type-A wiring (shielded twisted pair) up to 1900 m (6232 ft). Two additional types of
cabling are specified and are referred to as type C (multi-pair twisted without shield) and
type D (multi-core, no shield).
324
Practical Industrial Data Communications
Type C using #26 AWG cable is limited to 400 m (1312 ft) per segment and type D
with #16 AWG is restricted to segments less than 200 m (660 ft).
•
•
•
•
Type A: #18 AWG
1900 m (6232 ft)
Type B: #22 AWG
1200 m (3936 ft)
Type C: #26 AWG
400 m (1312 ft)
Type D: #16 AWG multi-core 200 m (660 ft).
The Foundation Fieldbus wiring is floating/balanced and equipped with a termination
resistor (RC combination) connected across each end of the transmission line
(Figure 15a.2). Neither of the wires should ever be connected to ground. The terminator
consists of a 100 Ω quarter Watt resistor and a capacitor sized to pass 31.25 kHz. As an
option, one of the terminators can be center-tapped and grounded to prevent voltage
buildup on the bus. Power supplies must be impedance matched. Off-the-shelf power
supplies must be conditioned by fitting a series inductor. If a ‘normal power supply’ is
placed across the line, it will load down the line due to its low impedance. This will cause
the transmitters to stop transmitting.
Figure 15a.2
Foundation Fieldbus physical layer
Foundation Fieldbus overview 325
Fast response times for the bus were one of the FF goals. For example, at 31.25 kbps on
the H1 bus, response times as low as 32 µs are possible. This will vary, based on the
loading of the system, but will average between 32 µs and 2.2 ms with an average of
approximately 1 ms.
Spurs can be connected to the ‘home run’. The length of the spurs depends on the type
of wire used and the number of spurs connected. The maximum length is the total length
of the spurs and the home run.
The upper layers are defined by the Fieldbus Foundation as the ‘communications stack’
and the ‘user layer’ (Figure 15a.3). The following sections will explore these upper
layers.
Figure 15a.3
The OSI model of the FF protocol stack
15a.3
The data-link layer
The communications stack, as defined by the FF, corresponds to OSI layers 2 and 7, the
data-link and applications layers. The DLL (data-link layer) controls access to the bus
through a centralized bus scheduler called the link active scheduler (LAS). The DLL
packet format is shown in Figure 15a.4.
The LAS controls access to the bus by granting permission to each device according
to predefined ‘schedules’. No device may access the bus without LAS permission.
There are two types of schedules implemented: cyclic (scheduled) and acyclic
(unscheduled). It may seem odd that one could have an unscheduled ‘schedule’, but
these terms actually refer to messages that have a periodic or non-periodic routine, or
‘schedule’.
326
Practical Industrial Data Communications
Figure 15a.4
Data-link layer packet format
The cyclic messages are used for information (process and control variables) that
requires regular, periodic updating between devices on the bus. The technique used for
information transfer on the bus is known as the publisher–subscriber method. Based on
the user predefined (programed) schedule, the LAS grants permission for each device in
turn access to the bus. Once the device receives permission to access the bus, it
‘publishes’ its available information. All other devices can then listen to the ‘published’
information and read it into memory (subscribe) if it requires it for its own use. Devices
not requiring specific data simply ignore the ‘published’ information.
The acyclic messages are used for special cases that may not occur on a regular basis.
These may be alarm acknowledgment or special commands such as retrieving diagnostic
information from a specific device on the bus. The LAS detects time slots available
between cyclic messages and uses these to send the acyclic messages.
15a.4
The application layer
The application layer in the FF specification is divided into two sub-layers: the
Foundation Fieldbus access sublayer (FAS) and the Foundation Fieldbus messaging
specification (FMS).
The capability to pre-program the ‘schedule’ in the LAS provides a powerful
configuration tool for the end-user since the time of rotation between devices can be
established and critical devices can be ‘scheduled’ more frequently to provide a form of
prioritization of specific I/O points. This is the responsibility and capability of the FAS.
Programing the schedule via the FAS allows the option of implementing (actually,
simulating) various ‘services’ between the LAS and the devices on the bus.
Three such ‘services’ are readily apparent such as:
1. Client/server with a dedicated client (the LAS) and several servers (the bus
devices)
2. Publisher/subscriber as described above
3. Event distribution with devices reporting only in response to a ‘trigger’ event,
or by exception, or other predefined criteria.
These variations, of course, depend on the actual application and one scheme need not
necessarily be ‘right’ for all applications, but the flexibility of the Foundation Fieldbus is
easily understood from this example.
The second sub-layer, the Foundation FMS, contains an ‘object dictionary’ that is a
type of database that allows access to Foundation Fieldbus data by tag name or an index
number. The object dictionary contains complete listings of all data types, data type
descriptions, and communication objects used by the application. The services allow the
object dictionary (application database) to be accessed and manipulated.
Foundation Fieldbus overview 327
Information can be read from or written to the object dictionary allowing manipulation
of the application and the services provided.
15a.5
The user layer
The Fieldbus Foundation specifies an eighth layer called the user layer that resides
‘above’ the application layer of the OSI model; this layer is usually referred to as layer
8. In the Foundation Fieldbus, this layer is responsible for three main tasks viz. network
management, system management, and function block/device description services.
Figure 15a.5 illustrates how all the layer’s information packets are passed to the
physical layer.
Figure 15a.5
The passage of information packets to the physical layer
The network management service provides access to the other layers for performance
monitoring and managing communications between the layers and between remote
objects (objects on the bus). The system management takes care of device address
assignment, application clock synchronization, and function block scheduling. This is
essentially the time coordination between devices and the software, and ensures correct
time stamping of events throughout the bus.
Function blocks and device description services provide pre-programed ‘blocks’, which
can be used by the end-user to eliminate redundant and time-consuming configuration. The
block concept allows selection of generic functions, algorithms, and even generic devices
from a library of objects during system configuration and programing. This process can
dramatically reduce configuration time since large ‘blocks’ are already configured and
simply need to be selected. The goal is to provide an open system that supports
interoperability and a device description language (DDL), which will enable multiple
vendors and devices to be described as ‘blocks’ or ‘symbols’. The user would select generic
devices then refine this selection by selecting a DDL object to specify a specific vendor’s
product. Entering a control loop ‘block’ with the appropriate parameters would nearly
328
Practical Industrial Data Communications
complete the initial configuration for the loop. Advanced control functions and mathematics
‘blocks’ are also available for more advanced control applications.
15a.6
Error detection and diagnostics
FF has been developed as a purely digital communications bus for the process industry
and incorporates error detection and diagnostic information. It uses multiple vendors’
components and has extensive diagnostics across the stack from the physical link up
through the network and system management layers by design.
The signaling method used by the physical layer timing and synchronization is
monitored constantly as part of the communications. Repeated messages and the reason
for the repetition can be logged and displayed for interpretation.
In the upper layer, network, and system management is an integral feature of the
diagnostic routines. This allows the system manager to analyze the network ‘on-line’ and
maintain traffic loading information. As devices are added and removed, optimization of
the LAS routine allows communications optimization dynamically without requiring a
complete network shutdown. This ensures optimal timing and device reporting, giving
more time to higher-priority devices and removing, or minimizing, redundant, or lowpriority messaging.
With the device description (DD) library for each device stored in the host controller
(a requirement for true interoperability between vendors), all the diagnostic capability of
each vendors’ products can be accurately reported and logged and/or alarmed to provide
continuous monitoring of each device.
15a.7
High-speed Ethernet (HSE)
High-speed Ethernet (HSE) is the Fieldbus Foundation’s backbone network running at
100 Mbits/second. HSE field devices are connected to the backbone via HSE-linking
devices. A HSE-linking device is a device used to interconnect H1 Fieldbus segments to
HSE to create a larger network. A HSE switch is an Ethernet device used to interconnect
multiple HSE devices such as HSE-linking devices and HSE field devices to form an
even larger HSE network. HSE hosts are used to configure and monitor the linking
devices and H1 devices. Each H1 segment has its own LAS located in a linking device.
This feature enables the H1 segments to continue operating even if the hosts are
disconnected from the HSE backbone. Multiple H1 (31.25 kbps) Fieldbus segments can
be connected to the HSE backbone via linking devices (Figure 15a.6).
Figure 15a.6
High-speed Ethernet and Foundation Fieldbus
Foundation Fieldbus overview 329
15a.8
Good wiring and installation practice
15a.8.1
Termination preparation
If care is taken in the preparation of the wiring, there will be fewer problems later and
minimal maintenance required.
A few points to be noted here are:
• Strip 50 mm of the cable sheathing from the cable and remove the cable foil.
• Strip 6 mm of the insulation from the ends. Watch out to avoid wire nicking or
cutting off strands of wire. Use a decent cable-stripping tool.
• Crimp a ferrule on the wire ends and on the end of the shield wire. Crimp
ferrules are preferable as they provide a gas-tight connection between the wire
and the ferrule that is corrosion resistant. It is the same metal as the terminal in
the wiring block.
• An alternative strategy is to twist the wires together and to tin them with solder.
Wires can be put directly into the wire terminal but make sure all strands are in
and they are not touching each other. Make sure that the strands are not
stretched to breaking point.
• Do not attach shield wires together in a field junction box. This can be the
cause of ground loops.
• Do not ground the shield in more than one place (inadvertently).
• Use good wire strippers to avoid damaging the wire.
15a.8.2
Installation of the complete system
Other system components can be installed soon after the cable is installed. This includes
the terminators, power supply, power conditioners, spurs and in some cases, the intrinsic
safety barriers. Some devices already have terminator built-in. In that case, be careful that
you are not doubling up with terminators.
Check whether the grounding is correct. There should only be one point for the shield
ground point. Once these checks have been performed, switch on the power supply and
check the wiring system.
The Fieldbus tester (or an alternative simpler device) can be used to indicate:
• Polarities are correct
• Power-carrying capability of the wire system is ok
• The attenuation and distortion parameters are within specification.
A few additional wiring tips and suggestions with reference to the diagram in
Figure 15a.7.
• It is not possible to run two homerun cables in parallel for redundancy under
the H1 standard. H1 Fieldbus is a balanced transmission line that must be
terminated at each end. In some cases, it is a good idea to run a parallel cable
for future use. In case of physical damage, you need to disconnect the damaged
cable and put in the undamaged one. Ensure that, if this is the philosophy, you
do not route both cables in the same cable tray.
• Do not ground the shield of the cable at each Fieldbus device. The shield of the
cable at the transmitter (for example) should be trimmed and covered with
insulating tape or heat-shrinkable tubing. The only ‘ground’ that occurs on the
segment is usually at the control room Fieldbus power conditioner.
330
Practical Industrial Data Communications
Figure 15a.7
Overall diagram of Fieldbus wiring configuration (courtesy Relcom Inc.)
• Note that the ground that is connected to the isolated terminator at the far end
of the segment does not connect the shields of the Fieldbus. It only allows for a
high-frequency path for AC currents.
• There has been no provision made for lightning strikes. However, you should
specify a terminator that has some type of spark gap arrestor, which will clamp
the shields to about 75 V in such a high-voltage surge.
• A quick way to check that the grounding is correct before powering up is,
doing a resistance measurement from the ground bolt on the power conditioner
to the earth ground connection point. This measurement should be of the order
of megohms. You can then connect the earth-protective ground to the power
conditioner bolt. Once this has been done, measure the resistance from the
cable shields on the isolated terminator at the far end of the segment to a nearby
earth ground point. A very low value of resistance should be seen.
• A standard power supply cannot be used to power a Fieldbus segment.
A standard power supply absorbs most of the Fieldbus signals due its low
internal impedance. It is possible for a standard power supply to provide power
to a Fieldbus power conditioning device as long as it has sufficient current is a
floating power supply and has very low ripple and noise.
• Use wiring blocks that hold the wiring securely and will not vibrate loose.
Regular testing of an operational Fieldbus network
A Fieldbus tester can be used to get a view on the operation of the network. It is generally
connected as follows:
• Red terminal to the (+) wire
• Black terminal to the (–) wire
• Green terminal to the shield.
When the network is operating, the tester will build up a record of operational devices
and then builds up a record of their signal characteristics. During later routine network
maintenance, the results will be compared. If there is deterioration, this will be indicated,
and could be wiring problems, additional noise, or a device, whose transmitter is starting
to fail.
Foundation Fieldbus troubleshooting
15b.1
Introduction
Estimates are that 70% of network downtime is caused by physical problems. Foundation
Fieldbus is more complicated to troubleshoot most networks because it can, and often
does, use the communication bus to power devices. The troubleshooter needs to know not
only whether the communication is working but also whether there is enough power for
the devices. Figure 15b.1 is a diagram of a typical system. Notice that the power supply
on the left is supplying power to the devices in the system.
Figure 15b.1
A typical Foundation Fieldbus system
When troubleshooting a Foundation Fieldbus system, it is necessary to first determine
whether the problem is a power problem or a communications problem. In new systems,
it may be found that the problem is both. In working systems it is usually one or the other.
332
Practical Industrial Data Communications
15b.2
Power problems
Power problems in a FF system can be divided into two types. One where the system is
new and never worked and the other where the system has been up and running for a
while. When new devices are added to an existing system and the communications
immediately fails, it is easy to realize that the new device had something to do with the
problem. If the system has never worked, then the problem could be anywhere and could
be caused by multiple devices. The problem could also be with the design itself.
The following need to be known when troubleshooting the power system of a FF
system.
•
•
•
•
•
What is the layout of the system? Does each device have at least 9 V DC?
What is the supply current?
What is the supply voltage?
What is the current draw of each device?
What is the resistance of each cable leg?
The easiest way of determining a power problem is to do the following (Figure 15b.2):
•
•
•
•
Check each device to see if the power light is on
Measure the voltage at each device
Check the connections for opens, corrosion or loose connections
Measure the current draw of each device to see if it conforms to the manufacturer’s
specifications.
Figure 15b.2
Testing the system (courtesy Relcom Inc.)
Notice that the home run in Figure 15b.3 connects the control room equipment with the
devices via common terminal blocks (chickenfoot or crowsfoot). The signal cable also
provides the power to the devices. There is a terminator at each end of the cable. Power
supplies require power conditioners.
Power example
Here is an example of the power requirements for a system:
• The power supply output is 20 V
• The two wires are 1 km long with 22 Ω per wire (44 Ω total)
Foundation Fieldbus troubleshooting
333
Figure 15b.3
Layout of a system
• Each device draws 20 mA
• Minimum voltage at each device 9 V (20 – 9 = 11 V)
• 11 V/44 Ω = 250 mA.
Therefore
• 250 mA/20 mA = 12 devices on the system.
15b.3
Communication problems
Once the power is ruled out as a problem, it can be assumed that the communication
system is at fault (Figures 15b.4 and 15b.5). Initially, it is important to check the
following items:
• Are the wires connected correctly?
• Is the shield continuous throughout the system?
• Is the shield grounded at only one place?
Figure 15b.4
Schematic of a terminal block (courtesy Relcom Inc.)
Once these basics are verified, the next step is to check to make sure that the cables are
not too long. To measure the losses through the cable, a FF transmitter device is placed
at one end and a receiver test device at the other. The maximum loss is usually around
–14 dB. The typical characteristics of a twisted-pair cable is:
• Impedance: 100 Ω
• Wire size: 18 GA (0.8 mm2)
334
Practical Industrial Data Communications
Figure 15b.5
Terminal block (courtesy Relcom Inc.)
• Shield: 90% coverage
• Capacitive unbalance: 2 nF/km
• Attenuation: 3 dB/km.
Using an ungrounded oscilloscope, it is possible to look at the signal. A good
transmitter signal might look like Figure 15b.6.
Figure 15b.6
A good transmitted signal (courtesy Relcom Inc.)
When it is received it might look like Figure 15b.7.
Notice that the waveform is a bit distorted and lower in amplitude but still good.
Figure 15b.8 is what the whole packet might look like.
Foundation Fieldbus troubleshooting
335
Figure 15b.7
Received FF signal (courtesy Relcom Inc.)
Figure 15b.8
Bipolar FF signal (courtesy Relcom Inc.)
15b.4
Foundation Fieldbus test equipment
There are a few manufacturers that have brought out test equipment specifically designed
for testing FF systems. Some of the test equipment can be used while the system is
working and others are used when the system is offline.
Some of the things the test equipment can check for are:
•
•
•
•
•
•
•
DC voltage levels
Link active scheduler probe node frame voltage
Number of devices on the network
If devices have been added or removed
The lowest voltage level transmitted by a device
Noise level between frames
Device response noise level.
One of the best troubleshooting tools are the LEDs provided on the devices. These
LEDs show many different conditions of the system. If the troubleshooter becomes
familiar with them, then LEDs can often indicate what is wrong with the system.
Modbus Plus protocol overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe the main features of Modbus Plus
• Fix problems with:
–
–
–
–
16a.1
Cabling
Grounding/earthing
Shielding
Terminations.
General overview
Besides the standard Modbus protocol, there exist two other Modbus protocol structures:
• Modbus Plus
• Modbus II.
The more popular one is the Modbus Plus. It is not an open standard as the classical
Modbus has become. Modbus II is not used much due to additional cabling requirements
and other difficulties.
The Modbus Plus protocol was developed to overcome the ‘single master’ limitation of
the Modbus protocol. As described earlier, in order to share information across various
Modbus networks, a designer would have had to employ a complex hierarchical network
structure in order to achieve system-wide networking.
Modbus Plus is a local area network system for industrial control applications. Networked devices can exchange messages for the control and monitoring of processes at
remote locations in the industrial plant. The Modbus Plus network was one of the first
token-passing protocol networks that pioneered the development of other more advanced
deterministic protocols of today.
Each device on a network segment must be assigned a unique network address within
the range 1 through 64 inclusive. Multiple network segments may be bridged together to
form large systems. Messages are passed from one device to another by the appropriate
use of a route within the message. The route will include the network address of the target
device, and any inter-network addresses required. The route field of a message may be up
to five layers deep, is terminated with the value 00, and all unused drops are also set to 00
(Figure 16a.1).
Modbus Plus protocol overview
337
Figure 16a.1
Format of Modbus Plus message frame
Each network supports up to 64 addressable node devices. Up to 32 nodes can connect
directly to the network cable over a length of 1500 ft (450 m). Repeaters can extend the
cable distance to its maximum of 6000 ft (1800 m) and the node count to its maximum of
64 (Figure 16a.2). Fiber-optic repeaters are available for longer distances.
Figure 16a.2
Typical Modbus Plus network with termination
Multiple networks can be inter-networked through bridge plus devices. Messages
originating on one network are routed through one or more bridges to a destination on
another network. Bridges do not permit deterministic timing of I/O processes. In networks
requiring deterministic I/O timing, messages remain on that network only, and do not pass
through bridges (Figure 16a.3).
Network nodes are identified by addresses assigned by the user. Each node’s address is
independent of its physical site location. Addresses are within the range of 1–64 decimal
and do not have to be sequential. Duplicate addresses are not allowed.
Network nodes function as peer members of a logical ring, gaining access to the
network upon receipt of a token frame. The token is a grouping of bits that is passed in a
rotating address sequence from one node to another. Each network maintains its own
token rotation sequence independent of the other networks. Where bridges join multiple
networks, the token is not passed through the bridge device.
While holding the token, a node initiates message transactions with other nodes. Each
message contains routing fields that define its source and destination, including its routing
path through bridges to a node on a remote network.
338
Practical Industrial Data Communications
Figure 16a.3
Modbus Plus network with bridges
When passing the token, a node can write into a global database that is broadcast to all
nodes on the network. Global data is transmitted as a field within the token frame. Other
nodes monitor the token pass and can extract the global data if they have been programed
to do so. Use of the global database allows rapid updating of alarms, setpoints, and other
data. Each network maintains its own global database, as the token is not passed through
a bridge to another network.
The network bus consists of twisted-pair shielded cable that is run in a direct path
between successive nodes. The two data lines in the cable are not sensitive to polarity;
however a standard wiring convention is followed in this guide to facilitate
maintenance.
On dual-cable networks (Figure 16a.4), the cables are known as cable A and cable B.
Each cable can be up to 1500 ft (450 m) long, measured between the two extreme end
devices on a cable section. The difference in length between cables A and B must not
exceed 500 ft (150 m), measured between any pair of nodes on the cable section.
The token passing sequence is determined by the node addresses. Token rotation begins
at the network’s lowest number active node, proceeding consecutively through each
higher number node, until the highest number active node receives the token. That node
then passes the token to the lowest one to begin a new rotation.
Modbus Plus protocol overview
339
Figure 16a.4
Typical Modbus Plus network with dual cabling
If a node leaves the network, a new token-passing sequence will be established to
bypass it, typically within 100 ms. If a new node joins, it will be included in the address
sequence, typically within 5 s (worst case time is 15 s). The process of deleting and
adding nodes is transparent to the user application.
Where multiple networks are inter-connected with bridges, tokens are not passed
through a bridge device from one network to another. Each network performs its token
passing process independently of the other networks.
While a node holds the token, it sends its application messages if it has any to transmit.
The other nodes monitor the network for incoming messages. When a node receives a
message, it sends an immediate acknowledgment to the originating node. If the message
is a request for data, the receiving node will begin assembling the requested data into a
reply message.
When the message is ready, it will be transmitted to the requestor when the node
receives a subsequent token granting it access to transmit. After a node sends all of its
messages, it passes the token on to the next node. Protocols for token passing and
messaging are transparent to the user application.
Modbus Plus troubleshooting
16b.1
Common problems and faults
The Modbus Plus network is a 3-wire (one pair and a shield) twisted-pair cable with the
nodes connected in a daisy-chained configuration. There is no polarity requirement at the
node’s transceiver, so the data cable pair may be connected either way at a node.
A 220 Ω terminator is required at each end of the network cable. There are limits upon
the maximum number of nodes per segment, the number of repeaters, and the lengths of
cable segments on the Modbus Plus network. For more information, refer to the Modicon
Modbus Plus network planning and installation guide (GM-MBPL-001).
The most common hardware-related problems have already been discussed in the
previous section. Please refer to the hardware-related troubleshooting list for the Modbus
protocol. You should configure the node address of the Modbus Plus port on the nodes
before connecting them to the network. This should avoid possible duplicate address
problems with other units on the network.
Most problem issues arise from the use of invalid slave addresses, illegal slave memory
addressing, illegal data formats and even perhaps use of unrecognized function codes.
Other issues are related to the actual configuration of the communication hardware itself,
for example, the SA85 Modbus Plus network interface card that is used to provide an
IBM compatible personal computer access to the Modbus Plus network.
The following are typical error messages reported by a controller:
•
•
•
•
•
Remote PC not listening for call
Slave xx reported an illegal data address
Slave xx reported an illegal data value
Slave xx reported an illegal function
Card SA85: 0 not found.
Often, problems could be easily be identified by referring to the user manual, and looking
up the meanings of the various status indicators provided on the front panel of the Modbus
Plus device. For example, there is usually a Modbus Active LED provided on the node.
Table 16b.1 is an example of such a Modbus Plus network status indicator for a specific
device.
16b.1.1
Modbus active LED
The Modbus active LED on the node flashes repetitive patterns to display node status.
Table 16b.1 lists typical Modbus active LED blinking patterns and their description.
Modbus Plus troubleshooting 341
Blinking Patterns
Flash every 160 ms
normal operation
Description
This node is successfully receiving and passing the token.
All nodes should blink at this rate under when working
properly
Flash every 1 s
This node is in the MONITOR_OFFLINE state. It must
remain in this state for 5 s and is not allowed to
transmit messages onto the network. While in this state,
the node is listening to all other nodes and building a
table of the known nodes
Two flashes, off 2 s This node is in the MAC_IDLE state. It is hearing other
nodes get the token but it is never receiving the token
itself. This node may have a bad transmitter
Three flashes, off
1.7 s
This node is not hearing any other nodes and it is
periodically claiming the token and finds no other
nodes to pass it to. This node may be the only active
node on the network or it may have a bad receiver
Four flashes, off
1.4 s
This node has heard a duplicate address packet.
There is another node on the network with the same
address as this node. This node is now in the
DUPLICATE_OFFLINE state where it will passively
monitor the link until it does not hear the duplicate
node for 5 s
Table 16b.1
Typical list of Modbus active LED blinking patterns
16b.2
Description of tools used
Apart from the standard set of tools that have been listed in the previous section while
discussing Modbus protocols to troubleshoot hardware-related problems, there are only
the manufacturer-supplied application packages that report any software errors that occur
in the Modbus Plus communication networks.
Section 16b.3 discusses the cause of some of the most common Modicon Modbus and
Modbus Plus error messages that you may encounter when attempting to establish
communications with one of the various off-the-shelf Modbus Plus-based applications
packages such as the Wonderware® Modicon Modbus or Modicon Modbus Plus I/O
server (Source: http://www.wonderware.com).
16b.3
Detailed troubleshooting
16b.3.1
Remote PC not listening for call
This error means that the Modbus or Modbus Plus I/O server is unable to ‘see’ a slave
device. That is, it is unable to get to the specified node(s), which has been defined in
the topic slave path. Either the slave path does not have the correct path defined or
342
Practical Industrial Data Communications
one or more of the node addresses in the slave path do not match the addresses of
the PLCs.
If you are unable to establish communications through more than one Modbus Bridge
Plus (BP), then start with the first bridge plus listed in the slave path and access a slave
device on that Modbus Plus network. For example, if your slave path is defined as
‘24.32.16.10’ and you are getting the ‘Remote PC not listening for call’ error, then begin
by establishing communications to a node through BP 24.
If you are addressing a slave device with an address of 25 on the first network, then set
the Slave Path to ‘24.25.0.0’. Then, establish communications to node 25 on the first
network (i.e. advise a point on node 25 and verify that the data is being retrieved). If you
are successful, then move onto the next BP and communicate to a node within that
network. Then, modify the slave path to ‘24.32.x.0’ where x represents the address of a
node within the next network. Repeat this step until the target PLC is reached.
By following this technique, it will help you to verify two things:
1. The correct node addresses are being used.
2. The slave path is defined correctly.
16b.3.2
Slave xx reported an illegal data address
This error message means that a tagname’s DDE item name has the correct format and
was accepted by the Modbus or Modbus Plus I/O Server, but the PLC (slave device)
rejected the DDE item name because the register has not been defined. ‘xx’ refers to the
node address of the slave device. That is, the referenced address, or DDE item name, was
not a defined address for the slave device.
For example, say we had the following configuration:
• The I/O table for a Modbus 984 PLC is defined up to register 45999
• A DDE topic in the I/O Server is defined with a slave type of ‘584/984’
• A DDE tagname is defined with an item of 49999.
Since register 49999 is not defined in the I/O table when the I/O server attempts to
advise this register, the error ‘slave xx reported an illegal data address’ is logged in the
Wonderware Logger (WWLogger).
To resolve this error, verify the DDE items to be advised, fall within the range of the
defined I/O table for the PLC.
Note that this error does not mean the same thing as getting an ‘Advised Failed’ error.
The ‘slave xx reported an illegal data address’ error means that the DDE item of a
tagname is not addressing a register defined in the PLC’s I/O map.
16b.3.3
Slave xx reported an illegal data value
(The xx refers to the node address of the slave device.) This error message means the data
which is being read from or written to a register in the slave device does not match the
expected data type for that register (‘xx’ refers to the node address of the slave). That is,
the data is not the same data type that the Modbus or Modbus Plus I/O server expects for
that register.
For example, if a I/O server topic is defined with a slave type of ‘584/984’ and the I/O
server tries to write to Coil 9999, but the data from the tagname is something other than a
discrete data type, then the error ‘slave xx reported an illegal data value’ occurs. This
error could also occur if the slave type selected is not correct for the type of PLC that is
Modbus Plus troubleshooting 343
being advised. For example, if you try to write data to a Modbus 984 PLC but the slave
type selected was ‘484’, then this error will occur.
16b.3.4
Slave xx reported an illegal function
This error message occurs when the slave device cannot perform a particular Modbus
function that was issued by the master node because either the slave device does not
understand the function or it is unable to execute the function. The computers that have
either the Modbus or Modbus Plus I/O server installed are considered to be a ‘master’
node. ‘xx’ refers to the node address of the slave device.
When receiving this type of error, verify that the slave device support the Modbus
function that it is being requested to execute. You can verify the Modbus function code
that the I/O server issued and the response that was received from the slave device by
enabling the I/O server’s ‘show send’ and ‘show receive’ menu options.
To enable the ‘show send’ and ‘show receive’ menu options, follow these steps:
• Edit the Win.ini file in the C:\Windows\System directory and depending on the
server you are using, i.e., the Modbus or Modbus Plus I/O server, locate either
[Modbus] or [MBPLUS] file section. Look for a ‘DebugMenu = x’ entry in the
file section. If it exists, modify the entry as follows:
DebugMenu = 1
• If it does not exist, add this entry to the end of the file section.
• Restart the Modbus or Modbus Plus I/O server and click the window’s control
menu box. You should now see the ‘show send’ and ‘show receive’ menu
options. Single click each option and the option ‘all topics’ (or a list of the
current active topics defined in the I/O server) will display to the right of the
‘show send’ or ‘show receive’ menu option. Enable either ‘all topics’ or one or
more of the topics for which the problem is occurring.
• Initiate communications to the slave device again.
• With the ‘show send’ and ‘show receive’ menu options enabled, the I/O server
will log all Modbus protocol messages in WWLogger when it attempts to
communicate ‘s’ to the slave device, such as the following send request
examples (those issued by the I/O server to the slave device):
S: :0301005601683D\r\n
S: :0101001101E20A\r\n
The Modbus and Modbus Plus I/O servers use the standard Modbus protocol and here
are the function codes that the I/O server will issue (Table 16b.2) (which are tied to the
protocol messages):
For information on how to correctly read the Modbus send requests in WWLogger and
decipher the Modbus function code that is listed as part of the message, see the document
titled ‘Interpreting the Modbus Protocol’ which is part of the Wonderware Knowledge
Base.
Or, you may go to the Modicon Website at http://www.modicon.com/.
Note: The ‘show send’ and ‘show receive’ menu options will generate frequent writes
to WWLogger, and therefore both should be enabled during troubleshooting only. If
‘show send’ and ‘show receive’ are left activated during the normal operation of the I/O
server, then the performance of your computer will slow considerably.
344
Practical Industrial Data Communications
Control Code
Read coil
Read contact
Read holding register
Read input register
Force coil
Load register
Force multiple coils
Load multiple registers
Read general reference
Write general reference
Function Code
01
02
03
04
05
06
15
16
20
21
Table 16b.2
Function codes issued by Modbus Plus I/O server
16b.3.5
Card SA85:0 not found, bad NCB_LANA_NUM – device not found
This error message means that the Modbus SA85 adapter card is not being acknowledged
by the Modbus Plus I/O server. This error may result when the SA85 adapter card does
not initialize during the startup of the computer. If this card does not initialize properly,
then the I/O server will not be able to communicate with any slave device on the Modbus
Plus network.
To resolve this error, if you are running Windows, Windows for Workgroups or
Windows 95, then make sure the following device statement is in the config.sys file
which is located in the root (C:\) directory:
Device = MBPHOST.SYS /mXXXX /nY /r2 /sZZ
Where XXXX represents the ‘memory window’ setting that is set with the switches
on the SA85 adapter card, Y represents the number of the adapter card that is
installed (examples: n0, n1), and ZZ is the hexadecimal address of the software
interrupt that is to be used by the device driver. Note the spaces between the
parameters in the device statement. Once this SA85 device statement is defined
correctly, the SA85 adapter card should initialize properly. (See tech note number 5,
solving Modbus Plus channel allocation errors for specific instructions on how to
define this device statement.)
Note that release 5.2a and later of the Modbus Plus I/O server will automatically add
this device statement to the config.sys file (earlier releases of the I/O server require that
you enter the device statement in the config.sys file, as well as edit the Modicon.ini file).
If you are running release 5.2a or later, then make sure that the Mbphost.sys file is located
in the root directory.
Note: Wonderware technical support suggests that you install version 3.4 or later of the
Mbphost.sys file instead of the SA85.sys file in the root directory. To obtain the latest
Mbphost.sys file, contact your Group Schneider/Modicon representative or go to the
Modicon website.
Verify that the Mbphost.sys file is being loaded to the SA85 Adapter card during
startup of the computer by manually stepping through the startup procedure of the
operating system. For Windows and Windows for Workgroups, you do this by
pressing the <F8> key when the operating system proceeds from its self-diagnostic
Modbus Plus troubleshooting 345
test to the execution of the config.sys file. For Windows 95, you do this by pressing
the <F8> key at the beginning of startup and select ‘step by step confirmation’ from
the Windows 95 startup menu screen. This will allow you to step through the
execution of each line of the config.sys and autoexec.bat files. (For Windows NT 3.51
and 4.0, see the section ‘Troubleshooting the SA85 adapter card under Windows NT’
below.)
After you enter the manual step through execution mode, look for information that is
displayed after the ‘Device=’ statement in which the Mbphost.sys file is executed. It
should indicate that the SA85 device driver was loaded correctly, as shown in this
example:
SA85 Device Driver Version 3.4
<<PG-MBPL-500>>
Copyright (c) 1989-1994 Modicon In. All Rights Reserved.
set for polled mode, SW int:0x5D Memory at 0xD000. LAN
adapter #0
node address = 31
This example shows that the SA85 adapter card is:
Set for polled mode (which is required for the SA85 Adapter
card);
Using interrupt 5D;
Using memory location D000;
Set to address 31 on the Modbus Plus network. (Note that for both ‘c’ and ‘d’,
memory and node address numbers must always match the dip switch settings
the SA85 adapter card.)
Following the procedure in this section, you will be able to help determine why
SA85 adapter card did not initialize properly, and thus, you will be able to resolve
error ‘Card SA85:0 not found, bad NCB_LANA_NUM – device not found’.
16b.3.6
the
on
the
the
Troubleshooting the SA85 adapter card under Windows NT
Troubleshooting the cause of the error ‘Card SA85:0 not found, bad NCB_LANA_NUM –
device not found’ under Windows NT 3.5 or 4.0 is performed differently than under
Windows or WFW.
Note: When installing or troubleshooting any communication problems with the Modbus
or Modbus Plus I/O server under Windows NT 3.5 or 4.0, make sure that you log on as
‘administrator’ to ensure that all files are copied and updated as necessary.
During the installation of the Modbus Plus I/O server, updates are added to the
Windows NT registry. If you are not logged on as ‘administrator’, then the registry will
not be updated, and thus, the I/O server will not work properly. The I/O server under
Windows NT does not rely on the config.sys file during startup of the computer. Instead,
during the installation of the I/O server, the file Mbplus.sys is installed into the
\%SystemRoot%\System32\Drivers directory. Then, once you configure the I/O server’s
adapter card settings and reboot the computer, the parameters for the Mbplus.sys file are
automatically defined and the registry will be updated.
Start up the Windows NT Event Viewer that is located under the Administrative Tools
program group. Verify that Windows NT acknowledged the SA85 Adapter card with no
errors. (That is, there are not ‘STOP’ errors for ‘MBPLUS’.)
346
Practical Industrial Data Communications
Further verify that Windows NT acknowledged the SA85 Adapter card by double
clicking the Devices program group from the Control Panel window. Then, scroll down
the Device list and locate the ‘MBPLUS’ device. Verify that the status of the device is
listed as ‘Started’ and the startup is defined as ‘Automatic’. If these statuses are not listed
and the Modbus Plus I/O server attempts to communicate with a slave device, then the
error ‘Card SA85:0 not found, bad NCB_LANA_NUM – device not found’ will result.
Data Highway Plus/DH485 overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe the main features of data highway plus
• Fix problems with:
–
–
–
–
–
Cabling
Grounding/earthing
Shielding
Terminations
Token passing timing.
17a.1
Allen Bradley Data Highway (Plus) protocol
17a.1.1
Overview of Allen Bradley protocol
There are three main protocol standards used in Allen Bradley data communications:
The Data Highway protocol
This is a local area network (LAN) that allows peer-to-peer communications up to
64 nodes. It uses a half-duplex (polled) protocol and rotation of link mastership. It
operates at 57.6 kbaud.
The Data Highway Plus protocol
This is similar to the data highway network although designed for fewer PCs and operates
at a data rate of 57.6 kbaud. This has peer-to-peer communications with a token passing
scheme to rotate link mastership among the nodes connected to that link.
Note that both protocol standards implement peer-to-peer communications through a
modified token passing system called the floating master. This is a fairly efficient
mechanism as each node has an opportunity to become a master at which time it can
immediately transmit without checking with each mode for the requisite permission to
commence transmission.
348
Practical Industrial Data Communications
The Allen Bradley Data Highway Plus uses the three layers of the OSI layer model:
• Hardware (a physical layer)
• Data-link layer protocol
• Application layer.
DH-485
This is used by the SLC range of Allen Bradley controllers and is based on RS-485.
17a.1.2
Physical layer (hardware layer)
This is based on twin axial cable with three conductors essentially in line with the
RS-485 specifications. The troubleshooting and specifications for RS-485 are covered in
an earlier chapter.
17a.1.3
Full-duplex data-link layer
Note that the asynchronous link can use either a full-duplex (unpolled) protocol or a
master slave communication through a half-duplex (unpolled) protocol. Although both
types of protocols are available the tendency today is to use the full-duplex protocol as
this explains the high performance nature of the link. Hence this protocol will be
examined in more detail in the following sections.
Full-duplex protocol is character orientated. It uses the ASCII control characters listed
in the following table, extended to eight bits by adding a zero for bit number seven
(i.e. the eighth bit).
The following ASCII characters are used (Table 17a.1):
Abbreviation
STX
ETX
ENQ
ACK
DLE
NAK
Hex Value
02
03
05
06
10
11
Table 17a.1
ASCII control characters
Full-duplex protocol combines these characters into control and data symbols.
Table 17a.2 lists the symbols used for full-duplex implementation.
Symbol
Type
DLE STX
Control symbol
DLE ETX
BCC/CRC
Control symbol
DLE ACK
Control symbol
DLE NAK
Control symbol
Description
Sender symbol that indicates the start of a
message
Sender signal that terminates a message
Responses symbol which signals that a
message has been successfully received
Response symbol which signals that a
message was not received successfully
Data Highway Plus/DH485 overview 349
Symbol
Type
DLE ENQ
Control symbol
APP DATA
Data symbol
DLE DLE
Data symbol
Description
Sender symbol that requests
retransmission of a response symbol
from the receiver
Single character data values between
00-OF and 11-FF includes data from
the application layer including user
programs and common application
routines
Symbol that represents the data value 10
Hex
Table 17a.2
Symbols used for full-duplex mode
Format of a message
Note that response symbols transmitted within a message packet are referred to as
embedded responses (Figure 17a.1).
Figure 17a.1
Protocol structure
The CRC-16 calculations is done using the value of the application layer data bytes and
the ETX byte. The CRC-16 results consists of two bytes.
Note that to transmit the data value of 10H, the sequence of data symbols DLE must be
used. Only one of these DLE bytes and no embedded responses are included in the CRC
value.
Message limitations
• Minimum size of a valid message is six bytes.
• Duplicate message detection algorithm – receiver compares the second, third,
fifth and sixth bytes of a message with the same bytes in the previous message.
P = Recovery procedure
T = Ready to transmit next message
* = Default values.
Depending on the highway traffic and saturation level, there may be a wait for a reply
from the remote node before transmitting the next message (Figure 17a.2).
350
Practical Industrial Data Communications
Figure 17a.2
Software logic for transmitter
Software logic for receivers
The following figures (Figures 17.3, 17.4 and 17.5) show typical events that occur in the
communications process. ‘xxxx’ indicates information transmitted while ‘x??x’ indicates
that errors have occurred during transmission.
Figure 17a.3
Software logic for receivers
Data Highway Plus/DH485 overview 351
Figure 17a.4
Normal message transfer
Figure 17a.5
Message transfer with NAK
There are two types of application programs:
1. Command initiators
2. Command executors.
Command initiators specify which command function to execute at a particular remote
node. The command executor must also issue a reply message for each command it
receives. If the executor cannot execute a command, it must send the appropriate error
code. The reply message may contain an error. The command initiator must check for this
condition and depending on the type of error, retransmit the message or notify the user.
If the command executor reply is lost due to noise, the command initiator should maintain
a timer for each outstanding command message. If the timer expires, the command initiator
should take appropriate action (notify the user or retransmit to executor).
If the application layer software cannot deliver a command message, it should generate
a reply message with the appropriate error code and send the reply to the initiator. If it
cannot deliver a reply message, the application layer should destroy the reply without
notification to the command executor (Figure 17a.6).
Note that not all command messages have FNC, ADR, SIZE or DATA bytes. Not all
reply messages have DATA or ETX STS bytes.
352
Practical Industrial Data Communications
Figure 17a.6
Application program
Explanation of bytes
•
•
•
•
DST: Destination byte. This contains the ultimate destination of the node
SRC: Source node of the message
CMD: Command byte
FNC: Function byte.
These together define the activity to be performed by the command message at the
destination node. Note that bit five of the command byte shall always be zero (normal
priority).
STS and ETX SYS – status and extended status bytes
In command messages, the STS byte is set to zero. In reply messages, the STS byte may
contain a status code. If the four high bits of the STS byte are ones, there is extended
status information in an ETX STS byte.
TNS – transaction bytes (two bytes)
The application level software must assign a unique 16-bit transaction number (generated
via a counter). When the command initiator receives reply to one of its command
messages, it can use the TNS value to associate the reply message with its corresponding
command.
Whenever the command executor receives a command from another node, it should
copy the TNS field of the command message into the same field of the corresponding
reply message.
ADDR
Address field contains the address of a memory location in the command executor where
the command is to begin executing. The ADDR field specifies a byte address (not a word
address as in PLC programing).
Size
The size byte specifies the number of data bytes to be transferred by a message.
Data Highway Plus/DH485 overview 353
Data
The data field contains binary data from the application programs.
PLC-5 command set message packet fields
• Packet offset: This field contains the offset between the DATA field of the
current message packet and the DATA field of the first packet in the
transmission.
• Total trans: This field contains the total number of PLC-5 data elements
transferred in all message packets initiated by a command.
Basic command set
The asynchronous link message packet formats to be used are delivered below (see
Figure 17a.7).
In the lists below, privileged commands are initiated by computer and executed by
PLCs. A PLC or a computer initiates non-privileged commands. The CMD values listed
are for non-priority command message packets.
Figure 17a.7
Basic command set for PLC-5
354
Practical Industrial Data Communications
Synchronous link status code (STS, ETX STS)
The TS bytes provide information about the execution or failure of the corresponding
command that was transmitted from the computer. If the reply returns a code of 00, the
command was executed at the remote node.
All other codes are divided into two types:
1. Local error: Local node is unable to transmit a message to the remote node.
2. Remote error: Remote node is unable to execute the command.
Local STS error code
Code
00
01
02
03
04
Description
Success – no error
Destination node out of buffer space
Remote node does not ACK command message
Duplicate token holder detected
Local port is disconnected
Remote STS error codes
Code
00
10
20
30
40
50
60
70
80
A0
B0
F0
C0 to E0
Description
Success – no errors
Illegal command or format
Host has a problem and will not communicate
Remote node is missing, disconnected
Host could not complete function due to hardware fault
Addressing problem or memory protect rungs
Function disallowed due to command protection selection
Processor is in program mode
Compatibility mode file is missing or communication zone problem
Not used
Remote node problems due to download
Error in the ETX STS byte
Not used
There is only an ETX STS byte if the STS code is F0. If the command code is 00–08,
there is not an ETX STS byte. Commands used in this implementation are in this range;
hence the ETX STS byte is not being used.
Diagnostic counter for each module
Diagnostic counters are bytes of information stored in RAM in each data highway and
data highway plus module. When using the ‘diagnostic read’ command, a dummy value
should be used for the address. The reply contains the entire counter block.
Data Highway Plus/DH485
troubleshooting
17b.1
Introduction
This section is broken down into the following sections:
• Data Highway Plus wiring troubleshooting
• Data Highway Plus network diagnostics.
Note that the rules for troubleshooting the physical side of these two cables are very
similar to that of RS-485. In fact, DH485 physical layer is identical to RS-485 whilst data
highway plus uses a transformer-isolated version.
The difficult part in diagnosing problems with the data highway plus is in the operation
of the protocol.
17b.2
Data Highway Plus wiring troubleshooting
One should inspect the cable closely for wiring problems if the operation of the network
appears intermittent. Typical problems that should be examined are:
• Damage to the cable
• No termination resistance (or removed) at the end of the line
• Screen removed or damaged.
A few suggestions to look for on the wiring are described below.
17b.2.1
Wiring recommendations
• Ensure that twin axial cable has been used for trunklines and drop lines (Belden
9463 or equivalent)
• Connect ground wire from station connector to the earth ground
• Route cables away from sources of electrical interference.
17b.2.2
Station connector wiring
• Connect trunklines to the upper side of the station connector
• Connect drop lines and earth to the lower connections.
356
Practical Industrial Data Communications
17b.2.3
Cable preparation
•
•
•
•
•
17b.2.4
Cut the drop line to required length
Remove 1 in. of insulation, foil, braid, and filler cord
Strip about one-eighth of an inch of insulation off each of the insulated wires
Insert the cable through hood of D-shell connector
Fit about half an inch of heatshrink tubing over each of the conductors.
Solder wires to connector
Solder as following:
•
•
•
•
•
Blue wire to Pin 6
Drain wire to Pin 7
Clear wire to Pin 8
Slide heatshrink over soldered connections and shrink with a heat gun
Assemble the 15-pin connector to the D-shell hood.
Prepare other end of the drop line cable
• Remove about 3 in. of insulation, foil, braid, and filler
• Free drain wire, clear and blue insulated wires
• Strip about 1/4 in. of insulation off each conductor.
17b.2.5
Trunk line preparation
Remove cover and terminal block from enclosure and mount enclosure where you want to
add the drop line. Cut trunk line cable. Feed incoming and outgoing cables through the
respective cable clamps at the top of the enclosure.
Wiring enclosure
• Twist the wires of the same color together from each trunk line cable
• Secure the wires under their respective screw clamp terminals.
Note: It is easier to do this with the terminal block out of the enclosure.
Terminating stations
• Install a 150 resistor at the first and last station connectors on the trunkline
• Use a 1 in. length of heatshrink to protect resistor and connect between terminals
1 and 3.
17b.3
Data Highway Plus network diagnostics
There are literally hundreds of possible errors, but there are a few important ones worth
highlighting in this section.
Many of them are the result of excessive noise on the network and can be corrected by
examining the actual wiring and removing the source of noise, if possible. If not (for
example, due to the data highway running parallel to a power cable in a cable tray),
consideration will have to be given to the use of fiber cabling as a replacement for the
copper-based cable.
Data Highway Plus/DH485 troubleshooting
357
A few errors (which are identified in the diagnostics registers on the interface module)
worth mentioning are:
• ACK Time out: This indicates where a sender has been transmitting packets
and has not received any acknowledgment responses. This is often due to long
cable or the presence of reflections and low-level noise.
• Contention: This occurs on noisy or overlength cables. A similar remedy to
the one above.
• False poll: This indicates the number of times this station has tried to
relinquish bus mastership, and the station that was expected to take over didn’t
do so. This is often the case on a noisy highway.
• Transmitted messages and received messages: While these are not errors,
they are worth examining (and recording) to see the level of activity (number of
packets sent and received) against the level of errors.
• Data pin allocation: Not all data highway plus equipment use the same pin
configuration on the 15-pin connector! With certain types of Allen Bradley
PLCs the allocation of pins 6 and 8 are transposed.
HART overview
Objectives
When you have completed study of this chapter, you will be able to:
• Describe the fundamental operation of HART
• Fix problems with:
– Cabling
– Configuration.
18a.1
Introduction to HART and smart instrumentation
Smart (or intelligent) instrumentation protocols are designed for applications where actual
data is collected from instruments, sensors, and actuators by digital communication
techniques. These components are linked directly to programmable logic controllers (PLCs)
and computers.
The HART (highway addressable remote transducer) protocol is a typical smart
instrumentation fieldbus that can operate in a hybrid 4–20 mA digital fashion.
HART is, by no means, the only protocol in this sphere. There are hundreds of smart
implementations produced by various manufacturers – for example, Honeywell – that
compete with HART. This chapter deals specifically with HART.
At a basic level, most smart instruments provide core functions such as:
• Control of range/zero/span adjustments
• Diagnostics to verify functionality
• Memory to store configuration and status information (such as tag numbers, etc.).
Accessing these functions allows major gains in the speed and efficiency of the
installation and maintenance process. For example, the time consuming 4–20 mA loop
check phase can be achieved in minutes, and the device can be readied for use in the
process by zeroing and adjustment for any other controllable aspects such as the damping
value.
18a.2
HART protocol
This protocol was originally developed by Rosemount and is regarded as an open standard,
available to all manufacturers. Its main advantage is that it enables an instrumentation
engineer to keep the existing 4–20 mA instrumentation cabling and to use, simultaneously,
HART overview
359
the same wires to carry digital information superimposed on the analog signal. This enables
most companies to capitalize on their existing investment in 4–20 mA instrumentation
cabling and associated systems and to add further capability of HART without incurring
major costs.
HART is a hybrid analog and digital protocol, as opposed to most fieldbus systems,
which are purely digital.
The HART protocol uses the frequency shift keying (FSK) technique based on the Bell
202 communications standard. Two individual frequencies of 1200 and 2200 Hz, representing digits 1 and 0 respectively, are used (Figure 18a.1). The average value of the sine
wave (at the 1200 and 2200 Hz frequencies), which is superimposed on the 4–20 mA
signal, is zero. Hence, the 4–20 mA analog information is not affected.
Figure 18a.1
Frequency allocation of HART protocol
The HART protocol can be used in three ways:
1. In conjunction with the 4–20 mA current signal in point-to-point mode
2. In conjunction with other field devices in multi-drop mode
3. In point-to-point mode with only one field device broadcasting in burst mode.
Traditional point-to-point loops use zero for the smart device polling address. Setting
the smart device polling address to a number greater than zero creates a multi-drop loop.
The smart device then sets its analog output to a constant 4 mA and communicates only
digitally.
The HART protocol has two formats for digital transmission of data:
• Poll/response mode
• Burst (or broadcast) mode.
In the poll/response mode the master polls each of the smart devices on the highway
and requests the relevant information. In burst mode the field device continuously
transmits process data without the need for the host to send request messages. Although
this mode is fairly fast (up to 3.7 times/s), it cannot be used in multi-drop networks.
The protocol is implemented with the OSI model using layers 1, 2, and 7. The actual
implementation is covered in this chapter.
18a.3
Physical layer
The physical layer of the HART protocol is based on two methods of communication.
• Analog 4–20 mA
• Digital frequency shift keying (FSK).
360
Practical Industrial Data Communications
The basic communication of the HART protocol is the 4–20 mA current system. This
analog system is used by the sensor to transmit an analog value to the HART PLC or
HART card in a PC (Figure 18a.2). In a 4–20 mA system, the sensor outputs a current
value somewhere between 4 and 20 mA that represents the analog value of the sensor.
For example, a water tank that is half full – say 3400 kL – would put out 12 mA.
The receiver would interpret this 12 mA as 3400 kL. This communication is always
point-to-point, i.e. from one device to another. It is not possible to do multi-drop
communication using this method alone. If two or more devices put some current on the
line at the same time, the resulting current value would not be valid for either device.
Figure 18a.2
HART point-to-point communications
18a.3.1
Digital multi-drop communications
For multi-drop communications, the HART protocol uses a digital/analog modulation
technique known as frequency shift keying (FSK). This technique is based on the Bell 202
communication standard (Figure 18a.3). Data transfer rate is 1200 baud with a digital
‘0’ frequency (2200 Hz) and a digital ‘1’ frequency (1200 Hz). Category 5 shielded,
twisted-pair wire is recommended by most manufacturers. Devices can be powered by the
bus or individually. If the bus powers the devices, only 15 devices can be connected. As the
average DC current of an ac frequency is zero, it is possible to place a 1200 or 2200 Hz tone
on top of a 4–20 mA signal. The HART protocol does this to allow simultaneous
communications on a multi-drop system.
Figure 18a.3
HART multi-point communications
HART overview
18a.3.2
361
The HART handheld communicator
The HART system includes a handheld control device (Figure 18a.4). This device can be
a second master on the system. It is used to read, write, range, and calibrate devices on
the bus. It can be taken into the field and used for temporary communications. The
battery-operated handheld has a display and key input for specific commands.
Figure 18a.4
HART handheld controller
The HART field controller in Figure 18a.5 is wired in series with the field device (valve
positioner or other actuator). In some cases, a bypass capacitor may be required across
the terminals of the valve positioner to keep the positioner’s series impedance below the
100 level required by HART specifications. Communications with the field controller
requires the communicating device (handheld terminal or PC) to be connected across a
loop impedance of at least 230 . Communications is not possible across the terminals of
the valve positioner because of its low impedance (100 ). Instead, the communicating
device must be connected across the transmitter or the current sense resistor.
Figure 18a.5
HART handheld connection method
362
Practical Industrial Data Communications
Figure 18a.6
HART protocol implementation of OSI layer model
18a.4
Data-link layer
The data-link frame format is shown in Figure 18a.7.
Figure 18a.7
HART data-link frame format
Two-dimensional error checking, including vertical and longitudinal parity checks, is
implemented in each frame. Each character or frame of information has the following
parameters:
•
•
•
•
18a.5
1 start bit
8 data bits
1 odd parity bit
1 stop bit.
Application layer
The application layer allows the host device to obtain and interpret field device data
(Figure 18a.8). There are three classes of commands:
1. Universal commands
2. Common practice commands
3. Device-specific commands.
Examples of these commands are listed below.
18a.5.1
Universal commands
• Read manufacturer and device type
• Read primary variable (PV) and units
• Read current output and percent of range
HART overview
•
•
•
•
•
•
Read up to four predefined dynamic variables
Read or write 8-character tag, 16-character descriptor, date
Read or write 32-character message
Read device range, units and damping time constant
Read or write final assembly number
Write polling address.
Figure 18a.8
HART application layer implementation
18a.5.2
Common practice commands
•
•
•
•
•
•
•
•
•
•
•
•
•
18a.5.3
Read selection of up to four dynamic variables
Write damping time constant
Write device range
Calibrate (set zero, set span)
Set fixed output current
Perform self-test
Perform master reset
Trim PV zero
Write PV units
Trim DAC zero and gain
Write transfer function (square root/linear)
Write sensor serial number
Read or write dynamic variable assignments.
Instrument specific commands
•
•
•
•
•
•
Read or write low flow cut-off value
Start, stop or clear totalizer
Read or write density calibration factor
Choose PV (mass flow or density)
Read or write materials or construction information
Trim sensor calibration.
363
364
Practical Industrial Data Communications
18a.5.4
Summary of HART benefits
•
•
•
•
•
•
•
•
18a.5.5
Simultaneous analog and digital communications
Allows other analog devices on the highway
Allows multiple masters to control same smart instrument
Multiple smart devices on the same highway
Long distance communications over telephone lines
Two alternative transmission modes
Flexible messaging structure for new features
Up to 256 process variables in any smart field device.
Hardware recommendations
Minimum cable size: 24 AWG, (0.51 mm diameter)
Cable type: Single-pair shielded or multiple-pair with overall shield.
HART troubleshooting
18b.1
Troubleshooting HART systems
Beside the actual instruments that require calibration, the only major problem that can
occur with HART is the cable length calculation.
The HART protocol is designed to work over existing analog signal cables but the
reliable length of cable depends on:
•
•
•
•
•
Loop load resistance
Cable resistance
Cable capacitance
Number and capacitance of field devices
Resistance and position of other devices in the loop.
The main reason for this is that network must pass the HART signal frequencies (1200
and 2200 Hz) without excessive loss or distortion. A software package such as H-Sim can
be used to calculate whether you are operating with the correct signal level. In addition,
you should ensure that you have the correct bandwidth of at least 2500 Hz. You can do
this by ensuring that the product of the cable resistance and capacitance is less than 65 µs.
Wireless technologies
19.1
Satellite systems
Satellite systems are a form of microwave radio transmission whereby the transmitter
sends the signal up to the satellite, where it is amplified and then transmitted back to the
earth on another frequency. This provides a point-to-multi-point transmission system
where each terminal accesses its signal out of the common beam. The satellite is located
in a geostationary earth orbit (GEO) at some 36 000 km above the earth’s equator where
it appears to be stationary relative to an observer on the earth. This is illustrated in
Figure 19.1. This means that the satellite antenna at all of the terminals can have fixed
alignment. In this chapter, the principles of satellite operation and methods of access will
be introduced and the design of very small aperture satellite (VSAT) systems will be
discussed.
Figure 19.1
Geostationary earth orbit satellite communication
19.1.1
Satellite-operating principles
Satellite electronics
The satellite comprises a number of transponders that receive the signal from the earth,
amplify it, convert it to a lower frequency and then amplify the signal for retransmission
back to earth. A block diagram showing these components is given in Figure 19.2.
Wireless technologies 367
The low noise amplifier (LNA) is one of the most critical components in the satellite link
since it is required to extract very low signals and the same time provide very large gain,
up to 50 dB. The high power amplifier (HPA) in the satellite transponder may produce
10–50 W depending on the configuration. This power is limited by international
agreement, to prevent interference with terrestrial microwave systems and such systems
are said to be ‘down-link limited’. By comparison, the equivalent HPA at the earth station
may be running at power levels between 100 and 600 W.
Figure 19.2
Block diagram of satellite transponder
Satellite frequency bands
The main frequency bands used for commercial satellite communication are listed in
Table 19.1. It will be noted that the up-link frequencies, which are used from the earth to
the satellite, are higher than the corresponding down-link frequencies. The reason for this
is that the higher frequencies have a greater path loss and therefore require greater
transmitter power. This is easily provided by the mains-powered earth stations, whereas
the satellite transmitters are reliant on limited battery and solar energy resources; so use
the lower frequencies.
Frequency Band Usage
L
S
C
X
Ku
Ka
Mobile
Military/Government
Commercial
Military/Government
Commercial
Military/Government
Uplink Frequency
(GHz)
1.550–1.600
5.925–6.055
5.2625–26.0265
26.900–8.400
12.265–18.10
226.00–31.00
Downlink
Frequency (GHz)
1.500–1.550
2.535–2.655
3.2600–4.800
26.250–26.2650
10.260–13.25
18.30–22.2
Table 19.1
Satellite frequency bands
The reasons for using particular frequency bands are as follows:
• C band: This is the most popular because it is least affected of all the bands
by man-made and natural noise. Because it is shared with terrestrial services,
satellite transmit power is limited to reduce interference.
• Ku band: As the C band is crowded, the Ku band is becoming more popular.
It is affected by rain attenuation and high natural noise levels. Greater data
rates are possible with higher frequencies.
368
Practical Industrial Data Communications
• L band: This is used for mobile low capacity terminals. It is affected by manmade noise. Less pointing accuracy and lower power is required.
• S, X and Ka bands: These are restricted to military, government, and
experimental users.
VSAT antenna diameter
The diameter of the antenna used at the earth station limits the amount of bandwidth that
a station is able to access. VSAT systems use antennae ranging from 5 to 11 m in
diameter at the hub and at the remote terminal antenna diameter ranges from 0.5 m
(1.6 ft) to 2.5 m (8 ft). The outbound traffic from the hub may reach 2 Mbps, while the
inbound traffic is typically 9600 bps.
Satellite access techniques
Satellite systems can be accessed in either the frequency domain (FDMA) or the time
domain (TDMA). This access can also be assigned permanently (i.e. dedicated) for heavy
traffic routes or dynamically as in demand-assigned multiple access (DAMA) systems. In
DAMA systems the master station assigns traffic circuits to an earth station on demand.
When the call terminates, the DAMA circuit is returned to the pool for re-assignment to
other users.
Frequency division multiple access (FDMA)
In FDMA systems, each transponder – which typically has 36 MHz bandwidth – assigned
divided into various frequency segments. Each earth station is assigned one or more
frequency segments. For example, 14 earth stations could access one 36 MHz transponder
with each having 24 analog voice channels. These allocations can be on a single channel
per carrier (SCPC) basis whereby an individual user can transmit and receive on
particular pre-allocated channels from his remote terminal. This is normally the simplest
method of providing access to large numbers of small users.
The advantages in using FDMA are that no network timing is required and that the
channel assignment is simple. The disadvantages are that up-link power levels need to be
closely coordinated for efficient use of transmitter RF output power and inter-modulation
becomes a problem as the number of separate RF carriers increases.
Time division multiple access (TDMA)
TDMA is used with digital transmission and is shared by the satellite transponder on a
time basis. Individual time slots are assigned to the earth stations in sequence and each
station has full and exclusive access to the whole of the transponder bandwidth during its
assigned time slot. During its access period, the earth station transmits a burst of digital
information. All stations on the network need precise time synchronization. This is done
using a synchronization burst at the start of each frame. A typical INTELSAT TDMA
system has a frame period of 2650 µs.
The advantages of TDMA are that only one earth station is providing input to the
satellite at any one instant, so inter-modulation does not occur, the system allows greater
flexibility with the different user’s EIRP and data rates, and is able to cope quickly with
the dynamically changing traffic patterns. The disadvantages of TDMA are that very
accurate timing is required for all stations on the network, there is some loss of
throughput due to the frame overhead, and large buffer storage is needed to cope with
long frame lengths.
Wireless technologies 369
19.1.2
Satellite system design concepts
Receiving system figure of merit (G/T )
The receiving system figure of merit (G/T ) describes the capability of a satellite or earth
station to receive a signal. This is defined as follows:
G/T = G – 10 log T
Where:
G/T is the figure of merit measured in dB per degree Kelvin (dB/K)
G is the net antenna gain up to the reference plane, which is normally the input to the
LNA, in dB
T is the effective noise temperature of the receiving system in degrees Kelvin.
Net antenna gain
The net antenna gain is the gross gain of the antenna less the losses of all of the
components making up the down-link receive chain, namely the radome loss, feed loss,
directional coupler loss, wave guide loss, the band pass filter loss, and transition losses.
These components are illustrated in Figure 19.3.
Figure 19.3
Components in receiver down-link chain
System noise temperature
The system noise temperature in degrees Kelvin is given by the following formula:
Tsys = Tsys + Tfeed + Trecv
Where:
Tsys is the sky noise
Tfeed is the thermal noise generated by the receive chain components
Trecv is the thermal noise generated by the LNA.
Sky noise
Sky noise varies in proportion to frequency and inversely in proportion to the elevation
angle. Some representative values of sky noise are given in Table 19.2. Earth stations
370
Practical Industrial Data Communications
generally have minimum elevation angles at which they will work satisfactorily. Below
the minimum angles the radio beam is subject to much greater atmospheric refraction and
there is greater noise pickup from terrestrial sources and the sky noise increases
dramatically. The minimum elevation angles are 5° at 4 GHz and 10° at 12 GHz.
Frequency (GHz)
4.0
4.0
26.5
26.5
11.26
11.26
20.0
20.0
20.0
Elevation Angle (°)
5
10
5
10
10
15
10
15
20
Sky Noise (K)
28
16
33
18
23
18
118
100
80
Table 19.2
Some sky noise figures
Noise figure
The noise figure (NF) in dB is the thermal noise generated by the losses of all devices
inserted in the receive chain and it can be related to the noise temperature by the
following formula:
T 

NF = 10 log  1 + e 

290 
Where Te is the effective noise temperature measured in degrees Kelvin (K).
19.1.3
Satellite down-link design procedure
To design the satellite down-link a similar process is followed to the design of microwave
radio links as discussed in Section 19.3.
The objective is to calculate the carrier-to-noise ratio (C/N0) using the following
methodology:
• The effective power produced by the satellite or earth station is calculated
(EIRP).
• The free space loss along the radio path is calculated (LFS).
• An allowance is made for the other losses along the radio path (Lother).
• The figure of merit for the receiver system is calculated (G/T ).
• Boltzmann’s constant (k) = − 228.6 dBW/Hz is subtracted.
• The algebraic sum of all the above gains and losses can be calculated as follows:
C/N 0 = EIRP – LFS – Lother + G/T – k dB
A fade margin of 4 dB is added to compensate for propagation anomalies and to allow
for aging in the components.
The other losses in the radio path can include:
• Polarization loss (0.5 dB) at both the earth station and the satellite (0.5 dB
each). This is to compensate for any antenna misalignment.
Wireless technologies 371
• Off-contour loss to compensate for variation in received signal strength at the
terminal location. This information is obtained from the satellite service
provider in the form of an antenna footprint, which shows the down-linkreceived signal strength (EIRP) for the specific spot or zone beam from the
satellite. An example is shown for the Aussat 160E satellite in Figure 19.4.
• Gaseous absorption loss is caused by atmospheric absorption and increases
with frequency, is inversely proportional to the elevation angle, and the
elevation of the earth station. At higher altitudes the air is less dense and
therefore the path loss is reduced. The elevation angle is compensated for by
allowing 0.5 dB for 4 GHz systems at less than eight degrees elevation and
1.0 dB for 20.25 GHz systems at less than 10°elevation.
• Rainfall attenuation is significant for systems operating above 10 GHz. The
amount of excess attenuation is dependent on the rainfall intensity and the
frequency.
Figure 19.4
Example of satellite down-link footprint
19.1.4
Applications of satellite systems
Satellite systems have a number of advantages plus a few disadvantages. The advantages
of satellite include:
• Systems only require a transmitting and a receiving site, and the distance
between them on the earth is irrelevant.
• Systems can be installed in very remote locations where the provision of other
services such as fiber-optic cables is impractical or prohibitively expensive.
• Systems can be installed on mobile equipment.
• System security involves only the protection of the transmitting and receiving
sites, without the need to protect cable along the total path length.
• System installation can be much quicker with only the two terminals to install.
372
Practical Industrial Data Communications
• VSAT systems are often installed to bypass the PSTN to create private data
networks, ensuring data security. They are also used for telemetry links in isolated
rural areas. An example of a VSAT network topology is shown in Figure 19.5.
Figure 19.5
Typical VSAT network configuration
The disadvantages of using satellite systems are as follows:
• Satellite systems have limited bandwidth, thereby restricting the achievable
data rates.
• The radio path is very long, and receive power is very low making systems
susceptible to interference.
• The length of the radio path, some 36 000 km each way, causes a propagation
delay of about 256 ms in one direction (512 ms return).
• Only limited radio frequencies are available in certain areas.
• GEO satellite orbital positions are limited and radio frequency interference is
possible if an earth station antenna is incorrectly aligned or has too wide a
beam width.
19.2
Wireless LANs (WLANs)
19.2.1
Topologies
A WLAN can be configured in three possible ways namely in star topology, peer-to-peer
topology or hierarchical topology. Each topology has its own advantages and the choice
of topology depends on the user’s needs.
Star topology
In a star topology, the communication takes place over two separate channels. The node
at the centre of the star is known as the central hub, while the surrounding nodes are
known as network nodes. The communication from the central hub to the network nodes
takes place on one channel while the other channel is used for communication in the
opposite direction.
Wireless technologies 373
It is important to note that star topology has only one hop between the network node
and the central hub. This characteristic provides the star topology with predictability and
extra reliability. However, if that connection is weak, then there are no alternatives. One
of the biggest advantages that a star topology WLAN offers is the use of central control
functions at the hub. These functions include channel estimations, routing, and resource
allocation.
Such centralized topology results in an efficient and reliable network service to the
client devices. Hence, most commercial wireless networks, for instance commercial
cellular and paging systems, prefer a star WLAN topology.
A hub-based LAN is said to operate in infrastructure mode.
Peer-to-peer topology
This topology interconnects a set of nodes, embedded with wireless adapters. The nodes
self-configure to form an integrated network utilizing distributed control. This is known
as ad hoc mode.
The connection between any two nodes in the network consists of one or more
peer-to-peer communication links. Peer-to-peer topology can use multiple hops for the
end-to-end connection, which results in the advantage of extending the network range.
However, if one of the hops fails the entire end-to-end connection is lost. This disadvantage is compensated for by the possibility of each node having multiple paths to
other nodes.
Peer-to-peer networks also do not require any existing infrastructure for establishing
the network. These are also easily configurable. These features have made peer-to-peer
networks ideal for military applications.
Hierarchical topology
As the name suggests, a hierarchical topology is ideal for networks that span a range of
coverage regions. These regions are related to each other in the form of a hierarchy, for
instance, town, city, state, and so forth.
Since the coverage regions define a natural hierarchy of the overall network,
hierarchical network architecture along with protocols, routing, and identifying locations
are more suited to this type of system.
19.2.2
WLAN components
The following components are necessary to set up a WLAN.
Wireless NIC
Wireless network interface cards interface the computer to the wireless network by
modulating the data signal with the spreading sequence and implementing a carrier sense
multiple access protocol (Figure 19.6). If a machine on the network wishes to transmit
data onto network, the NIC listens for other transmissions. If it does not sense any
transmission, the card will transmit its data in a frame. Other stations, listening to the
incoming data, capture the frame and check the address of the intended recipient. The
station whose address matches the frame address accepts the data frame, while the others
discard it.
Wireless NICs plug into the ISA bus of a desktop PC or into the PCMCIA slot of a
portable computer. Most wireless NICs have a built-in antenna, while some have
provision for an external antenna as well.
374
Practical Industrial Data Communications
Figure 19.6
Wireless NIC
Examples of commercial WLAN NICs include RangeLAN (Proxim Inc.) and
WaveLAN (Lucent Technologies).
Wireless network bridges (access points)
Network bridges play a vital role in any kind of network. They interconnect multiple
LANs at the MAC (medium access control) layer, the layer responsible for providing
access functions. Bridges interface multiple wireless and wired LANs, provided they are
identical at layer 2 of the OSI model. Another important function that a bridge provides is
packet filtering based on the MAC address. This function allows organizations to create
network segments in an enterprise network.
Assume that there are two network segments, A and B, both part of the enterprise
network. If workstation A1 in segment A wishes to send the data to workstation A2 in the
same segment A, then the bridge will not allow the packet (frame) to propagate to other
segments of the network or to the backbone of the enterprise network. On the other hand,
if A1 wishes to send data to workstation B1 in segment B2, the packet will be allowed to
pass through the bridge. This process is called segmentation. It makes the better use of
network bandwidth and increases overall performance.
Examples of commercial wireless bridges are RangeLAN Access Point (Proxim Inc.)
and WavePOINT (Lucent Technologies).
There are several parameters that need to be set up on an access point. On most new
models this can be done remotely via a browser.
Types of bridges
Based on the distances between the segments of an enterprise network that a bridge can
cover, it can be classified as local bridges or remote bridges. Local bridges connect
segments located at close proximities. For instance, two rooms in an office on a single
floor can be connected using a local bridge. Remote bridges, on the other hand, are
installed in pairs. They connect segments located a large distance apart, beyond the
LAN’s capability. For instance, two separate buildings can be connected using remote
bridges.
19.2.3
Cell coverage
In a wireless network, the two basic components are wireless network interface cards and
bridges (access points). These components can be used to create either a single cell or
multiple cell WLAN networks.
Wireless technologies 375
Single cell WLAN
This configuration is ideal for small regions such as single floor factories. Under ideal
conditions a single cell can cover up to 30 000 sq.m.
One of the characteristics of a single cell WLAN is the possible elimination of access
points (Figure 19.7). In the absence of access points, the NICs on the wireless devices
(PCs, laptops, etc.) can establish peer-to-peer connections with the other wireless devices
in the same cell coverage area. This, however, requires that there should not be to too
many nodes. It also eliminates some of the convenient functionalities of an AP, such as
dynamic host configuration (DHCP). Wireless NICs have to be specifically configured to
operate in ad hoc mode.
Figure 19.7
Access point
Multiple cell WLAN
If an organization requires its wireless device users to operate at greater distances, beyond
the scope of a single cell coverage area, a multiple cell scenario using wireless local
bridges can serve the purpose.
In a multiple cell configuration, a set of access points along with a wired network
backbone creates multiple cells, overlapping with each other. It results in increasing the
coverage of the wireless network to the desired extent. This configuration allows users,
belonging to different cells, communicate with each other. All the users have also the
privilege to access the common wired network resources. Multiple cell coverage is ideal
for multi-floor buildings, college campuses, and so forth.
In the multiple cell scenario, a portable device with a wireless LAN adapter can also
roam within the cells while maintaining a live connection to the corporate Intranet.
In the case of multiple cells, care must be taken not to operate adjacent access points at
the same frequency (channel). The channel selection has to be done on the access point
itself. Care must also be taken to eliminate interference where there are access points on
different floors as sufficient frequency diversity in a horizontal plane does not preclude
access points vertically separated (on different floors) from interfering with each other.
Roaming
Similar to the cellular networks, the concept of ‘roaming’ is applicable to WLANs. To
make this a reality, most commercial wireless network APs support a roaming protocol
that works in the MAC layer. The handoff process of a WLAN is similar to the handoff
process in cellular networks. There is also a roaming version of IP for use on WLANs. If,
on a fixed network, a workstation moves beyond a router on to a new network, its NetID
has to change. On a mobile station this is not practical, as a user may not even know that
he/she is on a different network. IP roaming enables the packet to be forwarded to the
host’s current location.
376
Practical Industrial Data Communications
19.2.4
Devices
There are many types of wireless devices that can operate in a WLAN environment.
At present, new WLAN-compatible devices are being rolled out everyday.
Some of the characteristics of WLAN devices include portability, a small screen size
limited storage capacities, and multimedia support. Some of the well-known devices that
satisfy these criteria include handheld (PDAs), laptops, tablet PCs (by Microsoft),
notebook PCs, and cell phones.
The area of WLAN devices is considered to have immense potential. In future, mobile
device manufactures are planning to develop their own range of WLAN devices.
19.2.5
Accessories
Antennas
These devices control the signal channels to ensure a clear, interference-free transmission
of all types of data including audio, video, and digital images. Most NICs have built-in
antennas giving a range of up to 100 m without amplification. Some have a socket for an
external antenna. These are available from various vendors and include Yagi antennas,
parabolic grids, and parabolic dishes. A properly designed parabolic antenna can improve
the range to about 8–10 km without amplification.
Power
WLAN access points need external power to operate. To obviate the need for separate
electrical wiring to power access points, the concept of PoE (power over Ethernet) has
been developed. This delivers power to the AP via the unused pairs of the Cat5 cabling.
19.2.6
IEEE 802.11 specifications
In 1996 the IEEE finalized the initial standard for wireless LANs, IEEE 802.11. This
standard specified a 2.4 GHz operating frequency with data rates of 1 and 2 Mbps. Users
had the option of using either frequency hopping spread spectrum (FHSS) or direct
sequence spread spectrum (DSSS).
Since the ratification of the initial 802.11 standard, the IEEE 802.11 working group
(WG) has made several revisions through various task groups. Task groups are designated
through a suffix after the 802.11, for example 802.11b designates task group B.
Here follows a brief summary of the various IEEE 802.11 standards.
IEEE 802.11a
IEEE 802.11a is a physical layer standard that specifies operating in the 5 GHz UNII
band using orthogonal frequency division multiplexing (OFDM). It supports data rates
ranging from 6 to 54 Mbps. It offers much less potential for radio frequency interference
than other PHYs (e.g. 802.11b and 802.11g) that utilize 2.4 GHz frequencies.
IEEE 802.11b
IEEE 802.11b uses direct sequence spread spectrum (DSSS) in the 2.4 GHz band.
It includes 5.5 and 11 Mbps data rates in addition to the 1 and 2 Mbps data rates of the
initial standard. To provide the higher data rates, 802.11b uses CCK (complementary
code keying), a modulation technique that makes efficient use of the radio spectrum.
Wireless technologies 377
IEEE 802.11b is the basis for Wi-Fi certification from the Wireless Ethernet
Compatibility Alliance (WECA).
IEEE 802.11c
IEEE 802.11c provides required information to ensure proper bridge operations. Product
developers utilize this standard when developing access points.
IEEE 802.11d
IEEE 802.11d deals with global harmonization in terms of 802.11. In order to support a
widespread adoption of 802.11, the 802.11d task group has an ongoing charter to define
PHY requirements that satisfy regulatory within countries other than the US, Europe, and
Japan, which already had the necessary regulations in place at the beginning. This is
especially important for operation in the 5 GHz bands because the use of these
frequencies differ widely from one country to another. This standard mostly applies to
companies developing 802.11 products.
IEEE 802.11e
This standard deals with MAC enhancements for quality of service (QoS). This is
required for optimum transmission of voice and video, which is achieved by mechanisms
to prioritize traffic within 802.11. The 802.11e task group refined the 802.11 medium
access layer to improve QoS for better support of audio and video such as MPEG-2
applications. Because 802.11e falls within the MAC Layer, it is common to all 802.11
PHYs and backward compatible with existing 802.11 wireless LANs.
IEEE 802.11f
This is the inter-access point protocol. The original 802.11 standard did not specify the
communications between access points in order to support users roaming from one access
point to another. The problem, however, was that access points from different vendors did
not always interoperate when supporting roaming. IEEE 802.11f specifies an inter-access
point protocol that provides the necessary information to support roaming.
19.2.7
WLANs (wireless local area networks) vs WPANs (wireless personal
area networks)
IEEE 802.11 is an example of a WLAN. However, Bluetooth (IEEE 802.15), a WPAN, is
gaining popularity as well. The key differences between a WLAN (IEEE 802.11) and a
WPAN (IEEE 802.15) are listed here. Bluetooth is essentially a replacement for infrared
technology and will be used to link computers, peripherals, and handheld devices in an
office environment.
WLAN
WPAN
Environment
It is appropriate for applications that require MAC
provisions (ability to support large numbers of
nodes involved in concurrent communication,
mechanism to avoid interference) and high range
(10–100+ meter) necessary to support LAN
applications
WPANs are appropriate for applications
which are limited to the WPAN range
(0–10 m), do not require the complexity
of a LAN-style MAC scheme, and are
extremely sensitive to power consumption
and per-unit cost
378
Practical Industrial Data Communications
WLANs support segments at least 100 m in length. The specifications for WPANs
focus on short-distance wireless networks with a range of 0–10 m.
Medium Access Control Mechanism (MAC)
IEEE 802.11 calls for a carrier sense
multiple access with collision
avoidance (CSMA/CA) MAC
scheme
The Task Group 1 (TG1 – Bluetooth)
specification calls for a time division
multiple access (TDMA) scheme. This
mechanism provides no ‘listen before
transmission’ type of interference
avoidance as in CSMA/CA
Cost Factor
No specific cost constraints for 802.11
Cost factors are critically important in the
WPAN arena; for example, the unit cost
to manufacturers for TG1 (Bluetooth)
devices is targeted in the $10 (or under)
range in volume
Power Consumption
No specific power consumption
constraints for 802.11
802.15 standards are intended to address
power consumption issues critical to this
class of small portable/mobile device
19.3
Radio and wireless communications
19.3.1
Introduction
A significant number of industrial protocols are transferred using radio telemetry systems.
Radio is often chosen in preference to using landlines for a number of reasons:
Costs of cable can far exceed that of radio telemetry systems
Radio systems can be installed faster than landline systems
Radio equipment is very portable and can be easily moved
Radio can be used to transmit the data in any format required by the user
Reasonably high data rates can be achieved compared to some landline
applications
• Radio can be used as a back-up for landlines.
•
•
•
•
•
This chapter briefly examines some of the issues that have to be considered if the
industrial protocols are used over a radio link. It is broken down into:
•
•
•
•
•
•
•
Components of a radio link
Radio spectrum and frequency allocation
Summary of radio characteristics for VHF/UHF radio telemetry systems
Radio modems
How to prevent inter-modulation problems
Implementing a radio link
Miscellaneous considerations.
Wireless technologies 379
19.3.2
Components of a radio link
A radio link consists of the following components:
•
•
•
•
•
•
Antennas
Transmitters
Receivers
Antenna support structures
Cabling
Interface equipment.
Figure 19.8 illustrates how these elements are connected together to form a complete
radio link.
Figure 19.8
Basic elements of a radio link
Antenna
This is the device used to radiate or detect the electromagnetic waves. There are many
different designs of antennas available. Each one radiates the signal (electromagnetic
waves) in a different manner. The type of antenna used depends on the application and on
the area of coverage required.
Transmitter
This is the device that converts the voice or data signal into a modified (modulated)
higher frequency signal. Then it feeds the signal to the antenna where it is radiated into
the free space as an electromagnetic wave at radio frequencies.
Receiver
This is the device that converts the radio frequency signals (fed to it from the antenna
detecting the electromagnetic waves from free space) back into voice or data signals.
Antenna support structure
An antenna support structure is used to mount antennas, in order to provide a height
advantage, which generally provides increased transmission distance and coverage.
It may vary in construction from a 3 m wooden pole to 1000 m steel structure.
A structure that has guy wires for support is generally referred to as a mast. A structure
that is free standing is generally referred to as a tower (Figure 19.9).
380
Practical Industrial Data Communications
Figure 19.9
Mast and tower
Cabling
There are three main types of cabling used in connecting radio systems:
1. Coaxial cable for all radio frequency connections
2. Twisted-pair cables for voice, data and supervisory connections
3. Power cables.
Interface equipment
This allows connection of voice and data into the transmitters and receivers from external
sources. It also controls the flow of information, timing of operation on the system, and
control and monitoring of the transmitter and receiver.
19.3.3
The radio spectrum and frequency allocation
There are very strict regulations that govern the use of various parts of the radio
frequency spectrum. Specific sections of the radio frequency spectrum have been
allocated for public use. All frequencies are allocated to users by a government regulatory
body. Table 19.3 illustrates the typical sections of the radio spectrum allocated for public
use around the world. Each section is referred to as a band.
Certain sections of these bands will have been allocated specifically for telemetry systems.
Mid band UHF
Ultra High Frequency (UHF)
Low band UHF
High band VHF
Very High Frequency (VHF)
Mid band VHF
Low band VHF
High Frequency (HF)
Table 19.3
The radio spectrum for public use
960 MHz
800 MHz
520 MHz
335 MHz
225 MHz
101 MHz
100 MHz
60 MHz
59 MHz
31 MHz
30 MHz
2 MHz
Wireless technologies 381
In some countries, a deregulated telecommunications environment has allowed sections
of the spectrum to be sold off to large private organizations to be managed, and then sold
to smaller individual users.
Application must be made to the government body, or independent groups that hold
larger chunks of the spectrum for on selling, to obtain a frequency, and no transmission is
allowed on any frequency unless a license is obtained.
19.3.4
Summary of radio characteristics of VHF/UHF
Tables 19.4 and 19.5 summarize the information discussed in this section.
19.3.5
Radio modems
Radio modems are suitable for replacing wire lines to remote sites or as a back-up to wire or
fiber-optic circuits, and are designed to ensure that computers and PLCs, for example, can
communicate transparently over a radio link without any specific modifications required.
Modern radio modems operate in the 400–900 MHz band. Propagation in this band requires
a free line of sight between transmitting and receiving antennae for reliable communications.
Radio modems can be operated in a network, but require a network management software
system (protocols) to manage network access and error detection. Often, a master station with
hot change-over, communicates with multiple radio field stations. The protocol for these
applications can use a simple poll/response technique (Figure 19.10).
Data rates
Low Band VHF
Mostly LOS
some surface
wave
600 baud
Diffraction properties
Excellent
Mid Band VHF
LOS
Minimal surface
wave
1200 baud
Very good
Natural noise
environment
High
Medium
Low
Affected by man-made
noise
Severe
Bad
Some
Penetration of solids
Excellent
Very good
Good
Fading by ducting
Long term
Medium term
Short term
Absorption by wet
vegetation
Negligible
Low
Some
Equipment availability
Minimal
Reasonable
Excellent
Relative equipment cost
Uses
High
– In forested
areas
– Mostly mobile
– Very hilly
Medium
– Very hilly and
Forested areas
– Mostly mobile
– Over water
Low
– Long distance/LOS/hilly
areas
– LOS links
– Mobile
– Borefields
– Over water
Propagation mode
Table 19.4
VHF radio characteristics
High Band VHF
LOS
2400 baud
Good
382
Practical Industrial Data Communications
UHF 1
Propagation mode
Data rates
Diffraction properties
Natural noise environment
Affected by man made noise
Penetration of solids
Reflection and absorption by solids
Absorption by wet vegetation
Interference by ducting
Equipment availability
Relative equipment costs
Uses
LOS
4800 baud (9600?)
Some
Low
Low
Low
Good (enhanced
multi-pathing)
High
Some
Excellent
Low
– Telemetry
– Mobile
UHF 2
LOS
9600 baud (19.2 K?)
Minimal
Negligible
Very low
Negligible
Excellent (excellent
multi-pathing)
Very high
Some
Reasonable
Medium
– Telemetry
– Mobile
– Links
Table 19.5
UHF radio characteristics
Figure 19.10
Radio modem configuration
The more sophisticated peer-to-peer network communications applications require a
protocol based on carrier sensing multiple access with collision detection (CSMA/CD) or
carrier sensing multiple access with collision avoidance (CSMA/CA). A variation on the
standard approach is to use one of the radio modems as a network watchdog to
periodically poll all the radio modems on the network and to check their integrity. The
radio modem can also be used as a relay station to communicate with other systems that
are out of the range of the master station.
Wireless technologies 383
The interface to the radio modem is typically RS-232 but RS-422, RS-485, and
fiber-optics are also options. Typical speeds of operation are up to 9600 bps. A buffer
is required in the modem and is typically a minimum of 32 kB. Hardware and
software flow control techniques are normally provided in the radio modem firmware,
ensuring that there is no loss of data between the radio modem and the connecting
terminal.
Typical modulation techniques are two level direct FM (1200–4800 bps) to three level
direct FM (9600 bps). A typical schematic of a radio modem is given in Figure 19.11.
Figure 19.11
Block diagram of a radio modem
The following terms are used in relation to radio modems:
PTT
RSSI
Noise squelch
RSSI squelch
Channel monitor
Soft carrier delay
RTS, CTS, DCD,
DCD, Clock, TxD,
RxD
Push to talk signal
Receive signal strength indicator – indicates the received signal
strength with a proportionally varying DC voltage
Attempts to minimize the reception of any noise signal at the
discriminator output
Opens the receive audio path when the signal strength of the RF
carrier is of a sufficiently high level
Indicates if the squelch is open
Allows the RF transmission to be extended slightly after the actual
end of the data message which avoids the end of transmission
bursts that occur when the carrier stops and the squelch almost
simultaneously disconnects the studio path
All relate to RS-232.
The radio modem has a basic timing system for communications between a terminal
and the radio modem, indicated in Figure 19.12.
384
Practical Industrial Data Communications
Figure 19.12
Radio modem timing diagram
Data transmission begins with the RTS line becoming active at the remote terminal
side. The radio modem then raises the CTS line to indicate that transmission can proceed.
At the end of the transmission, the PTT is kept active to ensure that the receiving side
detects the remaining useful data before the RF carrier is removed.
Modes of radio modems
Radio modems can be used either in point-to-point mode or in point-to-multi-point
mode.
A point-to-point system can operate in continuous RF mode, which has a minimal turn
on delay in transmission of data, and non-continuous mode where there is a considerable
energy saving. The RTS to CTS delay for continuous and switched carriers is of the order
of 10 and 20 ms respectively.
A point-to-multi-point system generally operates with only the master and one radio
modem at a time.
In a multi-point system when the data link includes a repeater, data regeneration must
be performed to eliminate signal distortion and jitter. Regeneration is not necessary for
voice systems where some error is tolerable.
Regeneration is performed by passing the radio signal through the modem, which
converts the RF analog signal back to a digital signal and then applies this output binary
data stream to the other transmitting modem, which repeats the RF analog signal to the
next location (Figure 19.13).
Wireless technologies 385
Figure 19.13
Regeneration of a signal with a radio modem
Features of a radio modem
Typical features that have to be configured in the radio modem are:
Transmit/receive radio channel frequency
In a point-to-point configuration running in a dual frequency/split channel assignment,
two radios will operate on opposing channel sets.
Host data rate and format
Data rate/character size/parity type and number of stop bits for RS-232 communications.
Radio channel data rate
Data rate across the radio channel defined by the radio and bandwidth capabilities. Note
that these specifications are generally set at the time of manufacture.
Minimum radio frequency signal level
Should not be set too low on the receiver otherwise noise data will also be read.
Supervisory data channel rate
Used for flow control and therefore should not be set too low otherwise the buffer on the
receiver will overflow. Typically one flow control bit to 32 bits of serial data is the
standard.
386
Practical Industrial Data Communications
Transmitter key up delay
The time for the transmitter to energize and stabilize before useful data is sent over the
radio link. Transmitter key up delay should be kept as low as possible to minimize
overheads.
Spread spectrum radio modems
Several countries around the world have allocated a section of bandwidth for use with
spread spectrum radio modems. In Australia and America, this is in the 900 MHz area.
In brief, a very wide band channel is allocated to the modem, for example,
approximately 3.6 MHz wide. The transmitter uses a pseudo random code to place
individual bits, or groups of bits, broadly across the bandwidth and the receiver uses the
same random code to receive them. Because they are random, a number of transceivers
can operate on the same channel and a collision of bits will be received as noise by a
receiver in close proximity.
The advantage of ‘spread spectrum’ radio modems is very high data security and data
speeds of up to 19.2 kbps. The disadvantage is the very inefficient use of the radio spectrum.
19.3.6
Inter-modulation and how to prevent it
Introduction
Besides noise and interference that emanates from man-made sources (cars, electrical
motors, switches, rectifiers, etc.), there are three other main causes of RF interference.
These are produced from other radio equipment. The first and the most obvious source, is
another radio user operating close by on the same frequency as the system suffering from
interference. Unfortunately, besides using special coding techniques to minimize the
problem there is little that can be done, short of complaining to the regulatory government
body that issues licenses, or finding out who it is and asking them to stop transmissions.
The second source of interference comes from noisy transmitters that emit spurious
frequencies outside their allocated bandwidth. These spurious emissions will tend to fall
on other users’ channel bandwidths and cause interference problems. Aging transmitters
and those that are not well maintained are normally the culprits.
The third source of interference is known as inter-modulation. This is normally the
most common source of interference and generally the most difficult to locate and the
most costly to eliminate. The following section will examine this phenomenon in more
detail.
Inter-modulation
Inter-modulation occurs where two or more frequencies interact in a non-linear device
such as a transmitter, receiver or their environs, or on a rusty bolted joint acting as an RF
diode to produce one or more additional frequencies that can potentially cause
interference to other users. When two electromagnetic waves meet and inter-modulate in
a non-linear device, they produce a minimum of two new frequencies – one being the
sum of the frequencies and the other being the difference of the frequencies.
A nearby receiver may be on or close to one of the inter-modulation frequency
products, receive it as noise and interference and then could retransmit it as further noise
and interference. For example, if two frequencies ‘a’ and ‘b’ interact, then they will
produce two new frequencies ‘c’ and ‘d’ where a + b = c and a – b = d. c and d are
referred to as inter-modulation products.
Wireless technologies 387
Of course c and d will be of significantly less magnitude than a and b and their exact
magnitude depends on the magnitude of a and b, at the point a and b meet, and on the
efficiency of the non-linear device at which the inter-modulation takes place.
Fortunately, this problem is only significant when the two transmitters for a and b are
within close proximity. However, consideration should be given to inter-modulation
products produced at a distant location as these have been known to cause noticeable
background noise. If there are more than two frequencies at one location then the number
of inter-modulation products possibly increases dramatically.
For example, if there are transmitters on frequencies a, b, and c at one location then the
inter-modulation products become:
a+b =
a+b =
b+c=
b–c=
a+c=
a–c=
a+b–c=
a+b–c=
a–b+c=
a–b–c=
f1
f2
f3
f4
f5
f6
f 26
f8
f9
f10
This illustrates that the number of potential inter-modulation products becomes
prodigious as the number of frequencies increases. Unfortunately, the scenario gets
worse. Each frequency from a transmitter will produce a significant harmonic at twice,
three times, four times, etc., its carrier frequency (this is particularly true with FM
systems). Each sequential harmonic will be of a lesser magnitude than the previous
one.
Therefore if the transmitter is operating on frequency a, then harmonics will be
produced at 2a, 3a, 4a, etc. The 2a and 3a harmonics can be quite large. These harmonics
are produced because of resonant properties of antennas, cables, buildings, and tuned
circuits in the receivers and transmitters themselves and also due to the harmonic side
bands produced in FM.
Taking these harmonics into account, inter-modulation products such as (2a – b) and
(3b – 2c) are examples of what can be produced. (i) is referred to as a third order product
and (ii) as a fifth order product. This refers to the total of multiples of each frequency.
These inter-modulation products can cause severe interference in radio systems that are
located at the same site. Taking these into account, there are numerous permutations of a
small number of base frequencies that may cause inter-modulation interference.
Generally, inter-modulation products greater than fifth orders are too small to be of
consequence and generally, radio systems are engineered only to take into account
possible inter-modulation distortion up to and including fifth order products. At sites
where there are many sensitive receivers on different channels in one building, then
calculations to seventh order may be carried out.
Software has been provided with this book for calculations of inter-modulation
products, from any number of transmitters at a site.
For transmitters that are 1 km or more apart, the inter-modulation products are
generally so small that it would not be a problem. The exception may be for first or
second order products.
388
Practical Industrial Data Communications
If a system is experiencing, unexplainable interference problems, a check should be
made for distant inter-modulation products.
Another source of harmonics sometimes comes from old buildings or masts where rusty
bolts or nails act as RF diodes (nonlinear devices) to produce inter-modulation products.
These are then detected by nearby receivers and may be retransmitted as noise and
interference.
A number of devices have been developed to assist in preventing the formation of intermodulation products and to prevent these products, spurious transmissions and harmonics
from causing interference to nearby receivers or transmitters. All these devices are
connected between the transmitter and the antenna. The following sections provide details
on these devices and how they are used to eliminate interference problems.
19.3.7
Implementing a radio link
There is an important methodology that must be followed when designing and
implementing a radio link if it is to work satisfactorily. It is relatively straightforward and
will provide successful radio communications if followed closely.
Software has been provided with this manual that carries out the necessary calculations.
Reference should be made to the book when using the software. In general, the software
will follow a procedure as is described in this section.
The design methodology in a sequential order is as follows:
•
•
•
•
•
•
•
Carry out a radio path profile
Calculate RF losses for the radio path
Calculate effects of transmitter power
Decide on required fade margin
Choose cable and antenna
Purchase equipment
Install equipment.
Path profile
The first requirement in establishing a successful radio link is to draw up a radio path
profile. This is basically a cross-sectional drawing of the earth for the radio propagation
path showing all terrain variations, obstructions, terrain type (water, land, trees,
buildings, etc.) and the masts on which the antenna are mounted. For distances less than
a kilometer or two, profiles are not normally required, as the RTU can quite often be
clearly seen from the master site (but all other calculations and choices described in the
design methodology must be carried out). The Figure 19.14 illustrates a typical path
profile.
The first step in this process is to obtain a contour map of the location. These survey
maps are readily available from government departments that oversee land administration
and private companies that carry out surveys and publish their material, for most areas of
developed countries and many areas in developing countries. It is recommended that the
map have a minimum of 20 m contour lines, with 2 m, 5 m, or 10 m being preferred.
Locate the RTU and master site locations on the map and draw a ruled line between the
two locations with a pencil. Then assuming that the master site is at distance 0 km, follow
the line along noting the kilometer marks and where a contour line occurs and at that
point, also note the contour height.
Wireless technologies 389
Figure 19.14
Typical path profile
The surface of the earth is of course not flat but curved. Therefore, to plot the points
you obtained from the map directly would not be a true indication of the path. The
formula below provides a height correction factor that can be applied to each point
obtained from the map to mark a true earth profile plot.
h =
(d1 × d 2 )
12.265K
Where:
H = height correction factor that is added to the contour height (in m)
d1 = the distance from a contour point to one end of the path (in km)
d2 = the distance from the same contour point to the other end of the path (in kilometers)
K = the equivalent earth radius factor.
The equivalent earth radius factor K is required to account for the fact that the radio
wave is bent towards the earth because of atmospheric refraction. The amount of bending
varies with changing atmospheric conditions and therefore the value of K varies to
account for this.
For the purposes of radio below 1 GHz it is sufficient to assume that for greater than
90% of the time K will be equal to 4/3. To allow for periods where a changing K will
increase signal attenuation, a good fade margin should be allowed for.
The K factor allows the radio path to always be drawn in a straight line and adjusts the
earth’s contour height to account for the bending radio wave. Once the height has been
calculated and added to the contour height, the path profile can be plotted.
From the plot it can now be seen if there are any direct obstructions in the path. As a
general rule, the path should have good clearance over all obstructions. There is an area
around the radio path that appears as a cone that should be kept as clearance for the radio
path. This is referred to as the ‘Fresnel zone’.
Fresnel zone clearance (Figure 19.15) is of more relevance to microwave path
prediction than to radio path prediction. The formula for the Fresnel zone clearance
required is:
1/2
 (d × d ) 
F = 0.55  1 2 
 (G × D) 
390
Practical Industrial Data Communications
Where:
F = Fresnel zone clearance in meters (i.e. radius of cone)
d1 = distance from contour point to one end of path (in km)
d2 = distance from contour point to other end of path (in km)
D = total length of path (in km)
G = frequency in MHz.
Figure 19.15
Fresnel zone clearance
If from the plot it appears that the radio path is going dangerously close to an
obstruction, then it is worth doing a Fresnel zone calculation to check for sufficient
clearance. Normally, the mast heights are chosen to provide a clearance of 0.6 times the
Fresnel zone radius. This figure of 0.6 is chosen because it firstly gives sufficient radio
path clearance and secondly assists in preventing cancellation from reflections. At less
than 0.6F, attenuation of the line of sight signal occurs. At 0.6F, there is no attenuation of
the line of sight signal and therefore there is no gain achieved by the extra cost of
providing higher masts.
Another important point to consider is that frequencies below 1 GHz have good
diffraction properties. The lower the frequency the more diffraction that occurs.
Therefore, for very long paths it is possible to operate the link with a certain amount of
obstruction. It is important to calculate the amount of attenuation introduced by the
diffraction and determine the affect it has on the availability (i.e. fade margin) of the radio
link. The mathematics for doing this calculation is relatively complex and has been
included in the software provided.
As an example, Figure 19.14 shows a hill obstructing the radio path. Therefore a
calculation needs to be carried out to determine the attenuation due to diffraction at this
hill. This would then be added to the total path loss to determine if the link will still
operate satisfactorily.
One further point of note is that this discussion and the formulas and software provided
are for fixed radio links only. Mobile (moving RTU) radio uses a completely different set
of criteria and formula. The greater majority of telemetry links are fixed and analysis of
mobile radio is not provided here.
RF path loss calculations
The next step is to calculate the total attenuation of RF signal from the transmitter
antenna to the receiver antenna.
This includes:
•
•
•
•
Free space attenuation
Diffraction losses
Rain attenuation
Reflection losses.
Wireless technologies 391
Free space attenuation and diffraction losses are calculated using the industry standard
formulas. Rain attenuation is negligible at frequencies below 1 GHz.
Reflection losses are difficult to determine. Firstly, the strength of the reflected
signal depends on the surface it is reflected off (for example, water, rock, sand).
Secondly, the reflected signal may arrive in phase, out of phase or at a phase angle in
between. So reflected waves can be anything from totally catastrophic to enhancing
the signal. Good engineering practice should always assume the worst case, which
would be catastrophic failure. Therefore, when designing a link, a check is made for
reflections and if they exist, measures should be taken to remove the problem
(Figures 19.16 and 19.17).
This can be done by moving antennas or masts to different locations and heights or by
placing a barrier in the path of the reflection to absorb it. For example, place the antenna
behind a hill, house, billboard, etc.
Figure 19.16
Removing potential reflections
With reference to Figure 19.10, the total loss would be calculated as follows:
(a) Free space loss = 32.5 + 20 log10 F + 20 log10 D
= 32.5 + 53.1 + 24.6
= 110.2 dB
(b) Diffraction loss = 23 dB
(c) Total loss
= (a) + (b)
= 133.2 dB
Figure 19.17
Example link
392
Practical Industrial Data Communications
Transmitter power/receiver sensitivity
The next step is to determine the gain provided by the transmitters. If in a link
configuration one transmitter operates with less power than the other, the direction with
the least power transmitter should be considered. Therefore, as the ACA regulation
requires that RTUs be allowed to transmit a maximum of 1 W into the antenna, while
master stations can transmit 5 W into the antenna (sometimes higher), the path direction
from the RTU to the master should be considered.
The transmit power should be converted to a dBm figure. For an RTU this would be as
follows:
Power = 10 log [1/10−3 ] dBm
= 10 log [103] dBm
= 30 dBm
The next step is to determine the minimum RF level at the receiver input that will open
the front end of the receiver (i.e. turn it on). This is referred to as the ‘receiver threshold
sensitivity level’ or sometimes as the ‘squelch level’. This figure can be obtained from the
manufacturer’s specification sheets.
For a radio operating at 450 MHz, this would be approximately –123 dBm. At this
level, the signal is only just above noise level and is not very intelligible. Therefore, as a
general rule, a figure slightly better than this is used as a receiver sensitivity level.
A de facto standard is used where the RF signal is at its lowest but still intelligible. This
level is referred to as the 12 dB SINAD level.
Again, this figure is obtained from manufacturer’s data sheets. For a typical 450 MHz
radio, this level is approximately –1126 dBm.
Using these figures, a simple calculation can be performed to determine the links
performance for the example link in Figure 19.10.
Tx Pwr
= Transmit power at RTU
Loss
= +30dBm
= RF Path attenuation
Rx Sen
= 133.2dB
= Receiver sensitivity for 12dB SINAD
Tx Pwr − Loss
= − 1126dB
= Available power at receiver
= +30 − 133.2 = − 103.2dBm
Since the receiver can accept an RF signal down to –1126 dBm, then the RF signal will
be accepted by the receiver. In this case, we have 13.8 dBm of spare RF power.
Signal to noise ratio and SINAD
The most common measure of the effect of noise on the performance of a radio system is
signal to noise ratio (SNR). This is a measure of the signal power level compared to the
noise power level at a chosen point in a circuit. In reality, it is the signal plus noise
compared to noise.
∴ SNR =
Psignal + Pnoise
Pnoise
Wireless technologies 393
SNR is often expressed in dBs. Therefore:
 Psignal + Pnoise 
SNR = 10 log10 

Pnoise


The real importance of this measurement is at the radio receiver. As the receive signal
at the antenna increases, the noise level at the audio output of the receiver effectively
decreases. Therefore, a measure of SNR at the receiver audio output for measured low
level of RF input to the receiver front end is a good measure of the performance of a
receiver.
The highest level of noise at the audio output will be when the RF input signal is at its
lowest level. That is when the RF input signal just opens the receiver. At this point, the
SNR will be between 3 and 6 dB.
A measurement has been developed as a de facto standard that measures the receiver
input signal level for an SNR of 12 dB at the audio output. It is often referred to as the
12 dB SINAD level (signal to noise and distortion). The measurement is made with a
device called SINAD meter.
The SINAD measurement is carried out by feeding the input of the receiver with a RF
signal that is modulated by a 1 kHz input audio signal. The 1 kHz signal will produce
harmonics and unwanted distortion in the audio output. A SINAD meter is placed at the
receiver audio output. The SINAD meter measures the power level of the 1 kHz plus the
noise and distortion. It then filters the 1 kHz signal and measures the broadband level of
noise power without the 1 kHz signal. It then divides the two measurements and provides
a reading of SNR in dBs. The level at the receiver RF input is slowly increased until the
SNR at the audio output is 12 dB. The RF input level is then noted and this is referred to
as the 12 dB SINAD level. Some manufacturers refer to it as sensitivity at 12 dB SINAD
or sometimes just as sensitivity. The latter can be very deceptive and care must be taken.
The equipment configuration for measurement of SINAD is shown in Figure 19.18.
Figure 19.18
Measuring SINAD
Some typical 12 dB SINAD sensitivity figures for modern radios in the VHF and UHF
bands are 0.25 to 0.35 µV. The receiver opening sensitivity is normally 0.18 to
0.2 µV (i.e. the squelch level).
Another measurement also used to determine receiver performance is the ‘20 dB
quieting’ measurement. This is not used as often as the 12 dB SINAD method.
Here, the receiver squelch is set so that it is just open. A level meter is connected to the
speaker and the noise level measured. The audio volume control is set for a convenient
level (0 dBm). A signal generator is then connected to the receiver input and an
unmodulated RF signal is fed into the receiver and increased slowly until the noise level
at the audio output has dropped by 20 dB.
394
Practical Industrial Data Communications
The disadvantage of this method is that it only measures the ability of the receiver to
receive an unmodulated RF carrier signal. Poor design, component aging or improper
alignment can provide a circuit response that admits the carrier signal perfectly but
provides poor quality reception of modulated signals.
Fade margin
Radio is statistical by nature and it is therefore impossible to predict with 100% accuracy
as to how it is going to perform. For example, due to degrading effects of reflections,
multi-pathing, ducting, and RF interference, a link may lose or gain signal by up to 15 dB
over short of long periods. It is because of this unpredictability, it is important to have a
safety margin to allow for intermittent link degradation. This safety margin (or spare RF
power) is generally referred to as the fade margin.
It should be the intention to design most links to have a fade margin of approximately
30 dB. This means that if there was a 30 dB drop in RF signal level then the RF signal at
the receiver input would drop below the 12 dB SINAD sensitivity. Therefore, in the
example in the previous section there is insufficient fade margin. This is overcome by
using high gain antennas. For example, if we use a 13 dB gain Yagi at the RTU and a
6 dB gain omni-directional antenna at the master site, we add an extra 19 dB gain to our
signal. Therefore, the total fade margin in our example would become:
13.8 + 19 = 32.8 dB
Finally, we must consider other losses introduced by cables, connectors, multi-couplers,
etc. In this example, if we have 20 m of 3 dB/100 m loss cable at each end, total
connector losses of 0.5 dB at each end and a multi-coupler loss of 3 dB at the master site
then:
Extra losses:
Cables: 2 (0.2 × 3 dB)
Connectors: 2 × 0.5 dB
Multi-coupler
Total extra losses
= 1.2 dB
= 1 dB
= 3 dB
= 3 + 1 + 1.2
= 5.2 dB
Therefore the fade margin for the link is:
32.8 – 5.2 = 226.6 dB
Summary of calculations
The following equation is a summary of the requirements for calculation of fade margin:
Fade margin = − (free space attenuation) − (diffraction losses) + (transmitter power) +
(receiver sensitivity) + (antenna gain at master site) + (antenna gain at RTU) − (cable
and connector loss at master) − (cable and connector loss at RTU) − (multi-coupler
filter or duplexer loss) + (receiver pre-amplifier gain).
Miscellaneous considerations
When implementing a radio link, there are other important considerations. The following
is a list of some of the main considerations:
• It is important to obtain specification sheets from radio supplier before
purchasing any equipment and ensuring that all parameters meet your
requirements.
Wireless technologies 395
• The audio frequency output and input for a radio is normally a balanced 600
connection. Depending on the equipment provided, it will accept levels of
between –30 dBm and +15 dB and will output levels from –15 to +15 dBm.
These are normally adjustable internally.
• Most radios operate at +12 V DC (or 13.2 V DC if it is floated across a
battery). Depending on the RF output power level the current consumption may
vary from 1 A for 1 W output to 10 A for 50 W output. This should be taken
into consideration when sizing power supplies and batteries.
• The size and weight of the radio equipment should be noted so that correct
mechanical mounting and rack space can be provided. Multi-couplers and
duplexers can be very large and may need to be mounted on walls outside the
radio equipment racks or in separate racks. In order to minimize intermodulation, multi-couplers and filters should be as close as possible to the
transmitter and receiver.
• It is beneficial to obtain mean time between failure (MTBF) figures from
manufacturers because if the radio is to be placed in a remote location that is
difficult to regularly access, it is vital that the radio is reliable.
• If the telemetry communication protocol requires the radio to be switched on
and off at regular intervals, it is best to avoid having relays in the inline RF
circuit. Discrete transistor RF switching is preferred.
System design methodology
20.1
Introduction
Ideally, the project should be implemented with a systems engineering approach as described
in Chapter 1. This involves drawing up a functional specification first, followed by the
validation of any techniques or technologies of which perfect operation is not a foregone
conclusion. After this come the detailed design, procurement, installation/integration,
commissioning/testing, and long-term maintenance.
With that in mind, the functional requirements have to be established up front.
It involves specifying environmental requirements as well as the inputs, outputs, and
transfer functions of all systems and sub-systems to be procured. If any systems or
subsystems (for example, data acquisition units) have to be built from scratch for some or
other reason, then the EC and ST structures (in other words, how the system is put
together and how it operates) will also be designed.
Often more than one solution satisfies the functional requirements. If this is the case, and
the cost of the system is substantial, there may be a requirement that an economic analysis be
done as well. For each system, all expenditures (capital expenditures as well as operating
costs) will be estimated and then referred to the present time by means of a process called
discounting. This will be done at an interest rate called the ‘cost of capital’ or the ‘hurdle rate’.
The final decision is made, for example, on the internal rate of return (IRR) or the net present
value (NPV). Unfortunately, these techniques are totally beyond the scope of this book.
As part of the specification process, the following will need to be formalized.
20.1.1
Environment
Before embarking on a design of the system itself, the environment in which it will
operate has to be specified, as this imposes very specific constraints on the system design.
The environment involves everything surrounding, affecting, or supporting the system and
its sub-systems, but does not include the system itself. For the purpose of this discussion we
will view the system as all communicating devices as well as the interconnecting medium.
The following questions need to be asked. The points listed here are not in any specific
order:
• What is the range of temperatures to be experienced by individual components?
Whereas a network device (e.g. Ethernet switch) in an enclosure within a
control room may be kept more or less permanently at 25 ºC, for example, the
same device mounted outside in the open (with maybe some form of covering
to keep out rain) may experience from –10 ºC in winter to +50 ºC in summer.
System design methodology 397
• What is the maximum level of humidity/water to be experienced? Some
equipment for use on oilrigs may need to be immersable, for example, whilst
components mounted outside in a partially open enclosure with a roof will need
to be able to withstand occasional raindrops being blown in from the side. This
applies to devices as well as cable assemblies and connectors.
• Is the environment very dusty? In the vicinity of mines, especially open (surface)
mines, the dust levels in an unfiltered environment can be immense.
• How corrosive is the atmosphere? In marine environments, the salt in the air can
create large-scale corrosion. Wiring suitable for use in a building will not last
long on a ship.
• Is there a possibility of people, equipment or other objects (e.g. stones) impacting
on the equipment or its enclosure and thereby damaging it?
In the presence of dust, moisture, and impact as described above, the appropriate IP
ratings for individual components have to be determined:
• What power infrastructure is available, if any? Is there alternating current
(AC)? If so, is it 110 V or 220 V? Is there direct current (DC)? If so, is it 12 V
or 24 V? Is there a proper earth ground available? Is there a proper protective
(chassis) ground available? How ‘clean’ is the latter? Is there any need for or
provision of redundant supplies? Are there differences in the earth potentials?
• Are cables running outside, e.g. between two buildings? If so, are they exposed
to water in some or other form? Are they exposed to UV radiation (sunlight)?
Are they running in a protected channel or do they need to be armored?
• What is the possibility of a nearby lightning strike?
• Are there any existing (‘legacy’) systems to be considered?
• Are there any corporate directives/policies in terms of preferred vendors and/or
technologies?
• Is there a risk of explosion caused by electrical sparks? This could be the case
where flammable vapors or a high concentration of airborne combustible dust
(e.g. sawdust) is present. In this case the data communications electronics will
have to be intrinsically safe, i.e. the energy in the signals will have to be limited
to such an extent that sparks cannot be created if a physical connection is made
or broken.
• Are there any cost/budgetary constraints?
• What level of vendor support is available locally?
• Is there an existing maintenance infrastructure, and if so, which technologies
are supported?
Once these facts have been established, we can turn our attention to the functional
requirements of the actual data communications systems.
20.2
Point-to-point links
20.2.1
Functional requirements
If there is any point-to-point (or point-to-multipoint) communications required, the following
needs to be ascertained:
• Is it purely point-to-point communication between two nodes, a multi-drop (bus)
configuration with only one transmitter but several receivers, or a multidrop
configuration with several transmitters and receivers?
398
Practical Industrial Data Communications
• Is the communication simplex, half-duplex or full-duplex?
• What distance needs to be covered? Is it a few meters? A few hundred meters?
A few kilometers?
• If the communicating devices are a few kilometers apart are the two sites
within line-of-sight? What is the terrain like in terms of the path profile? Is
there any existing communications infrastructure?
• If line-of-sight operation is not possible, are both stations within the footprint
of a specific satellite?
• Are they far apart but can, for some reason, not be be serviced by satellite?
• What transmission rate is required (bits per second)?
• What data rate (on average) is expected?
• Are remote stations stationary or mobile?
Once these facts are known, the options could be as follows.
20.2.2
RS-232
If the two communicating devices are close together (say, in the same building), there is
very little electrical interference (noise) present and there is a good grounding system
with both end systems at the same reference potential, RS-232 is an option.
Refer to the chapter on RS-232 for permissible distances. The distance has to be
reduced as the bit (baud) rate goes up, and at 110 baud, a distance of 850 m is possible
but at 115 kbaud, this distance is reduced to 20 m. Charts, similar to the one for RS-485
shown further on, are available for RS-232 on different types of wire.
Since RS-232 uses separate lines for TX and RX, full-duplex is possible in theory as long
as the upper layer protocols can support it. Keep in mind that RS-232 only implements
layer 1 (the physical layer) of the OSI model.
20.2.3
RS-422/485
If the two end stations are up to 1200 m (4000 ft) apart and/or there is a fair amount of
noise present, RS-422 or 485 may be a solution. However, at 1200 m a data rate of no
more than 90 kbps may be possible. The data rate decreases at a straight line on a log/lin
scale until at 10 Mbps a distance of no more than 15 m may be possible.
Refer to Figure 20.1, which shows the data-signaling rate vs cable length for 24 AWG
twisted-pair wire with a shunt capacitance of 16 pF/ft and terminated in 100 Ω. This
curve is based on signal quality requirements of a signal rise and fall time equal to or less
than one-half unit interval at the applicable bit rate, and a maximum voltage loss between
driver and load of 6 dB. Simple charts like this enable users to select the proper cable
without a technical understanding of the cable parameters. The charts for RS-422 and
RS-485 are identical.
The losses in a transmission line are a combination of AC losses (skin effect) in the
conductor, DC losses in the conductor, leakage, and AC losses in the dielectric. In highquality cable, the losses in the conductor and the losses in the dielectric are more or less the
same. Figure 20.2 shows the difference in losses (in terms of attenuation vs frequency)
between different types of cable. Polyethylene cables offer much lower attenuation than
PVC cables.
System design methodology 399
Figure 20.1
Data-signaling rate vs cable length
Figure 20.2
Attenuation vs frequency for various cable types
Another approach to choosing the cable is the so-called ‘E-GRADE’ program,
established by Anixter Bros. Inc., a worldwide distributor of wiring products. Under this
system, there are four categories viz.:
•
•
•
•
E-GRADE 1: Limited distance
E-GRADE 2: Standard distance
E-GRADE 3: Extended distance
E-GRADE 4: Maximum distance.
For reliable operation, it is imperative that surge suppressors be installed on both sides.
However, the capacitance of the suppression devices may adversely affect the maximum
bit rate of the link. If this becomes a problem, fiber might be the only way out.
If the earth potentials between the two end sites are significantly different, it might also
be necessary to revert to fiber.
400
Practical Industrial Data Communications
An RS-422 system only supports simplex operation. RS-485 with one pair of wires only
supports half-duplex, while RS-485 with two pairs of wire supports full-duplex. However,
as in the case of RS-232, this depends on the capabilities of the upper layer communication
protocols since RS-422 and 485 only implement layer 1 of the OSI model.
20.2.4
Fiber
If the earth potentials of the two end stations are significantly different, or there is a
severe noise problem, or the cable runs outside and there is a high incidence of lightning
strikes, then it will be necessary to revert to fiber.
The cheapest fiber solution is to use two low-cost (1 mm) plastic fibers with an RS-232
to fiber converter at each end as a replacement for the copper link. Power stealing
converters (drawing power from the control lines on the RS-232 connector) will be the
cheapest option but will limit the distance to around 10 m. Using external power supplies
for the converters will increase this distance to around 40 m.
A slightly more expensive solution will be to use multimode fiber (e.g. 50/120/900)
with appropriate interfaces. This will operate typically up to 1.8–2 km.
If larger distances are required, single-mode fiber will be necessary and, depending on
the driver and the wavelength used, segments of up to 100 km can be implemented.
Ultimately, distance is not a limitation as repeaters can be used to link segments; the only
problem is to supply power to the repeaters. At present the high-speed Internet links
between the East Coast of the USA and South Africa, for example, are undersea fiberoptic links.
A single fiber can only support simplex operation as the fiber plus is transmitter and
receiver only allows light to travel in one direction. For full- or half-duplex operation,
two fibers are necessary.
20.2.5
Other solutions
Here is a brief summary of alternative methods used to transfer data from one location to
another. The cost and reliability will differ from method to method.
• Cable modems: Cable modems can create a permanent connection between
two devices (DTEs) across a twisted-pair cable (landline). The cable can be
privately owned and installed between two locations, or it can be leased from a
telephone utility. The hosts (DTEs) communicate with the modems (DCEs) via
RS-232, whilst the two modems use a different protocol, such as the pointto-point protocol (PPP) between themselves.
• Wireless modems: Wireless modems are suitable for replacing wireless lines
to remote sites or as a back-up to wire or fiber-optic circuits. They are designed
to ensure that computers and PLCs, for example, can communicate
transparently over a radio link without any specific modifications required. The
interface to the radio is typically RS-232, but RS-22 or 485 are also options.
Typical speeds of operation are 9600 bps. A buffer is built into the modem and
is typically 32 kB in size. Typical modulation techniques are two level FM
(1200 to 4800 bps) to three level FM (9600 bps).
• IEEE 802.11 wireless LAN with suitable long-range antennas: With suitable
antennas (40–50 cm parabolic dish) a remote station can communicate with an
IEEE 802.11 wireless hub over a range of several kilometers without the need
for power amplification. The data rate depends on the version (802.11, 802.1b
System design methodology 401
or 802.11a) and the quality of the signal, but 1–10 Mbps over distances up to
8 km is attainable.
• VSAT (very small aperture terminals): This technology is a solution where
remote stations have to link up to a base station in a remote area where the
distances are large (beyond line of sight) and there is no established
infrastructure. A typical example would be data acquisition systems used by
geologists in the oilfields in the north of Canada.
The following methods use third-party links supplied by telecommunications utilities.
They are not truly ‘industrial’ links, but could nevertheless be used to link industrial sites
at the enterprise level.
• Cellular data systems such as CDPD (cellular digital packet data) and GPRS
(general packet radio service).
• Other public transport network (WAN) technologies such as T1, E1, Switched
56, ISDN-BRI (integrated services data network-basic rate interface), ISDNPRI (ISDN primary rate interface), Frame Relay, B-ISDN (broadband ISDN),
X.25, ATM (synchronous transfer mode), and SDH/SONET (synchronous
digital hierarchy/synchronous optical network). The costs of the various
technologies vary, commensurate with the data rate.
• The Internet as the ‘cloud’. In this case it would be advisable to use an
industrial Internet service provider (iISP) instead of a commercial ISP, as well
as proper authentication and encryption schemes. The creation of a virtual
private network (VPN) might be advisable for security reasons, as all
communication through the Internet will take place in logical ‘tunnels’, which
make it impossible to intercept the data.
20.3
Networked systems
Networked systems in a plant can be placed conceptually in one of the three levels. These
areas are not rigorously defined, so it is possible for two vendors to place heir products at
slightly different positions in this hierarchy.
The lowest (bottom layer) is often referred to as the device layer. Systems at this level
are deployed on the plant floor and link sensors and actuators, both digital and analog,
with controllers. Required response times could be fast, and often a high degree of
deterministic behavior is required. Data packets (frames) are generally small, and often
bit-oriented. Typical systems at this level include DeviceNet, Foundation Fieldbus H1,
Profibus DP/PA, HART, and AS-i. This layer is not necessarily implemented by one
large system, but can consist of several smaller systems, linked with bridges, routers, or
gateways. For example, smaller AS-i automation islands can be connected via gateways
to a larger Profibus DP network. Systems at this level could be involved in PID control.
The middle layer is referred to as the control layer, and this layer typically interconnects
PLCs, SCADA systems, and HMIs. Typical systems at this level are ControlNet,
Ethernet/IP, Foundation Fieldbus HSE, ProfiNet, Modbus Plus, and Data Highway Plus.
Systems at this level are involved in supervisory control, and there is less emphasis on
deterministic behavior. Response times can be slower, and data packets (frames) are
generally larger, and byte-oriented.
The upper layer is referred to as the enterprise or information layer. Typical networks at
this level include Ethernet and Token ring, connecting various computer systems
(mainframes, etc.). Systems interconnected include financial systems, MRP systems, and
402
Practical Industrial Data Communications
various other database-type applications. There is no need for deterministic behavior and
messages could be large.
Ideal solutions would offer the following:
• The use of common protocols across all three layers, for example, TCP/IP
• The same network standard (e.g. Ethernet) across all three layers, or at least the
upper two. This simplifies maintenance, and facilitates the use of off-the-shelf
network components (e.g. routers and switches).
Keeping the above model in mind, the following questions need to be asked:
• At what level is the system being designed (device, operator, or enterprise)?
• Inputs: What types, e.g. digital input, analog input? How many? How
distributed across the plant?
• Outputs: What types, e.g. analog output, digital output (relay contact or open
collector transistor, etc.)? How many? How dispersed across the plant?
• Functionality required:
–
–
–
–
–
Real-time control (PID)? If so, how many loops?
Logging?
SCADA functions?
What degree of determinism is required?
What degree of redundancy is required and if any where? Redundant
links? Redundant network devices? Redundant controllers?
– Is it a real-time application? If so, what constitutes ‘real time’? On the
steering system of an oil tanker, a 30 s response time is real time. On a
computer numerically controlled milling machine, a 5 ms response time
might be ‘real time’.
– Is power via the bus a requirement? If so, to what degree? Is it only for
networking devices or also for field devices?
– Are built-in OPC servers a requirement? If so, which ones? DA, DX, etc.?
OPC DX allows controllers from multiple vendors to be configured via a
standard web interface.
Once these requirements have been finalized, it is time to look at the various options.
20.3.1
Device level
Ethernet has truly come of age and is often used at the device level in applications as
demanding as marine applications (for example, controlling the position of the floats of
the barge used to raise the submarine Kursk), on ships and on avionics systems for fighter
aircraft. Applications like this tend to use deterministic Ethernet with dual redundant
switched fiber rings. Ethernet interfaces are available right down to sensor level. Limited
device power is available with PoE (Power over Ethernet).
However, Ethernet is not a cure-all. It cannot supply large amounts of current via the
bus (due to the small wire gage) and it implements only the bottom two layers of the OSI
model. Agreed, TCP/IP runs well over Ethernet, but still only accounts for layers 3 and 4.
There is no agreed upper layer control protocol in layers 5–7, and no ‘user’ layer above it
so there are no functional building blocks for process control applications. Ethernet
frames have a large overhead, but the high speeds (typically 100 Mbps for industrial
applications) compensate for that.
For smaller applications with primarily digital inputs, a very strong need for
determinism and a fast cycle rate, AS-i could be an option. Note that there are a
System design methodology 403
somewhat limited number of inputs, and the maximum diameter of the network is
relatively small; so that in many applications it has to be bridged into another network,
e.g. Profibus to make data accessible throughout the plant.
For applications with intrinsic safety requirements, the choices are narrowed down to
systems with IEC 61158-2 capability. This includes Foundation Fieldbus H1 and Profibus
PA. Note that Profibus PA encompasses only the physical and data-link layers, it still
needs Profibus PA or FMS for its ‘intelligence’.
For applications requiring power on the bus, DeviceNet or Foundation Fieldbus could
be options, with DeviceNet allowing up to 8 A of current and FF up to 2.5 A. However,
DeviceNet supports up to 500 kbps while FF only allows 31.25 kbps due to its intrinsic
safety design.
The degree of determinism required is a contentious issue, more so because many
people do not really understand the issues and are confused by proponents of the various
technologies as well as misleading advertising. Deterministic does not mean fast! The
degree of determinism is an indicator of the predictability of the access time of a given
device to the bus, not the actual waiting time before it gains access. For example, on a
token passing network with several hundred nodes the access time could be guaranteed to
be a maximum of 200 ms. Whereas the operation could be labeled as deterministic, it is
by no means fast. On the other hand, on an IEEE 802.1p/Q Ethernet VLAN switch, a
specific node could always gain immediate access by virtue of the fact that it has the
highest priority, yet many people do not regard Ethernet as being deterministic!
Full-duplex operation (i.e. simultaneous reading from and writing to a device) may also
be an issue for some high-speed applications. Ethernet (100 Mbps and up) is the ONLY
base band network technology capable of this since it uses a hub topology with separate
transmit and receive paths. Virtually all device level networks use a bus topology, which
makes full-duplex impossible.
20.3.2
Control level
Systems at this level include Modbus Plus, Data Highway Plus, ControlNet, Profibus
FMS, etc. Ethernet features strongly here, though not on its own but rather as the OSI
layers 1 and 2 of other control networks. For example, Rockwell automation has brought
out its Ethernet/IP (Ethernet/Industrial Protocol), which is essentially an Ethernet version
of DeviceNet. The Profibus organization brought out ProfiNet, which is essentially
Profibus FMS running on Ethernet (although its functionality goes way beyond that of
profibus). In similar fashion, the Fieldbus Foundation brought out Foundation Fieldbus
HSE, which is essentially Foundation Fieldbus on Ethernet. An added feature of these
Ethernet-based systems are that they support TCP/IP. This makes it possible to access
devices freely across the network, and even from outside the network, provided that the
required access devices and security measures are in place. In addition, the products
mentioned here will also support OPC DX, which means that all controllers on the
network, regardless of vendor, can be configured in exactly the same way via a common
browser interface. In the final analysis, it only makes sense to complement the device
layer systems with control layer systems from the same vendor where possible. In this
way we will have DeviceNet supplemented with Ethernet/IP, Profibus with ProfiNet, and
FF H1 with FF HSA.
Whatever the case, it is important to be able to source an off-the-shelf device (gateway)
to connect the device level network with the control level network.
All the new products support TCP/IP. TCP/IP not only supports OPC, but also HTTP so
web servers can be embedded in devices such as PLCs. In addition, de facto protocols
like Modbus (Modbus over TCP/IP) can be supported.
404
Practical Industrial Data Communications
Enterprise network
Typical technologies employed at the upper (enterprise) layer include Token Ring and
Ethernet, although for new applications Ethernet would be the preferred choice, for
several reasons:
• It is by far the fastest LAN technology available today.
• If Ethernet is used at the control level, interconnection between the middle and
upper layers can be established through standard off-the-shelf switches and
routers.
• The protocol suite used here would also be TCP. Note that the network at this
level is a generic ‘IT’-type installation and that none of the industrial
prerequisites apply.
Detailed planning
Once a decision has been made on the product to be used, extreme care has to be taken
with the design.
Detailed planning and installation manuals are available for the various systems. For
example, Allen Bradley publishes an 80-page guide on DeviceNet installation planning.
The wiring types, maximum wire lengths and connector types are no cause for concern as
they are strictly defined by the standards.
Installation, commissioning,
troubleshooting
21.1
Introduction
The following section offers some guidelines for getting a system to work. It does assume
that the troubleshooter has the appropriate tools for the job and a thorough understanding
of the technology involved. Troubleshooting, for example, a TCP/IP-based system
requires the use of a suitable protocol analyzer as well as a thorough understanding of the
interrelationships of the various protocols (e.g. IP, ICMP, ARP) and the composition of
the various headers.
21.2
Methodology
21.2.1
Old vs new systems
The approach to troubleshooting will vary slightly depending on whether it is a new
system being commissioned or an old system being troubleshooted. In a new system the
wiring has to be checked to confirm that it has been installed according to specifications.
However, in an operational system it can be assumed that the wiring has been installed
correctly and that any wiring problems could be attributed to a damaged cable.
21.2.2
Systems approach
A good idea is to use a systems approach when commissioning a plant or trying to find a
fault. View a system in terms of its environment, divide the overall system into
subsystems, and then view the subsystems in terms in inputs, outputs, and transfer
functions.
When troubleshooting, check that the following environmental factors do not create
problems:
•
•
•
•
•
•
Water/humidity
Electrical noise
Dirt/dust
Vibration
Temperature
Power (no power, under voltage, over voltage, power fluctuations, voltage
transients).
406
Practical Industrial Data Communications
Then view the system as a collection of subsystems interconnected with the appropriate
media. If individual subsystems are operating correctly and the media has been properly
installed, the subsystems should be able to communicate with each other.
21.2.3
OSI model
Follow the sequence of the OSI layers when troubleshooting instead of approaching the
problem in a haphazard fashion. If two systems cannot communicate with each other at
the application (client/server) level, start by checking the medium (‘layer 0’). If the
medium is in order, proceed by checking the physical layer (layer 1) in the case of
RS-232/485 or the network interfaces (layers 1 and 2) in the case of networks such as
Ethernet. If it function correctly, the problem could be protocol-related. Proceed by
checking out the protocol layers in ascending numerical order, in other words, first
confirm that the routing (layer 3) is operational – if it is present. If there is no problem at
this level, proceed to check the transport layer protocols (layer 4), etc. If all protocol
layers seem to be functioning correctly, the problem must be at the application level.
(Note: not to be confused with the seventh layer of the OSI model!)
It should be obvious that the higher one ventures into the OSI layer, the more complex
the problem and the more difficult the search for it becomes. It is therefore imperative to
start at the bottom. It is estimated that 60–70% of problems can be traced to the medium
and layer 1.
Miscellaneous
Here are some additional tips:
•
•
•
•
•
Remove all non-critical devices from the system under test.
Use onboard diagnostics wherever they are available.
Be systematic.
Good documentation, especially wiring diagrams, is critical.
Baseline statistics, i.e. system measurements such as traffic density (bytes
per second) under normal operating conditions are helpful. In the case of a
suspected problem, the current measurements can be compared against the
baseline measurements and that can point towards the problem.
21.3
Common problems
21.3.1
Media problems
These problems have been dealt with in detail in other chapters, but a summary at this
point in time is in order. The following problems may be encountered:
•
•
•
•
•
Damaged or broken cables
Connector-related problems
Excessive attenuation
Insufficient bandwidth
Reflections/ringing.
Problems related to attenuation, bandwidth, and reflections can usually be eliminated
altogether by choosing the correct type of cable and installing it as per the suppliers’
guidelines using appropriate tools. When using an external installer, it is wise to insist on
a certificate of approval from the cable manufacturer. This may not be obtainable in all
cases but it is normally done with Cat5 cabling for network installations.
Installation, commissioning, troubleshooting
407
Most problems at this level can be detected with a multi-meter and/or a handheld cable
tester. Persistent reflection-related problems might require the use of an oscilloscope or
time domain reflectometer.
Network interface problems
Problems at this level can be categorized as follows:
• Total failure (layer 1): This is very rare but it can happen when a network
interface simply stops working due to an electronic fault. More often, however,
this damage is caused by a high voltage transient, for example, due to lightning
finding their way into the circuitry. Any cabling prone to high voltage
transients should be fitted with transient protection devices to protect the
circuitry.
• Incorrect set-up: In this case, the circuitry does not work but it is fully
operational. It simply has not been set up properly. This is a very common
problem with 10/100 Ethernet cards that have not been set up correctly or
where the auto negotiation process has failed.
• Traffic-related problems (layer 2): If too many devices try to access the
medium or the aggregate traffic levels become too high, problems may occur
(e.g. packets dropped or too many collisions). Nothing in the system has failed
but the situation has arisen due to an oversight on the part of the system
designer. This can only be observed with the help of a protocol analyzer and
similar tools.
Protocol problems
These are software problems related to layers 3–7 of the OSI model as well as ‘layer 8’ as
defined by various vendors. Problems at this level require a thorough understanding of
the protocols used and require the use of a protocol analyzer.
AS-interface (AS-i), 271
data-link layer, 274–6
operating characteristics, 276
physical layer, 272–4
AS-interface troubleshooting tools:
addressing handheld, 277–8
monitor, 278
service book, 278
service device, 278
slave simulator, 278
Allen Bradley Data Highway (plus) protocol:
Data Highway Plus protocol, 347–8
Data Highway protocol, 347
DH-0–0
full-duplex data-link layer, 348
format of a message, 349
message limitations, 349
software logic for receivers, 350–1
physical (hardware) layer, 348
Application layer message functions, 181
application function codes, 182
internal indicators, 187–8
object header, 188–9
qualifier and range fields, 190–4
request function codes, 182–6
response functions, 186
Bytes, explanation of, 352
ADDR, 352
basic command set, 353
data, 353
diagnostic counter for each module, 354
local STS error code, 354
PLC-5 command set message packet fields, 353
remote STS error codes, 354
size, 352
STS and ETX SYS – status and extended status
bytes, 352
synchronous link status code, 354
TNS – transaction bytes, 352
Cable characteristics:
factors affecting performance, 27
attenuation, 28
characteristic impedance, 28
crosstalk, 28–9
packaging, 27
selection, 29
factors, 31
function and location, 30
structure, 26
cable sheath, 27
conductor, 26
insulation, 26–7
Cable spacing, 21–2
filtering, 24
grounding requirements, 22
suppression techniques, 22–4
CCITT V.24 interface standard see RS-232
interface standard
Coaxial cables:
construction, 31
braid shield, 32
carrier wire, 31
foil shield, 31
insulation, 31
sheath, 32
Index
performance, 32–3
advantages, 33
disadvantages, 33
faults, 33
Connector standards, 41–2
Current loop and RS-485 converters:
configuration jumpers, 110–11
current loop (20 mA), 107
digital ground/shield, 111
DTE/DCE settings, 111
failsafe termination, 111
network transmit, 110
serial interface converters, 108–9
troubleshooting converters, 110
Data Highway Plus network diagnostics, 356–7
Data Highway Plus wiring troubleshooting, 355
cable preparation, 356
solder wires to connector, 356
station connector wiring, 355
trunk line preparation, 356
wiring recommendations, 355
DeviceNet, 279–80
application layer, 295
cable budgets, 283
cable description:
flat cable, 288
thick cable, 287
thin cable specification, 287
connectors, 281
hard-wired (unsealed) connection, 282
micro (sealed) connector, 282–3
mini (sealed) connector, 282
pluggable (unsealed) connector, 281
data-link layer:
fragmentation, 294
frame format, 292–3
medium access, 293–4
device taps:
IDC taps, 284
multi-port open taps, 286
open style taps, 285–6
power taps, 286
sealed taps, 284
network power:
errors and power supply options, 291
general approach, 288–9
single supply – center connected, 289–90
single supply – end connected, 289
physical layer:
topology, 280–1
signaling, 292
system grounding, 291–2
409
DeviceNet troubleshooting, 296
faultfinding procedures:
common mode voltage problems, 300–1
incorrect cable lengths, 298–9
incorrect signal voltage levels, 300
incorrect termination, 301
node communication problems, 301–2
noise, 301
power and earthing problems, 299–300
tools:
CAN bus monitor, 298
controller software, 298
handheld analyzers, 297
intelligent wiring components, 297
multimeter, 296
oscilloscope, 297
DNP3 message structure:
application layer, 154
control field, 173–5
description, 171–2
message formats, 173
message sequences, 172–3
message transaction flow diagrams, 175–81
build-up, 153–4
data-link layer, 154
CRC error codes, 168–9
description, 157
how it works, 160–54
services provided, 157–8
summary, 169
terminology, 158–9
transmission procedures, 164–8
physical layer, 154
description of, 155
procedures, 156–7
services provided by, 155
topologies, 156
pseudo-transport layer, 154
summary, 171
transport header, 170
transport protocol data unit, 170
summary, 155
Earthing/grounding, 43–5
Enhance performance architecture (EPA), 150–1
Ethernet, 231–2
10 Mbps:
design rules, 242–4
frame reception, 238
frame transmission, 238
IEEE802.2 LLC, 241
MAC frame format, 239–40
media systems, 232–7
410
Index
Ethernet, (Continued)
medium access control, 237–8
reducing collisions, 241–2
signaling methods, 237
100 Mbps, 244–5
auto-negotiation, 246–7
media access: full duplex, 245–6
troubleshooting, 270
Gigabit, 247–8
design considerations, 248–50
full-duplex repeaters, 248
troubleshooting, 270
industrial, 250
active electronics, 253
connectors and cabling, 250–1
deterministic vs stochastic operation, 251
fast and gigabit, 253–4
industrial Ethernet architecture for high
availability, 254
noise and interference, 251–2
partitioning of the network, 252
size and overhead of Ethernet frame, 251
switching technology, 252–3
TCP/IP and industrial systems, 254
see also Industrial Ethernet
troubleshooting
Fiber optic:
cable components, 51
cladding, 52
fiber core, 51
fiber-optic buffer, 52
sheath, 52
strength members, 52
cable parameters:
attenuation, 53
bandwidth, 53
diameter, 53
dispersion, 53
wavelength, 53
cable types:
aerial, 55
indoor, 56
sub-aqueous, 56
underground, 55–6
cleaning optical connectors:
connecting fibers:
connection losses, 57
connector handling, 59
connectors, 58
optical couplers, 59
splicing fibers, 57–8
installation rules:
cable bending radius, 64
cable tension, 64–5
locating broken fibers:
continuity testing, 65–6
insertion loss testing, 66–7
optical time domain reflectometer, 67–70
splicing trays/organizers and termination
cabinets:
splicing enclosures, 60–1
splicing trays, 59–60
termination in patch panels and distribution
frames, 61–2
troubleshooting, 62–3
standard approach, 63
tools required, 63
types of, 54–5
Foundation fieldbus troubleshooting, 231
communication problems, 333–4
foundation fieldbus test equipment, 335
power problems, 332–3
Foundation fieldbus, 322–3
application layer, 326–7
data-link layer, 325–6
error detection and diagnostics, 328
good wiring and installation practice:
installation of the complete system, 329–30
termination preparation, 329
high speed Ethernet (HSE), 328
physical layer and wiring rules, 323–5
user layer, 327–8
Frequency division multiple access (FDMA), 368
HART and smart instrumentation, 358
HART protocol, 358–9
application layer, 362
common practice commands, 363
hardware recommendations, 364
instrument specific commands, 363
summary of benefits, 364
universal commands, 362–3
data-link layer, 362
physical layer, 359–60
digital multi-drop communications, 360
HART handheld communicator, 361
HART troubleshooting, 365
IEC, 60870-5 standard:
application layer, 217
ASDU structure, 217–26
overall message structure, 217
data-link layer, 203
balanced transmission procedures, 213–15
data-link security, 216
Index
frame format, 203–5
link layer concepts, 206–9
link vs application data, 216
order of information, 205
unbalanced transmission procedures, 209–13
information elements, 228–9
quality bits, 230
message addressing and routing, 226–8
overall structure, 195
description of contents, 197–9
development of standards, 196
obtaining standards, 196–7
physical layer, 201
communications interface, 201–2
network configurations, 202–3
protocol architecture:
EPA and OSI reference models,
199–200
selections from standards, 200
Industrial Ethernet troubleshooting:
common problems and faults, 255
problems and solutions:
AUI problems, 264
fiber-optic problems, 263
host-related problems, 268
hub-related problems, 269
NIC problems, 264–8
noise, 257–8
thick coax problems, 260–3
thin coax problems, 258–9
UTP problems, 260
tools of the trade:
fiber-optic cable testers, 256
handheld cable testers, 256
handheld frame analysis, 257
hardware based protocol analyzers, 257
multimeters, 255
RMON probes, 256
software protocol analyzers, 257
traffic generators, 256
Ingress protection, 24–5
Internet layer protocols (packet transport):
address resolution protocol (ARP),
117–18
address resolution cache, 118
datagram, 118–20
host-to-host layer: end to end reliability:
TCP header, 124–6
ICMP, 120
datagrams, 120–1
IP version, 4 (IPv4), 114
IPv4 addressing, 116–17
IPv4 header, 114–16
411
routing:
autonomous systems, 123–4
direct vs indirect delivery, 122
static vs dynamic, 122–3
UDP, 126
header, 126
Media, 11–12
Message transaction flow diagrams:
collision with unsolicited response, 178–9
confirmation timeouts, 176–7
effects of network delays, 177–8
multi-fragment response, 176
rules for unsolicited message transactions,
179–81
typical message transaction, 175
Modbus:
function codes, 135
force multiple coils/digital outputs, 140
force multiple registers, 140–1
force single coil, 138
loopback test, 140
preset single register, 139
read coil or digital output status, 136
read digital input status, 136
read exception status, 139
read holding registers, 136–7
reading input registers, 137–8
overview, 132–4
protocol structure, 134–5
Modbus plus protocol, 336–9
Modbus plus protocol troubleshooting:
common problems and faults, 340
description of tools used, 341
detailed troubleshooting:
card SA85:0 not found, bad
NCB_LANA_NUM – device not foud,
344–5
remote PC not listening for call, 341–2
slave xx reported an illegal data address, 342
slave xx reported an illegal data value, 342–3
slave xx reported an illegal function, 343
troubleshooting the SA85 adapter card under
Windows NT, 345–6
Modbus active LED, 340
Modbus troubleshooting:
common problems and faults, 142
description of tools used, 142
faulty communication interfaces:
RS-232 driver failed, 146
RS-232 receiver failed, 146–7
RS-485 driver failed, 147
RS-485 receiver failed, 147
412
Index
Modbus troubleshooting: (Continued)
mis-wired communication cabling:
grounding, 146
RS-232 wiring for three-wire point-to-point
mode, 143–4
RS-485 wiring for two-wire, multi-drop mode
only, 144–6
RS-485/RS-422 wiring for four-wire, repeat
or multi-drop modes, 144
software-related problems, 147
Model layers, functions of:
application layer, 153
data-link layer, 152
physical layer, 152
pseudo-transport layer, 152–3
Networked systems, 401–2
control level, 403
detailed planning, 404
device level, 402–3
enterprise network, 404
Noise:
cable ducting/raceways, 21
electrical coupling of, 15
electrostatic/capacitive coupling, 16–18
good shielding performance ratios, 20–1
impedance coupling, 15
magnetic/inductive coupling, 18–19
radio frequency radiation, 19–20
shielding, 20
sources of electrical, 15
OSI, 7 layer model, 149–50
OSI layer services, 5–6
application layer, 6
data link layer, 8
network layer, 7
physical layer, 8
presentation layer, 6–7
session layer, 7
transport, 7
OSI reference model, 2–5
Physical connections, 12
Profibus, 303–4
communication model, 311
communication objects, 312–13
performance, 313
protocol stack, 305
application layer, 308
data-link layer, 306–8
fieldbus management layer (FMA), 311
fieldbus message specification (FMS), 308–10
lower layer interface (LLI), 310
physical layer, 305–6
relationship between application process and
communication, 311–12
system operation:
configuration, 313–14
data transfer between DPM1 and the
DP-slaves, 315
mixed operation of FMS and DP stations, 316
safety and protection of stations, 315–16
synchronization and freeze modes, 315
Profibus troubleshooting:
tips, 320–1
tools:
built-in diagnostics, 318–19
bus monitors, 319–20
configuration utilities, 318
D-type connectors with built-in terminators,
317–18
handheld testing device, 317–18
Protocols, 12–15
Radio and wireless communications, 378
components of radio link, 379
antenna support structure, 379
antenna, 379
cabling, 380
interface equipment, 380
receiver, 379
transmitter, 379
fade margin, 394
implementing a radio link, 388
inter-modulation, 386–8
miscellaneous considerations, 395
path profile, 388–90
radio modems, 381–4
features of, 385–6
modes of, 384
spread spectrum, 386
radio spectrum and frequency allocation, 380–1
RF path loss calculations, 390–1
signal to noise ratio and SINAD, 392–4
summary of characteristics of VHF/UHF, 381
transmitter power/receiver sensitivity, 392
RS-232 interface standard, 71–2
EIA/TIA-232 revisions, 80
half-duplex operation of the, 78–80
limitations, 81
major elements:
electrical signal characteristics, 72–5
functional description of interchange circuits, 77
mechanical characteristics of the interface,
75–6
Index
RS-232 troubleshooting:
summary, 90–1
test equipment, 83–4
breakout box, 84
loop back plug, 85
null modem, 84–5
protocol analyzer, 85
typical approach, 82–3
typical problems, 86–7
mechanical, 87–9
noise, 89–90
setup, 89
RS-485 troubleshooting:
installation, 98–9
interface standard, 92–6
noise problems, 99
common mode noise, 100
induced noise, 100
reflections and ringing, 101
shielding, 103
unbalancing the line, 101–2
summary, 106
test equipment:
multimeter, 103–4
oscilloscope, 104–5
protocol analyzer, 105–6
vs RS-0–0, 98
Satellite access techniques, 368
Satellite electronics, 366–7
Satellite frequency bands, 367–8
Satellites systems:
applications of satellite systems,
371–2
satellite down-link design procedure,
370–1
satellite-operating principles, 366–8
satellite system design concepts,
369–70
State transition structure, 10
conceptual phase, 11
detailed design:
implementation, 11
maintenane/troubleshooting, 11
system life cycle, 10
validation phase, 11
System design methodology:
environment, 396–7
poinit-to-point links:
fiber, 400
functional requirements, 397–8
RS-0–03, 398
RS-422/4486, 398–400
413
System installation, commissioning and
troubleshooting:
common problems:
media problems, 406–7
network interface problems, 407
protocol problems, 407
methodology:
old vs new systems, 405
OSI model, 406
systems approach, 405–6
Systems engineering approach:
system specifications, 8–9
functions, 10
inputs, 9
outputs, 9–10
TCP/IP:
host-to-host layer, 114
Internet layer, 113
process/application layer, 114
see also Internet layer protocols
(packet transport)
TCP/IP troubleshooting:
common problems, 127
network problems:
duplicate IP addresses, 129
host is unable to obtain an automatically
assigned IP address, 128
incorrect network ID, 129
incorrect/absent default gateway(s), 129
IP address of a device not known to user, 130
MAC address of a device not known to
user, 130
remote host, 128
reserved IP addresses, 128
TCP/IP protocol stack not properly installed
on local host, 128
wrong IP address, 130
tools of the trade:
hardware-assisted protocol analyzers, 127
software protocol analyzers, 127
TCP/IP utilities, 127
third party utilities, 127
transport layer problems:
incorrect port number, 130–1
no connection established, 130
Termination, 45–6
Time division multiple access (TDMA), 368
Transient protection, 47–9
Troubleshooting:
fast Ethernet, 270
Gigabit Ethernet, 270
switched networks, 269–70
414
Index
Twisted-pair cable, 34–5
advantages of, 38
components of:
conductor wires, 35
sheath, 35–6
shield, 35
disadvantages of, 38
distribution/installation standards,
39–41
EIA/TIA, 568 cable categories, 36–7
problems with unshielded twisted pairs:
attenuation, 38
connector problems, 39
crossed pairs, 38–9
crosstalk, 39
split pair problems, 39
termination problems, 39
VSAT antenna diameter, 368
Wireless LANs (WLANs):
accessories:
antennas, 376
power, 376
cell coverage, 374
multiple cell WLAN, 375
roaming, 375
single cell WLAN, 375
components:
types of bridges, 374
wireless network bridges (access points), 374
wireless NIC, 373–4
devices, 376
IEEE802.11 specifications, 376–7
topologies:
hierarchical, 373
peer-to-peer, 373
star, 372–3
vs WPANs, 377–8
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement