Modbus Messaging Implementation Guide v1

Modbus Messaging Implementation Guide v1
MODBUS MESSAGING ON TCP/IP
IMPLEMENTATION GUIDE
Rev 1.0 8May02
A Community for MODBUS Users by MODBUS Users
MODBUS MESSAGING IMPLEMENTATION GUIDE 1.0
8-May-02
Key words :
(5-10 words)
Summary :
(3-5 lines)
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Table of Contents
1.
INTRODUCTION................................ ................................ ........................ 6
1.1
OBJECTIVES................................ ................................ ................................ .6
1.2
CLIENT / SERVER MODEL ................................ ................................ ........... 6
1.3
REFERENCE DOCUMENTS................................ ................................ .......... 8
2.
ABBREVIATIONS ................................ ................................ ..................... 8
3.
CONTEXT................................ ................................ ................................ ..8
3.1
PROTOCOL DESCRIPTION................................ ................................ .......... 8
3.1.1 General communication architecture ................................ ................................ ................ 8
3.1.2 MODBUS On TCP/IP Application Data Unit ................................ ................................ ......9
3.1.3 MBAP Header description................................ ................................ ................................ 10
3.2
4.
MODBUS FUNCTIONS CODES DESCRIPTION................................ ......... 11
FUNCTIONAL DESCRIPTION................................ ................................ 12
4.1
MODBUS COMPONENT ARCHITECTURE MODEL................................ ... 12
4.2
TCP CONNECTION MANAGEMENT................................ ........................... 15
4.2.1 Connections management Module ................................ ................................ ................. 15
4.2.1.1 General description .......................................................................................................15
4.2.1.2 Connection management description ............................................................................17
4.2.2 Impact of Operating Modes on the TCP Connection ................................ ..................... 18
4.2.2.1 Communication break between two operational end points:...........................................18
4.2.2.2 Crash and Reboot of the Server end point.....................................................................18
4.2.2.3 Crash and Reboot of the Client .....................................................................................18
4.2.3 Access Control Module................................ ................................ ................................ ....19
4.3
USE of TCP\IP STACK ................................ ................................ ................ 19
4.3.1 Use of BSD Socket interface................................ ................................ ............................ 20
4.3.2 TCP layer parameterization ................................ ................................ ............................ 22
4.3.3 IP layer parameterization ................................ ................................ ................................ .23
4.3.3.1 IP Parameters ...............................................................................................................23
4.4
COMMUNICATION APPLICATION LAYER................................ ................. 24
4.4.1 MODBUS Client ................................ ................................ ................................ ................ 24
4.4.1.1 MODBUS client design .................................................................................................24
4.4.1.2 Build a MODBUS Request ............................................................................................25
4.4.1.3 Process MODBUS Confirmation ...................................................................................27
4.4.1.4 Time-out managing .......................................................................................................29
4.4.2 MODBUS Server ................................ ................................ ................................ ............... 30
4.4.2.1 MODBUS Server Design...............................................................................................30
4.4.2.2 MODBUS PDU Checking ..............................................................................................32
4.4.2.3 MODBUS service processing ........................................................................................33
4.4.2.4 User Application Interface (Backend Interface)..............................................................34
4.4.2.5 MODBUS Response building ........................................................................................34
5.
IMPLEMENTATION GUIDELINE ................................ ............................ 35
5.1
OBJECT MODEL DIAGRAM ................................ ................................ ....... 36
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5.1.1
5.1.2
5.1.3
5.1.4
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TCP management package ................................ ................................ .............................. 37
Configuration layer package ................................ ................................ ........................... 38
Communication layer package ................................ ................................ ........................ 39
Interface classes................................ ................................ ................................ ............... 40
5.2
IMPLEMENTATION CLASS DIAGRAM ................................ ....................... 40
5.3
SEQUENCE DIAGRAMS................................ ................................ .............. 42
5.4
CLASSES AND METHODS DESCRIPTION................................ ................ 45
5.4.1 MODBUS server class ................................ ................................ ................................ ...... 45
Class CMODBUSServer ................................ ................................ ................................ ............. 45
5.4.2 MODBUS Client Class ................................ ................................ ................................ ...... 46
Class CMODBUSClient ................................ ................................ ................................ .............. 46
5.4.3 Interface Classes ................................ ................................ ................................ .............. 47
5.4.3.1 Interface Indication class...............................................................................................47
Class CInterfaceIndicationMsg ................................ ................................ ................................ .47
5.4.3.2 Interface Response Class..............................................................................................47
Class CInterfaceResponseMsg ................................ ................................ ................................ .47
5.4.4 Connexion Management class ................................ ................................ ........................ 48
Class CConnexionMngt................................ ................................ ................................ ............. 48
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List of Figures
Figure 1: MODBUS TCP/IP communication architecture ................................ ............... 9
Figure 2: General MODBUS frame ................................ ................................ .................... 9
Figure 3: MODBUS request/response over TCP/IP................................ ......................... 9
Figure 4: MODBUS Messaging Service Conceptual Architecture ............................... 12
Figure 5 MODBUS Data Model with separate blocks................................ .................... 13
Figure 6 MODBUS Data Model with only 1 block ................................ .......................... 13
Figure 7: TCP connection management activity diagram ................................ ............ 16
Figure 8: MODBUS TCP connection establishment ................................ ..................... 17
Figure 9: MODBUS Exchanges................................ ................................ ....................... 21
Figure 10: MODBUS Client unit ................................ ................................ ...................... 24
Figure 11: MODBUS Client Activity Diagram ................................ ................................ .25
Figure 12: Request building activity diagram ................................ ............................... 26
Figure 13: Process MODBUS Confirmation activity diagram ................................ ...... 29
Figure 14: MODBUS Server unit ................................ ................................ ..................... 30
Figure 15: Process MODBUS Indication activity diagram ................................ ............ 31
Figure 16: MODBUS PDU Checking activity diagram ................................ ................... 32
Figure 17: MODBUS service processing activity diagram ................................ ........... 33
Figure 18: MODBUS Messaging Service Object Model Diagram ................................ .36
Figure 19: MODBUS TCP management package ................................ .......................... 37
Figure 20: MODBUS Configuration layer package................................ ........................ 38
Figure 21: MODBUS Communication Application layer package................................ 39
Figure 22: Class Diagram................................ ................................ ................................ 41
Figure 23: MODBUS client sequence diagram ................................ .............................. 42
Figure 24: MODBUS server Diagram ................................ ................................ .............. 44
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1. INTRODUCTION
1.1 OBJECTIVES
The objective of this document is to present the MODBUS messaging service over TCP/IP , in order
to provide reference information that helps software developers to implement this service. The
encoding of all MODBUS function codes are not described in this document, for this information
please read the MODBUS application protocol specification [2].
This document gives accurate and comprehensive description of a MODBUS messaging service
implementation. Its purpose is to facilitate the interoperability between the devices using the
MODBUS messaging service.
This document comprises mainly three parts:
• An overview of the MODBUS over TCP/IP protocol
• A functional description of a MODBUS client, server and gateway implementation
• An implementation guideline that proposes the object model of an MODBUS
implementation example.
1.2 CLIENT / SERVER MODEL
The MODBUS messaging service provides a Client/Server communication between devices
connected on an Ethernet TCP/IP network.
This client / server model is based on four type of messages:
•
•
•
•
MODBUS Request,
MODBUS Confirmation,
MODBUS Indication,
MODBUS Response
MODBUS Client
Request
Indication
Confirmation
Response
MODBUS Server
A MODBUS Request is the message sent on the network by the Client to initiate a transaction,
A MODBUS Indication is the Request message received on the Server side,
A MODBUS Response is the Response message sent by the Server,
A MODBUS Confirmation is the Response Message received on the Client side
The MODBUS messaging services (Client / Server Model) are used for real time information
exchange:
• between two device applications,
• between device application and other device,
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• between HMI/SCADA applications and devices,
• between a PC and a device program providing on line services.
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1.3 REFERENCE DOCUMENTS
This chapter gives a list of documents that are interesting to read before this one:
[2]
[4]
Modbus Application Protocol Specification
RFC 1122
2. ABBREVIATIONS
ADU
IETF
IP
MAC
MB
MBAP
PDU
PLC
TCP
BSD
MSL
Application Data Unit
Internet Engineering Task Force
Internet Protocol
Medium Access Control
MODBUS
MODBUS Application Protocol
Protocol Data Unit
Programmable Logic Controller
Transport Control Protocol
Berkeley Software Distribution
Maximum Segment Lifetime
3. CONTEXT
3.1 PROTOCOL DESCRIPTION
3.1.1
General communication architecture
A communicating system over MODBUS TCP/IP may include different types of device:
• A MODBUS TCP/IP Client and Server devices connected to a TCP/IP network
• The Interconnection devices like bridge, router or gateway for interconnection between the
TCP/IP network and a serial line sub-network which permit connections of MODBUS Serial
line Client and Server end devices.
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Modbus
Client
Serial Line
Modbus
Client
TCP/IP
Modbus
Client
TCP/IP
Client TCP/IP
gateway
Modbus TCP /
IP
Modbus
Server
TCP/IP
Modbus
Server
Serial Line
Modbus
Server
TCP/IP
Modbus
Server
Serial Line
Server TCP/IP
gateway
Modbus Serial line
Figure 1: MODBUS TCP/IP communication architecture
The MODBUS protocol defines a simple Protocol Data Unit (PDU) independent of the underlying
communication layers. The mapping of MODBUS protocol on specific buses or networks can
introduce some additional fields on the Application Data Unit (ADU).
ADU
Additional address
Function code
Data
Error check
PDU
Figure 2: General MODBUS frame
The client that initiates a MODBUS transaction builds the MODBUS Application Data Unit. The
function code indicates to the server which kind of action to perform.
3.1.2
MODBUS On TCP/IP Application Data Unit
This section describes the encapsulation of a MODBUS request or response when it is carried on a
MODBUS TCP/IP network.
MODBUS TCP/IP ADU
MBAP Header
Function code
Data
PDU
Figure 3: MODBUS request/response over TCP/IP
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A dedicated header is used on TCP/IP to identify the MODBUS Application Data Unit. It is called the
MBAP header (MODBUS Application Protocol header).
This header provides some differences compared to the MODBUS RTU application data unit used on
serial line:
§ The MODBUS ‘slave address’field usually used on MODBUS Serial Line is replaced by
a single byte ‘Unit Identifier’ within the MBAP Header. The ‘Unit Identifier’ is used to
communicate via devices such as bridges, routers and gateways that use a single IP
address to support multiple independent MODBUS end units.
§ All MODBUS requests and responses are designed in such a way that the recipient can
verify that a message is finished. For function codes where the MODBUS PDU has a
fixed length, the function code alone is sufficient. For function codes carrying a variable
amount of data in the request or response, the data field includes a byte count.
§ When MODBUS is carried over TCP, additional length information is carried in the
MBAP header to allow the recipient to recognize message boundaries even if the
message has been split into multiple packets for transmission. The existence of explicit
and implicit length rules, and use of a CRC-32 error check code (on Ethernet) results in
an infinitesimal chance of undetected corruption to a request or response message.
3.1.3
MBAP Header description
The MBAP Header contains the following fields:
Fields
Length
Description -
Client
Server
Transaction Identifier
2 Bytes
Identification of a
MODBUS Request /
Response transaction.
Initialized by the
client
Recopied by the
server from the
received request
Protocol Identifier
2 Bytes
0 = MODBUS protocol
Initialized by the
client
Recopied by the
server from the
received request
Length
2 Bytes
Number of following
bytes
Initialized by the
client ( request)
Initialized by the
server (
Response)
Unit Identifier
1 Byte
Identification of a remote
slave connected on a
serial line or on other
buses.
Initialized by the
client
Recopied by the
server from the
received request
The header is 7 bytes long:
• Transaction Identifier - It is used for transaction pairing, the MODBUS server copies in the
response the transaction identifier of the request.
• Protocol Identifier – It is used for intra-system multiplexing. The MODBUS protocol is identified
by the value 0.
• Length - The length field is a byte count of the following fields, including the Unit Identifier and
data fields.
• Unit Identifier – This field is used for intra-system routing purpose. It is typically used to
communicate to a MODBUS or a MODBUS+ serial line slave through a gateway between an
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Ethernet TCP-IP network and a MODBUS serial line. This field is set by the MODBUS Client in the
request and must be returned with the same value in the response by the server.
All Modbus/TCP ADU are sent via TCP on registered port 502.
Remark : the different fields are encoded in Big-endian.
3.2 MODBUS FUNCTIONS CODES DESCRIPTION
Standard function codes used on MODBUS application layer protocol are described in details in the
MODBUS Application Protocol Specification [2].
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4. FUNCTIONAL DESCRIPTION
The MODBUS Component Architecture presented here is a general model including both MODBUS
Client and Server Components and usable on any device.
Some devices may only provide the server or the client component.
In the first part of this chapter a brief overview of the MODBUS messaging service component
architecture is given, followed by a description of each component presented in the architectural
model.
4.1 MODBUS COMPONENT ARCHITECTURE MODEL
USER
APPLICATION
Ressource Management
&
Flow Control
Communication
Application
Layer
Modbus Client
Interface
Modbus Client
Modbus Backend
Interface
Modbus Server
TCP
Management
Stack
parmeterization
Connection
Management
Access Ctl
TCP/IP Stack
Figure 4: MODBUS Messaging Service Conceptual Architecture
• Communication Application Layer
A MODBUS device may provide a client and/or a server MODBUS interface.
A MODBUS backend interface can be provided allowing indirectly the access to user application
objects.
Four areas can compose this interface: input discrete, output discrete (coils), input registers and
output registers. A mapping between this interface and the user application data has to be done (local
issue).
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Primary tables
Object type
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Discretes Input
Single bit
Type of
access
Read-Only
Coils
Single bit
Read-Write
Input Registers
16-bit word
Read-Only
Holding Registers
16-bit word
Read-Write
Comments
This type of data can be provided by an I/O system.
This type of data can be alterable by an application program.
This type of data can be provided by an I/O system
This type of data can be alterable by an application program.
Device application memory
Device application memory
MODBUS access
MODBUS access
Input Discrete
Input Discrete
R
Coils
W
MODBUS Request
R
Input Registers
W
Output Registers
MODBUS Request
Input Registers
Output Registers
MODBUS SERVER DEVICE
MODBUS SERVER DEVICE
Figure 5
Coils
MODBUS Data Model with
separate blocks
Figure 6
MODBUS Data Model with only 1
block
Ø MODBUS Client
The MODBUS Client allows the user application to explicitly control information exchange with a
remote device. The MODBUS Client builds a MODBUS request from parameter contained in a
demand sent by the user application to the MODBUS Client Interface.
The MODBUS Client uses a MODBUS transaction whose management includes waiting for and
processing of a MODBUS confirmation.
Ø MODBUS Client Interface
The MODBUS Client Interface provides an interface enabling the user application to build the
requests for various MODBUS services including access to MODBUS application objects. The
MODBUS Client interface (API) is not specified in this specification, although an example is described
in the implementation model.
Ø MODBUS Server
On reception of a MODBUS request this module actives a local action to read, to write or to achieve
some other actions. The processing of these actions is done totally transparently for the application
programmer. The main MODBUS server functions are to wait for a MODBUS request on 502 TCP
port, to treat this request and then to build a MODBUS response depending on device context.
Ø MODBUS Backend Interface
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The MODBUS Backend Interface is an interface from the MODBUS Server to the user application in
which the application objects are defined.
• TCP Management layer
One of the main functions of the messaging service is to manage communication establishment and
ending and to manage the data flow on established TCP connections.
Ø Connection Management
A communication between a client and server MODBUS Module requires the use of a TCP
connection management module. It is in charge to manage globally messaging TCP connections.
Two possibilities are proposed for the connection management. Either the user application itself
manages TCP connections or the connection management is totally done by this module and
therefore it is transparent for the user application. The last solution implies less flexibility.
The listening TCP port 502 is reserved for MODBUS communications. It is mandatory to listen by
default on that port. However, some markets or applications might require that another port is
dedicated to MODBUS over TCP. This is the case when interoperability is required with non =S=
products, such as in Building Control. For that reason, it is highly recommended that the clients and
the servers give the possibility to the user to parameterize the MODBUS over TCP port number. It is
important to note that even if another TCP server port is configured for MODBUS service in
certain applications, TCP server port 502 must still be available in addition to any application
specific ports.
Ø Access Control Module
In certain critical contexts, accessibility to internal data of devices must be forbidden for undesirable
hosts. That’s why a security mode is needed and security process may be implemented if required.
• TCP\IP Stack layer
The TCP/IP stack can be parameterized in order to adapt the data flow control, the address
management and the connection management to different constraints specific to a product or to a
system. Generally the BSD socket interface is used to manage the TCP connections.
§ Resource management and Data flow control
In order to equilibrate inbound and outbound messaging data flow between the MODBUS client and
the server, data flow control mechanism is provided in all layers of MODBUS messaging stack.
The resource management and flow control module is first based on TCP internal flow control added
with some data flow control in the data link layer and also in the user application level.
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4.2 TCP CONNECTION MANAGEMENT
4.2.1
Connections management Module
4.2.1.1 General description
A MODBUS communication requires the establishment of a TCP connection between a Client and a
Server.
The establishment of the connection can be activated either explicitly by the User Application module
or automatically by the TCP connection management module.
In the first case an application-programming interface has to be provided in the user application
module to manage completely the connection. This solution provides flexibility for the application
programmer but it requires a good expertise on TCP/IP mechanism.
In the second case the TCP connection management is completely hidden to the user application that
only sends and receives MODBUS messages. The TCP connection management module is in charge
to establish a new TCP connection when it is required.
The definition of the number of TCP client and server connections is not on the scope of this
document (value n in this document). Depending on the device capacities the number of TCP
connections can be different.
Implementation Rules :
1) Without explicit user requirement, it is recommended to implement the automatic TCP connection
management
2) It is recommended to keep the TCP connection opened with a remote device and not to open and close
it for each MODBUS/TCP transaction,
Remark: However the MODBUS client must be capable of accepting a close request from the server
and closing the connection. The connection can be reopened when required.
3) It is recommended for a MODBUS Client to open a minimum of TCP connections with a remote
MODBUS server (with the same IP address). One connection per application could be a good choice.
4) Several MODBUS transactions can be activated simultaneously on the same TCP Connection.
Remark: If this is done then the MODBUS transaction identifier must be used to uniquely identify the
matching requests and responses.
5) In case of a bi-directional communication between two remote MODBUS entities ( each of them is client
and server), it is necessary to open separate connections for the client data flow and for the server data
flow.
6) A TCP frame must transport only one MODBUS ADU. It is advised against sending multiple MODBUS
requests or responses on the same TCP PDU
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Iddle
Wait
Client
[Request to a remote device________
____]
[______Event on a socket ]
[else]
[<n connections]
Server
[data]
[connection established]
[connection request]
[no access ctl]
[n connections]
[access ctl]
[IP forbidden]
[IP authorized]
Oldest_unused_
connection_closed
Connection_refused
Connection_accepted
[n connections]
[< n connections]
Connection_establishment
[Connection NOK]
[Connection OK]
Oldest_unused_
no_prioritary_
connection_closed
Active_Connection
Connection_established
network_transmission
Request_treatment
Figure 7: TCP connection management activity diagram
1.
Explicit TCP connection management
The user application module is in charge of managing all the TCP connections: active and passive
establishment, connection ending… . This management is done for all MODBUS communication
between a client and a server. The BSD Socket interface is used in the user application module to
manage the TCP connection. This solution offers a total flexibility but it implies that the application
programmer has sufficient TCP knowledge.
A limit of number of client and server connections has to be configured taking into account the device
capabilities and requirement.
2. Automatic TCP connection management
The TCP connection management is totally transparent for the user application module. The
connection management module may accept a sufficient number of client and server connections.
Nevertheless a mechanism must be implemented in case of exceeding the number of authorized
connection. In such a case we recommend to close the oldest unused connection.
A connection with a remote partner is established at the first packet received from a remote client or
from the local user application. This connection will be closed if a termination arrived from the
network or decided locally on the device. On reception of a connection request, the access control
option can be used to forbid device accessibility to unauthorized clients.
The TCP connection management module uses the Stack interface (usually BSD Socket interface) to
communicate with the TCP/IP stack.
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In order to maintain compatibility between system requirements and server resources, the TCP
management will maintain 2 pools of connection.
§ The first pool ( priority connection pool) is made of connections that are never closed on a
local initiative. A configuration must be provided to set this pool up. The principle to be
implemented is to associate a specific IP address with each possible connection of this pool. The
devices with such IP addresses are said to be “marked”. Any new connection that is requested by
a marked device must be accepted, and will be taken from the priority connection pool. It is also
necessary to configure the maximum number of Connections allowed for each remote device to
avoid that the same device uses all the connections of the priority pool.
§ The second pool (non-priority connection pool) contains connections for non marked devices.
The rule that takes over here is to close the oldest connection when a new connection request
arrives from a non-marked device and when there is no more connection available in the pool.
A configuration might be optionally provided to assign the number of connections available in each
pool. However (It is not mandatory) the designers can set the number of connections at design time if
required.
4.2.1.2 Connection management description
• Connection establishment :
The MODBUS messaging service must provide a listening socket on Port 502, which permits to
accept new connection and to exchange data with other devices.
When the messaging service needs to exchange data with a remote server, it must open a new client
connection with a remote Port 502 in order to exchange data with this distant. The local port must be
higher than 1024 and different for each client connection.
Device@
IP1
Client
Ports
Device@
n
(n>1024)
Server
Port
Connection (@ IP1 n,
@IP2 502)
n
(n>1024)
502
502
IP2
Client
Ports
Server
Port
Figure 8: MODBUS TCP connection establishment
If the number of client and server connections is greater than the number of authorized connections
the oldest unused connection is closed. The access control mechanism can be activated to check if
the IP address of the remote client is authorized. If not the new connection is refused.
• MODBUS data transfer
A MODBUS request has to be sent on the right TCP connection already opened. The IP address of
the remote is used to find the TCP connection. In case of multiple TCP connections opened with the
same remote, one connection has to be chosen to send the MODBUS message, different choice
criteria can be used like the oldest one, the first one. The connection has to maintain open during all
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the MODBUS communications. As described in the following chapters a client can initiate several
MODBUS transactions with a server without waiting the ending of the previous one.
§ Connection closing
When the MODBUS communications are ended between a Client and a Server, the client has to
initiate a connection closing of the connection used for these communications.
4.2.2
Impact of Operating Modes on the TCP Connection
Some Operating Modes (communication break between two operational End Points, Crash and
Reboot on one of the End Point, … ) may have impacts on the TCP Connections. A connection can be
seen closed or aborted on one side without the knowledge of the other side. The connection is said to
be "half-open".
This chapter describes the behavior for each main Operating Modes. It is assumed that the Keep
Alive TCP mechanism is used on both end points (See Chapter 4.3.2)
4.2.2.1 Communication break between two operational end points:
The origin of the communication break can be the disconnection of the Ethernet cable on the Server
side. The expected behavior is:
• If no packet is currently sent on the connection:
The communication break will not be seen if it lasts less than the Keep Alive timer value. If the
communication break lasts more than the Keep Alive timer value, an error is returned to the TCP
Management layer that can reset the connection.
• If Some packets are sent before and after the disconnection:
The TCP retransmission algorithms (Jacobson's, Karn's algorithms and exponential backoff See
Chapter 4.3.2) are activated. This may lead to a stack TCP layer Reset of the Connection before
the Keep Alive timer is over.
4.2.2.2 Crash and Reboot of the Server end point
After the crash and Reboot of the Server, the connection is "half-open" on Client side. The expected
behavior is:
•
If no packet is sent on the half-open connection:
The TCP half-open connection is seen opened from the Client side as long as the Keep Alive
timer is not over. After that an error is returned to the TCP Management layer that can reset the
connection.
• If some packets are sent on the half-open connection:
The Server receives data on a connection that doesn't exist anymore. The stack TCP layer sends
a Reset to close the half-open connection on the Client side
4.2.2.3 Crash and Reboot of the Client
After the crash and Reboot of the Client, the connection is "half-open" on Server side. The expected
behavior is:
• No packet is sent on the half-open connection:
The TCP half-open connection is seen opened from the Server side as long as the Keep Alive timer
is not over. After that an error is returned to the TCP Management layer that can reset the
connection.
• If the Client opens a new connection before the Keep Aliver timer is over :
Two cases has to be studied:
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§ The connection opening has the same characteristics than the half-open connection on the
server side (same source and destination Ports, same source and destination IP Addresses),
therefore the connection opening will fail at the TCP stack level after the Time-Out on
Connection Establishment (75s on most of Berkeley implementations). To avoid this long TimeOut during which it is not possible to communicate, it is advised to ensure that different source
port numbers than the previous one are used for a connection opening after a reboot on the
client side.
§ The connection opening has not the same characteristics as the half-open connection on
server side (different source Ports, same destination Port, same source and destination IP
Address ), therefore the connection is opened at the stack TCP level and signaled to the Server
TCP Management layer.
If the Server TCP Management layer only supports one connection from a remote Client IP
Address, it can close the old half-opened connection and use the new one.
If the Server TCP Management layer supports several connections from a remote Client IP
Address, the new connection stays opened and the old one also stays half-opened until the
expiration of the Keep Alive Timer that will return an error to the TCP Management layer. After
that the TCP Management layer will be able to Reset the old connection.
4.2.3
Access Control Module
The goal of this module is to check every new connection and using a list of authorized remote IP
addresses the module can authorize or forbid a remote Client TCP connection.
In critical context the application programmer needs to choose the Access Control mode in order to
secure its network access. In such a case he needs to Authorize/forbid access for each remote @IP.
The user needs to provide a list of IP addresses and to specify for each IP address if it’s authorized or
not. By default, on security mode, the IP addresses not configured by the user are forbidden.
Therefore with the access control mode a connection coming from an unknown IP address is closed.
4.3 USE of TCP\IP STACK
A TCP/IP stack provides an interface to manage connections, to send and receive data, and also to
do some parameterizations in order to adapt the stack behavior to the device or system constraints.
Msg
Modbus
The goal of this chapter is to give an
overview of the Stack interface and also
some
information
concerning
the
parameterization of the stack. This
overview focuses on the features used by
the MODBUS Messaging.
TCP
ICMP
ARP
IP
Network Access
Ethernet II and 802.3 layer
For more information, the advice is to read the RFC 1122 that provides guidance for vendors and
designers of Internet communication software. It enumerates standard protocols that a host connected
to the Internet must use as well as an explicit set of requirements and options.
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The stack interface is generally based on the BSD (Berkeley Software Distribution) Interface that is
described in this document.
.
4.3.1
Use of BSD Socket interface
Remark : some TCP/IP stack proposes other types of interface for performance issues. A MODBUS
client or server can use these specific interfaces, but this use will be not described in this
specification.
A socket is an endpoint of communication. It is the basic building block for communication. A
MODBUS communication is executed by sending and receiving data through sockets. The TCPIP
library provides only stream sockets using TCP and providing a connection-based communication
service.
The Sockets are created via the socket () function. A socket number is returned, which is then used
by the creator to access the socket. Sockets are created without addresses (IP address and port
number). Until a port is bound to a socket, it cannot be used to receive data.
The bind () function is used to bind a port number to a socket. The bind () creates an association
between the socket and the port number specified.
In order to initiate a connection, the client must issue the connect () function specifying the socket
number, the remote IP address and the remote listening port number (active connection
establishment).
In order to complete a connection, the server must issue the accept () function specifying the socket
number that was specified in the prior listen () call (passive connection establishment). A new socket
is created with the same properties as the initial one. This new socket is connected to the client’s
socket, and its number is returned to the server. The initial socket is thereby free for other clients that
might want to connect with the server.
After the establishment of the TCP connection the data can be transferred. The send() and recv()
functions are designed specifically to be used with sockets that are already connected.
The setsockopt () function allows a socket’s creator to associate options with a socket. These options
modify the behavior of the socket. The description of these options is given in the chapter § 4.3.2.
The select () function allows the programmer to test events on all sockets.
The shutdown () function allows a socket user to disable send () and/or receive () on the socket.
Once a socket is no longer needed, its socket descriptor can be discarded by using the close ()
function.
The above figure describes a full MODBUS communication between a client and a s server. The
Client establishes the connection and sends 3 MODBUS requests to the server without waiting the
response of the first one. After receiving all the responses the Client closes the connection properly.
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CLIENT
( IP1)
SERVER
(IP2)
fd=socket()
fd'=socket()
bind(fd,n)
bind(fd',502)
connect(fd,IP2,502)
listen(fd')
SYN J
SYN K, ACK J+1
ACK K+1
fd''=accept(fd')
recv(fd'')
send(fd)
send(fd)
recv(fd)
MOD
BUS
Requ
est P
DU 1
MOD
BUS
Requ
est P
DU i
MODBUS Resp
send(fd)
recv(fd)
recv(fd'')
onse PDU 1
send(fd")
MOD
BUS
Requ
est P
DU N
MODBUS Resp
recv(fd)
onse PDU i
MODBUS Respon
se PDU N
recv(fd'')
send(fd")
send(fd")
close(fd)
FIN
ACK of FIN
FIN
close(fd")
ACK of FIN
Figure 9: MODBUS Exchanges
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TCP layer parameterization
Some parameters of the TCP/IP stack can be adjusted to adapt its behavior to the product or system
constraints. The following parameters can be adjusted in the TCP layer:
• Parameters for each connection:
SO-RCVBUF, SO-SNDBUF:
These parameters allow setting the high water mark for the Send and the Receive Socket. They can
be adjusted for flow control management. The size of the received buffer is the maximum size
advertised window for that connection. Socket buffer sizes must be increased in order to increase
performances. Nevertheless these values must be smaller than internal driver resources in order to
close the TCP window before exhausting internal driver resources.
The received buffer size depends on the TCP Windows size, the TCP Maximum segment size and
the time needed to absorb the incoming frames. With a Maximum Segment Size of 300 bytes (a
MODBUS request needs a maximum of 256 bytes + the MBAP header size), if we need 3 frames
buffering, the socket buffer size value can be adjusted to 900 bytes. For biggest needs and bestscheduled time, the size of the TCP window may be increased.
TCP-NODELAY:
Small packets (called tinygrams) are normally not a problem on LANs, since most LANs are not
congested, but these tinygrams can lead to congestion on wide area networks. A simple solution,
called the "NAGLE algorithm", is to collect small amounts of data and sends them in a single segment
when TCP acknowledgments of previous packets arrive.
In order to have better real-time behavior it is recommended to send small amounts of data directly
without trying to gather them in a single segment. That is why it is recommended to force the TCPNODELAY option that disables the "NAGLE algorithm" on client and server connections.
SO-REUSEADDR:
When a MODBUS server closes a TCP connection initialized by a remote client, the local port
number used for this connection cannot be reused for a new opening while that connection stay in the
"Time-wait" state (during two MSL : Maximum Segment Lifetime).
It is recommended specifying the SO-REUSEADDR option for each client and server connection to
bypass this restriction. This option allows the process to assign itself a port number that is part of a
connection that is in the 2MSL wait for client and listening socket.
SO-KEEPALIVE:
By default on TCP/IP protocol no data are sent across an idle TCP connection. Therefore if no
process at the ends of a TCP connection is sending data to the other, nothing is exchanged between
the two TCP modules. This assumes that either the client application or the server application uses
timers to detect inactivity in order to close a connection.
It is recommended to enable the KEEPALIVE option on both client and server connection in order to
poll the other end to know if the distant has either crashed and is down or crashed and rebooted.
Nevertheless we must keep on mind that enabling KEEPALIVE can cause perfectly good connections
to be dropped during transient failures, that it consumes unnecessary bandwidth on the network if the
keep alive timer is too short.
• Parameters for the whole TCP layer:
Time Out on establishing a TCP Connection:
Most Berkeley-derived systems set a time limit of 75 seconds on the establishment of a new
connection, this default value should be adapted to the real time constraint of the application.
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Keep Alive parameters:
The default idle time for a connection is 2 hours. Idles times in excess of this value trigger a keep
alive probe. After the first keep alive probe, a probe is sent every 75 seconds for a maximum number
of times unless a probe response is received.
The maximum number of keep Alive probes sent out on an idle connection is 8. If no probe response
is received after sending out the maximum number of keep Alive probes,TCP signal an error to the
application that can decide to close the connection
Time-out and retransmission parameters:
A TCP packet is retransmitted if its lost has been detecting. One way to detect the lost is to manage a
Retransmission Time-Out (RTO) that expires if no acknowledgement have been received from the
remote side.
TCP manages a dynamic estimation of the RTO. For that purpose a Round-Trip Time (RTT) is
measured after the send of every packet that is not a retransmission. The Round-Trip Time (RTT) is
the time taken for a packet to reach the remote device and to get back an acknowledgement to the
sending device. The RTT of a connection is calculated dynamically, nevertheless if TCP cannot get
an estimate within 3 seconds, the default value of the RTT is set to 3 seconds.
If the RTO has been estimated, it applies to the next packet sending. If the acknowledgement of the
next packet is not received before the estimated RTO expiration, the Exponential BackOff is
activated. A maximum number of retransmissions of the same packet is allowed during a certain
amount of time. After that if no acknowledgement has been received, the connection is aborted.
The maximum number of retransmissions and the maximum amount of time before the abort of the
connection (tcp_ip_abort_interval) can be set up on some stacks.
Some retransmission algorithms are defined in TCP standards :
§ The Jacobson's RTO estimation algorithm is used to estimate the Retransmission Time-Out
(RTO),
§ The Karn's algorithm says that the RTO estimation should not be done on a retransmitted
segment,
§ The Exponential BackOff defines that the retransmission time-out is doubled for each
retransmission with an upper limit of 64 seconds.
§ The fast retransmission algorithm allows retransmitting after the reception of three duplicate
acknowledgments. This algorithm is advised because on a LAN it may lead to a quicker
detection of the lost of a packet than waiting for the RTO expiration.
The use of these algorithms is recommended for a MODBUS implementation.
4.3.3
IP layer parameterization
4.3.3.1 IP Parameters
The following parameters must be configured in the IP layer of a MODBUS implementation :
• Local IP Address : the IP address can be part of a Class A, B or C.
• Subnet Mask, : Subnetting an IP Network can be done for a variety of reasons : use of different
physical media (such as Ethernet, WAN, etc.), more efficient use of network addresses, and the
capability to control network traffic. The Subnet Mask has to be consistent with the IP address
class of the local IP address.
• Default Gateway: The IP address of the default gateway has to be on the same subnet as the
local IP address. The value 0.0.0.0 is forbidden. If no gateway is to be defined then this value is
to be set to either 127.0.0.1 or the Local IP address.
Remark : The Modbus messaging service doesn't require the fragmentation function in the IP layer.
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The local IP End Point shall be configured with a local IP Address and with a Subnet Mask and a
Default Gateway (different from 0.0.0.0) .
4.4 COMMUNICATION APPLICATION LAYER
4.4.1
MODBUS Client
Modbus
Client
Serial Line
Modbus
Client
TCP/IP
Modbus
Client
TCP/IP
Client TCP/IP
gateway
Modbus TCP / IP
Modbus
Server
TCP/IP
Modbus
Server
Serial Line
Modbus
Server
TCP/IP
Modbus
Server
Serial Line
Server TCP/IP
gateway
Modbus Serial line
Figure 10: MODBUS Client unit
4.4.1.1 MODBUS client design
The definition of MODBUS/TCP protocol allows a simple design of a client. The following activity
diagram describes the main treatments that are processed by a client to send a MODBUS request
and to treat a MODBUS response.
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Idle
Wait
[Reveive_Response_from_TCP_Mgt]
[Request_from_the_user application]
Waiting_response_timer_expires
Build MODBUS
request
Find out pending
transaction
Process
MODBUS
Confirmation
[Confirmation error]
[Retries number not reached]
Send MODBUS request
To TCP management
[Confirmation OK]
[Retries number reached]
Send positive
confirmation to
User Application
[Send Not OK]
Send negative
confirmation to user
Application
[Send OK]
Set Waiting
response timer
Figure 11: MODBUS Client Activity Diagram
A MODBUS client can receive three events:
§ A new demand from the user application to send a request, in this case a MODBUS request has
to be encoded and be sent on the network using the TCP management component service. The
lower layer ( TCP management module) can give back an error due to a TCP connection error, or
some other errors.
§ A response from the TCP management, in this case the client has to analyze the content of the
response and send a confirmation to the user application
§ The expiration of a Time out due to a non-response. A new retry can be sent on the network or a
negative confirmation can be sent to the User Application.
Remark : These retries are initiated by the MODBUS client, some other retries can also be done
by the TCP layer in case of TCP acknowledge lack.
4.4.1.2 Build a MODBUS Request
Following the reception of a demand from the user application, the client has to build a MODBUS
request and to send it to the TCP management.
Building the MODBUS request can be split in several sub-tasks:
§ The instantiation of a MODBUS transaction that enables the Client to memorize all required
information in order to bind later the response to the request and to send the confirmation to the
user application.
§ The encoding of the MODBUS request (PDU + MPAB header). The user application that initiates
the demand has to provide all required information which enables the Client to encode the
request. The MODBUS PDU is encoded according to the MODBUS Application Protocol (MB
function code, associated parameters and application data [2] ). All fields of the MBAP header are
filled. Then, the MODBUS request ADU is built prefixing the PDU with the MBAP header
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§ The sending of the MODBUS request ADU to the TCP management module which is in charge of
finding the right TCP socket towards the remote Server. In addition to the MODBUS ADU the
Destination IP address must also be passed.
The following activity diagram describes, more deeply than in figure 17, the request building phase.
Instantiate a MB
transaction
[No Transaction available]
[Transaction available]
Initialize the
transaction
Encode the MB
request PDU
function code
Encode the
MBAP header
Send a
negative
confirmation to
the user
application
Send MB
request to TCP
Mgt
Figure 12: Request building activity diagram
The following example describes the Modbus request ADU encoding for reading 1 word at the
address 05 in a remote server :
♦
MODBUS Request ADU encoding :
Description
MBAP Header
Size
Example
Transaction Identifier Hi
1
0x15
Transaction Identifier Lo
1
0x01
Protocol Identifier
2
0x0000
Length
2
0x0006
Unit Identifier
1
0xFF
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Function Code (*)
1
0x03
Starting Address
2
0x0005
Quantity of Registers
2
0x0001
(*) please see the MODBUS Application Protocol Specification [2]
• Transaction Identifier
The transaction identifier is used to associate the future response with the request. So, at a time,
on a TCP connection, this identifier must be unique. There are several manners to use the
transaction identifier:
- For example, it can be used as a simple "TCP sequence number" with a counter which is
incremented at each request.
- It can also be judiciously used as a smart index or pointer to identify a transaction context
in order to memorize the current remote server and the pending MODBUS request.
Normally, on MODBUS serial line a client must send one request at a time. This means that the client
must wait for the answer to the first request before sending a second request. On TCP/MODBUS,
several requests can be sent without waiting for a confirmation to the same server. The
MODBUS/TCP to MODBUS serial line gateway is in charge of ensuring compatibility between these
two behaviors.
The number of request accepted by a server depends on its capacity in term of number of resources
and size of the TCP windows. In the same way the number of transactions initialized simultaneously
by a client depends also on its resource capacity. This implementation parameter is called
"NumberMaxOfClientTransaction" and must be described as one of the MODBUS client features.
Depending of the device type this parameter can take a value from 1 to 16.
• Unit Identifier
This field is used for routing purpose when addressing a device on a MODBUS or MODBUS+
serial line sub-network. In that case, the “Unit Identifier” carries the MODBUS slave address of
the remote device:
If the MODBUS server is connected to a MODBUS+ or MODBUS Serial Line sub-network
and addressed through a bridge or a gateway, the MODBUS Unit identifier is necessary to
identify the slave device connected on the sub-network behind the bridge or the gateway.
The destination IP address identifies the bridge itself and the bridge uses the MODBUS Unit
identifier to forward the request to the right slave device.
The MODBUS slave device addresses on serial line are assigned from 1 to 247 (decimal).
Address 0 is used as broadcast address.
On TCP/IP, the MODBUS server is addressed using its IP address; therefore, the MODBUS
Unit Identifier is useless. The value 0xFF has to be used.
When addressing a MODBUS server connected directly to a TCP/IP network, it’s
recommended not using a significant MODBUS slave address in the “Unit Identifier” field. In
the event of a re-allocation of the IP addresses within an automated system and if a IP
address previously assigned to a MODBUS server is then assigned to a gateway, using a
significant slave address may cause trouble because of a bad routing by the gateway. Using
a non-significant slave address, the gateway will simply discard the MODBUS PDU with no
trouble. 0xFF is recommended for the “Unit Identifier" as non-significant value.
Remark : The value 0 is also accepted to communicate directly to a MODBUS/TCP device.
4.4.1.3 Process MODBUS Confirmation
When a response frame is received on a TCP connection, the Transaction Identifier carried in the
MBAP header is used to associate the response with the original request previously sent on that TCP
connection:
§ If the Transaction Identifier doesn't refer to any MODBUS pending transaction, the response must
be discarded.
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§ If the Transaction Identifier refers to a MODBUS pending transaction, the response must be
parsed in order to send a MODBUS Confirmation to the User Application (positive or negative
confirmation)
Parsing the response consists in verifying the MBAP Header and the MODBUS PDU response:
§ MBAP Header
After the verification of the Protocol Identifier that must be 0x0000, the length gives the size of
the MODBUS response.
If the response comes from a MODBUS server device directly connected to the TCP/IP network,
the TCP connection identification is sufficient to unambiguously identify the remote server.
Therefore, the Unit Identifier carried in the MBAP header is not significant (value 0xFF) and must
be discarded.
If the remote server is connected on a Serial Line sub-network and the response comes from a
bridge, a router or a gateway, then the Unit Identifier (value != 0xFF) identifies the remote
MODBUS server which has originally sent the response.
§ MODBUS Response PDU
The function code must be verified and the MODBUS response format analyzed according to the
MODBUS Application Protocol:
• if the function code is the same as the one used in the request, and if the response format is
correct, then the MODBUS response is given to the user application as a Positive
Confirmation.
• If the function code is a MODBUS exception code (Function code + 80H), the MODBUS
exception response is given to the user application as a Positive Confirmation.
• If the function code is different from the one used in the request (=non expected function
code), or if the format of the response is incorrect, then an error is signaled to the user
application using a Negative Confirmation.
Remark: A positive confirmation is a confirmation that the command was received and responded to
by the server. It does not imply that the server was able to successfully act on the command (failure to
successfully act on the command is indicated by the MODBUS Exception response).
The following activity diagram describes, more deeply than in figure 17, the confirmation processing
phase.
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Find Out pending
MB transaction
[PendingTransaction ]
[No Pending Transaction ]
Use MB transaction to
bind with the request
Discard
Response
Analyse MBAP header
[Other_protocol]
[Modbus_protocol]
Analyse Response
PDU
[MB response OK]
[Incorrect Response]
[MB Exception response]
Extract MB
Response
Send negative
Confirmation to
user Application
Process_MB_
exception
Send positive
confirmation to user
application
Wait
Figure 13: Process MODBUS Confirmation activity diagram
4.4.1.4 Time-out managing
There is deliberately NO specification of required response time for a transaction over
MODBUS/TCP.
This is because MODBUS/TCP is expected to be used in the widest possible variety of
communication situations, from I/O scanners expecting sub-millisecond timing to long distance radio
links with delays of several seconds.
From a client perspective, the timeout must take into account the expected transport delays across
the network, to determine a ‘reasonable’response time. Such transport delays might be milliseconds
for a switched Ethernet, or hundreds of milliseconds for a wide area network connection.
In turn, any ‘timeout’ time used at a client to initiate an application retry should be larger than the
expected maximum ‘reasonable’ response time. If this is not followed, there is a potential for
excessive congestion at the target device or on the network, which may in turn cause further errors.
This is a characteristic, which should always be avoided.
So in practice, the client timeouts used in high performance applications are always likely to be
somewhat dependent on network topology and expected client performance.
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Applications which are not time critical can often leave timeout values to the normal TCP defaults,
which will report communication failure after several seconds on most platforms.
4.4.2
MODBUS Server
Modbus
Client
Serial Line
Modbus
Client
TCP/IP
Modbus
Client
TCP/IP
Client TCP/IP
gateway
Modbus TCP / IP
Modbus
Server
TCP/IP
Modbus
Server
TCP/IP
Modbus
Server
Serial Line
Modbus
Server
Serial Line
Server TCP/IP
gateway
Modbus Serial line
Figure 14: MODBUS Server unit
The role of a MODBUS server is to provide access to application objects and services to remote
MODBUS clients.
Different kind of access may be provided depending on the user application :
§ simple access like get and set application objects attributes
§ advanced access in order to trigger specific application services
The MODBUS server has:
§ To map application objects onto readable and writable MODBUS objects, in order to get or set
application objects attributes.
§ To provide a way to trigger services onto application objects.
In run time the MODBUS server has to analyze a received MODBUS request, to process the required
action, and to send back a MODBUS response.
4.4.2.1 MODBUS Server Design
The MODBUS Server design depends on both :
§ the kind of access to the application objects (simple access to attributes or advanced access to
services)
§ the kind of interaction between the MODBUS server and the user application (synchronous or
asynchronous).
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The following activity diagram describes the main treatments that are processed by the Server to
obtain a MODBUS request from TCP Management, then to analyze the request, to process the
required action, and to send back a MODBUS response.
Iddle
[Server init]
Wait
[Invocation to User application done]
[MB Indication discarded]
Invoke back
end interface
[Response from user Application]
[Reception of a MODBUS indication
from TCP Mgt]
[MB Transaction refused] MODBUS_PDU_Checking...
[MODBUS transaction accepted]
[Processing not complete]
[Need user Application processing]
MODBUS_Service_Processing
Response
processing
[Processing NOK]
[Processing Not OK]
[Processing OK]
[Processing OK]
Build a MODBUS
response
Build a MODBUS
Exception
[MB Exception OK]
[MB Response OK]
Send response
to TCP_Mgt
[processing end]
Release the
MODBUS server
transaction
Figure 15: Process MODBUS Indication activity diagram
As shown in the previous activity diagram:
§ Some services can be immediately processed by the MODBUS Server itself, with no direct
interaction with the User Application ;
§ Some services may require also interacting explicitly with the User Application to be processed ;
§ Some other advanced services require invoking a specific interface called MODBUS Back End
service. For example, a User Application service may be triggered using a sequence of several
MODBUS request/response transactions according to a User Application level protocol. The
Back End service is responsible for the correct processing of all individual MODBUS
transactions in order to execute the global User Application service.
A more complete description is given in the following chapters.
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The MODBUS server can accept to serve simultaneously several MODBUS requests. The maximum
number of simultaneous MODBUS requests the server can accept is one of the main characteristics
of a MODBUS server. This number depends on the server design and its processing and memory
capabilities. This implementation parameter is called "NumberMaxOfSeverTransaction" and must
be described as one of the MODBUS server features. It may have a value from 1 to 16 depending on
the device capabilities.
The behavior and the performance of the MODBUS server are significantly affected by the
"NumberMaxOfTransaction" parameter. Particularly, it's important to note that the number of
concurrent MODBUS transactions managed may affect the response time of a MODBUS request by
the server.
4.4.2.2 MODBUS PDU Checking
The following diagram describes the MODBUS PDU Checking activity.
Parse the
MBAP header
MB Indication
discarded
[Error on MBAP ]
[MBAP OK]
Instanciate a
MB Transaction
[No Transaction av ailable]
[Transaction av ailable]
Parse The MB
PDU
[Error on MB PDU]
MB Transaction
refused
[OK]
MB Transaction
accepted
Figure 16: MODBUS PDU Checking activity diagram
The MODBUS PDU Checking function consists of first parsing the MBAP Header. The Protocol
Identifier field has to be checked :
§ If it is different from MODBUS protocol type, the indication is simply discarded.
§ If it is correct (= MODBUS protocol type; value 0x00), a MODBUS transaction is instantiated.
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The maximum number of MODBUS transactions the server can instantiated is defined by the
"NumberMaxOfTransaction" parameter ( A system or a configuration parameter).
In the case of no more transaction is available, the server builds a MODBUS exception response
(Exception code 6 : Server Busy).
If a MODBUS transaction is available, it's initialized in order to memorize the following information:
• The TCP connection identifier used to send the indication (given by the TCP Management)
• The MODBUS Transaction ID (given in MBAP Header)
• The Unit Identifier (given in MBAP Header)
Then the MODBUS PDU is parsed. The function code is first controlled :
§ in case of invalidity a MODBUS exception response is built (Exception code 1 : Invalid function).
§ If the function code is accepted, the server initiates the "MODBUS Service processing" activity.
4.4.2.3 MODBUS service processing
Transaction_accepted
Response_from_user_Appli
Response
processing
Analyse
requested service
[Local processing ]
[Need User Appli. processing]
Local Service
processing
[Processing not completed]
[Completed]
Send an invocation to
User Application
through MB Backend
interface
[Processing OK]
[Processing Not OK]
[Processing OK]
[Processing Not OK]
Build Modbus
response
Build Modbus
Exception Response
Figure 17: MODBUS service processing activity diagram
The processing of the required MODBUS service can be done in different ways depending on the
device software and hardware architecture as described in the hereafter examples :
• Within a compact device or a mono-thread architecture where the MODBUS server can
access directly to the user application data, the required service can be processed "locally" by
the server itself without invoking the Back End service.
The processing is done according to the “MODBUS Application Protocol Specification”. In
case of an error, a MODBUS exception response is built.
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• Within a modular multi-processor device or a multi-thread architecture where the
"communication layers" and the "user application layer" are 2 separate entities, some trivial
services can be processed completely by the Communication entity while some others can
require a cooperation with the User Application entity using the Back End service.
To interact with the User Application, the MODBUS Backend service must implement all appropriate
mechanisms in order to handle User Application transactions and to manage correctly the User
Application invocations and associated responses.
4.4.2.4 User Application Interface (Backend Interface)
Several strategies can be implemented in the MODBUS Backend service to achieve its job although
they are not equivalent in terms of user network throughput, interface bandwidth usage, response
time, or even design workload.
The MODBUS Backend service will use the appropriate interface to the user application :
§ Either a physical interface based on a serial link, or a dual-port RAM scheme, or a simple I/O
line, or a logical interface based on messaging services provided by an operating system.
§ The interface to the User Application may be synchronous or asynchronous.
The MODBUS Backend service will also use the appropriate design pattern to get/set objects
attributes or to trigger services. In some cases, a simple "gateway pattern" will be adequate. In some
other cases, the designer will have to implement a "proxy pattern" with a corresponding caching
strategy, from a simple exchange table history to more sophisticated replication mechanisms.
The MODBUS Backend service has the responsibility to implement the protocol transcription in order
to interact with the User Application. Therefore, it can have to implement mechanisms for packet
fragmentation/reconstruction, data consistency guarantee, and synchronization whatever is required.
4.4.2.5 MODBUS Response building
Once the request has been processed, the MODBUS server has to build the response using the
adequate MODBUS server transaction and has to send it to TCP management component.
Depending on the result of the processing two types of response can be built :
§ A positive MODBUS response :
§ The response function code = The request function code
§ A MODBUS Exception response :
§ The objective is to provide to the client relevant information concerning the error
detected during the processing ;
§ The response function code = the request function code + 0x80 ;
§ The exception code is provided to indicate the reason of the error.
Exception
Code
MODBUS name
Comments
01
02
03
04
05
Illegal Function Code
Illegal Data Address
Illegal Data Value
Server Failure
Acknowledge
06
Server Busy
0A
0B
Gateway problem
Gateway problem
The function code is unknown by the server
Dependant on the request
Dependant on the request
The server failed during the execution
The server accepted the service invocation but the service
requires a relatively long time to execute. The server therefore
returns only an acknowledgement of the service invocation
receipt.
The server was unable to accept the MB Request PDU. The
client application has the responsibility of deciding if and when
to re-send the request.
Gateway paths not available.
The targeted device failed to respond. The gateway generates
this exception
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The MODBUS response PDU must be prefixed with the MBAP header which is built using data
memorized in the transaction context.
• Unit Identifier
The Unit Identifier is copied as it was given within the received MODBUS request and
memorized in the transaction context.
• Length
The server calculates the size of the MODBUS PDU plus the Unit Identifier byte. This value is
set in the "Length" field.
• Protocol Identifier
The Protocol Identifier field is set to 0x0000 (MODBUS protocol), as it was given within the
received MODBUS request.
• Transaction Identifier
This field is set to the "Transaction Identifier" value that was associated with the original
request and memorized in the transaction context.
Then the MODBUS response must be returned to the right MODBUS Client using the TCP connection
memorized in the transaction context. When the response is sent, the transaction context must be
free.
5. IMPLEMENTATION GUIDELINE
The objective of this chapter is to propose an example of a messaging service implementation.
The model describes below can be used as a guideline during a client or a server implementation of a
MODBUS messaging service.
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5.1 OBJECT MODEL DIAGRAM
User Application
InterfaceUserApplication
Communication application layer
Configuration layer
InterfaceIndicationMsg
InterfaceResponseMsg
TCP management
Figure 18: MODBUS Messaging Service Object Model Diagram
Four main packages composes the Object Model Diagram:
• The Configuration layer which configures and manages operating modes of components of
other packages
• The TCP Management which interfaces the TCP\IP stack and the communication application
layer managing TCP connection. It implies the management of socket interface.
• The Communication application layer which is composed by the MODBUS client on one side
and the MODBUS server on the other side. This package is linked with the user application.
• The User application which correspond to the device application, it is completely dependent of
the device therefore it will be not described in this document.
This model is independent of implementation choices like the type of OS, the memory management,
.. In order to guarantee this independence generic Interface layers are used between the TCP
management layer and the communication layer and between the communication layer and the user
application layer.
Different implementations of this interface can be realized: Pipe between two tasks, shared memory,
serial link interface, procedural call, .. .
Some hypothesis has been taken to defined the hereafter implementation model :
• Static memory management
• Synchronous treatment of the server
• One task to process the receptions on all sockets.
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TCP management package
InterfaceIndicationMsg
(from Logical View)
InterfaceResponseMsg
(from Logical View)
----------------------------------------------------------------------------------------------------------------------------------------------ConnexionMgt
NbConnectionSupported : Int
NbLocalConnection : Int
NbRemoteConnection : int
m_sendData()
m_Receivedata()
m_isConnectionAuthorized()
InterfaceConnex ion
GetObjectConnexion()
FreeObjectConnexion()
RetreivingObject Connexion()
TCP management
package
TCPConnexion
MBAP
ItemConnexion
SocketDescript : Int
IPSource : Long
PortSource : Long
PortDest : Long
IPdestination : long
BufferIn : Char*
BufferOut : Char*
IsConnexionRequest ()
OpenConnexion()
AcceptConnexion()
CloseConnexion()
IsEtablishedConnexion()
IsMdbHeader()
GetMessagelength()
WriteTransactionId()
ReadTransactionId()
StackTCP IP
Socket interface()
Figure 19: MODBUS TCP management package
The TCP management package comprises the following classes :
CInterfaceConnexion: The role of this class consists in managing memory pool for connections.
CItemConnexion: This class contains all information needed to describe a connection.
CTCPConnexion:, This class provides methods for managing automatically a TCP connection
(Interface socket is provided by CStackTCP_IP).
CConnexionMngt: This class manages all connections and send query/response to MODBUS
Server/MODBUS Client through CinterfaceIndicationMsg and CInterfaceResponseMsg. This class
also treats the Access control for the connection opening.
CMBAP: This class provides methods for reading/writing/analyzing the MODBUS MBAP.
CStackTCP_IP: This class Implements socket services and provides parameterization of the stack.
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Configuration layer package
TCP management
(from Logical View)
Communication application layer
(from Logical View)
------------------------------------------------------------------------------------------------------------------------------------------------
Operating Mode
m_Configure()
m_Start()
m_Stop()
m_Reset()
Configuartion layer
package
ConfigurationObject
GlobalState : char
MyModbusAddress : Int
MyIPAddress : long
MyPortNumber : Long
NumberAuthorized_IP : int
ListAuthorized_IP : int
NumberForbidden_IP : Int
ListForbidden_IP : long()
NumberConnect ionSupported : int
Figure 20: MODBUS Configuration layer package
The Configuration layer package comprises the following classes :
TConfigureObject: This class groups all data needed for configuring each other component. This
structure is filled by the method m_Configure from the class CoperatingMode. Each class needing to
be configured gets its own configuration data from this object. The configuration data is
implementation dependent therefore the list of attributes of this class is provided as an example.
COperatingMode: The role of this class is to fill the TConfigureObject (according to the user
configuration) and to manage the operating modes of the classes described below:
§ CMODBUSServer
§ CMODBUSClient
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§ CconnexionMngt
5.1.3
Communication layer package
InterfaceUserApplication
(from Logical View)
-------------------------------------------------------------------------------------------------------------------------------------------------------ModbusPDU
IsMdbAnalysisOfDataCorrect()
m_BuildModbusException()
m_WritePDU()
Communication
Application layer
package
ModbusServer
ModbusClient
m_ServerReceivingMessage()
m_ServerModbus()
m_BuildResponse()
m_InitServerfunction()
m_ClientReceivingResponse()
m_ClientReceivingMessage()
m_ClientModbus()
Transaction
TransactionId : int
TimeSnapShot : int
IsTransactionTimeOut()
m_WriteTransactionID()
m_StartTimesnapShoot()
-------------------------------------------------------------------------------------------------------------------------------------------------------InterfaceResponseMsg
(from Logical View)
InterfaceIndicationMsg
(from Logical View)
Figure 21: MODBUS Communication Application layer package
The Communication Application layer package comprises the following classes :
CMODBUSServer: MODBUS query is received from class CInterfaceIndicationMsg (by the method
m_ServerReceivingMessage). The role of this class is to build the MODBUS response or the
MODBUS Exception according the query (incoming from network). This class implements the Graph
State of MODBUS server. Response can be built only if class COperatingMode has sent both user
configuration and right operating modes.
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CMODBUSClient: MODBUS query is read from class CInterfaceUserApplication, The client task
receives query by the method m_ClientReceivingMessage. This class implements the State Graph of
MODBUS client and manages transaction for linking query with response (from network). Query can
be sent over network only if class CoperatingMode has sent both user configuration and right
operating modes.
CTransaction: This class implements methods and structures for managing transactions.
5.1.4
Interface classes
CInterfaceUserApplication: This class represents the interface with the user application, it provides
two methods to access to the user data. In a real implementation this method can be implemented in
different way depending of the hardware and software device capabilities (equivalent to an end-driver,
example access to PCMCIA, shared memory, etc).
CInterfaceIndicationMsg: This Interface class is proposed for sending query from Network to the
MODBUS Server, and for sending response from Network for the Client. This class interfaces
TCPManagement and ‘Communication Application Layer’ packages (From Network). The
implementation of this class is device dependent.
CInterfaceResponseMsg: This Interface class is used for receiving response from the Server and
for sending query from the client to the Network. This class interfaces packages ‘Communication
Application Layer’and package ‘TCPManagement’(To Network). The implementation of this class is
device dependent.
5.2 IMPLEMENTATION CLASS DIAGRAM
The following Class Diagram describes the complete diagram of a proposal implementation.
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Figure 22: Class Diagram
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5.3 SEQUENCE DIAGRAMS
Two Sequence diagrams are described hereafter in order to illustrate a Client MODBUS transaction and a
Server MODBUS transaction.
Figure 23: MODBUS client sequence diagram
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General comments for a better understanding of the Client sequence diagram:
First step: A Reading query comes from User Application (method m_Read).
Second Step: The ‘Client’ task receives the MODBUS query (method m_ClientReceivingMessage).
This is the entry point of the Client. To associate the query with the corresponding response when it will
arrive, the Client uses a Transaction resource (Class CTransaction). The MODBUS query is sent to the
TCP_Management
by
calling
the
class
interface
CInterfaceResponseMsg
(method
m_MODBUSRequest)
Third Step: If the connection is already established there is nothing to do on connection, the message
can be send over the network. Otherwise, a connection must be opened before the message can be sent
over the network.
At this time the client is waiting for a response (from a remote server)
Fourth step: Once a response has been received from the network, the TCP/IP stack receives data
(method m_EventOnSocket is implicitly called).
If the connection is already established, then the MBAP is read for retrieving the connection object
(connection object gives memory resource and other information).
Data coming from network is read and confirmation is sent to the client task via the class Interface
CInterfaceIndicationMsg (method m_MODBUSConfirmation). Client task receives the MODBUS
Confirmation (method m_ClientReceivingResponse).
Finally the response is written to the user application (method m_WriteData), and transaction resource is
freed.
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Hereafter is an example of a MODBUS Server exchange.
Figure 24: MODBUS server Diagram
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General comments for a better understanding of the Server sequence diagram:
First step: a client has sent a query (MODBUS query) over the network.
The TCP/IP stack receives data (method m_EventOnSocket is implicitly called).
Second step: The query may be a connection request or not (method m_IsConnexionRequest).
If the query is a connection request, the connection object and buffers for receiving and sending the
MODBUS frame are allocated (method m_GetObjectConnexion). Just after, the connection access
control must be checked and accepted (method m_AcceptConnexion)
Third step: If the query is a MODBUS request, the complete MODBUS Query can be read (method
m_ReceiveData). At this time the MBAP must be analyzed (method m_IsMdbHeaderCorrect). The
complete frame is sent to the Server task via the CinterfaceIndicationMessaging Class (method
m_MODBUSIndication).
Server
task
receives
the
MODBUS
Query
(method
m_ServerReceivingMessage) and analyses it.
If an error occurs (function code not supported, etc), a MODBUSException frame is built
(m_BuildMODBUSException), otherwise the response is built.
Fourth Step: The response is sent over the network via the CinterfaceResponseMessaging (method
m_MODBUSResponse). Treatment on the connection object is done by the method m_SendData
(retrieve the connection descriptor, etc) and data is sent over the network.
5.4 CLASSES AND METHODS DESCRIPTION
5.4.1
MODBUS server class
Class CMODBUSServer
class CMODBUSServer
Stereotype implementationClass
Provides methods for managing MODBUS Messaging in Server Mode
Field Summary
protected char GlobalState
state of the Modbus Server
Constructor Summary
CMODBUSServer(TConfigureObject * lnkConfigureObject)
Constructor : Create internal object
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Method Summary
protected void m_InitServerFunctions (void )
Function called by the constructor for filling array of functions 'm_ServerFunction'
bool m_Reset(void )
Method for Reseting Server, return true if reseted
int m_ServerReceivingMessage (TItemConnexion * lnkMODBUS)
Interface with CindicationMsg::m_MODBUSIndication for receiving Query from
NetWork return negative value if problem
bool m_Start(void )
Method for Starting Server, return true if Started
bool m_Stop(void )
Method for Stopping Server, return true if Stopped
protected void m_tServerMODBUS(void )
Server MODBUS task ...
5.4.2
MODBUS Client Class
Class CMODBUSClient
class CMODBUSClient
Provides methods for managing MODBUS Messaging in Client Mode
Stereotype implementationClass
Field Summary
protected char GlobalState
State of the MODBUS Client
Constructor Summary
CMODBUSClient(TConfigureObject * lnkConfigureObject)
Constructor : Create internal object , initialize to 0 variables.
Method Summary
int m_ClientReceivingMessage (TItemConnexion * lnkMODBUS)
Interface provided for receiving message from application Layer Typically : Call
CinterfaceUserApplication::m_Read for reading data call
CInterfaceConnexion::m_GetObjectConnexion for getting memory for a transaction
Return negative value if problem
int m_ClientReceivingResponse (TitemConnexion * lnkTItemConnexion)
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bool
bool
bool
protected void
5.4.3
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Interface with CindicationMsg::m_Confirmation for receiving response from network
return negative value if problem
m_Reset(void )
Method for Reseting component return true if reseted
m_Start(void )
Method for Starting component return true if started
m_Stop(void )
Method for Stoppping component return true if stopped
m_tClientMODBUS(void )
Client MODBUS task....
Interface Classes
5.4.3.1 Interface Indication class
Class CInterfaceIndicationMsg
Direct Known Subclasses:
CConnexionMngt
class CInterfaceIndicationMsg
Class for sending message from TCP_Management to MODBUS Server or Client
Stereotype interface
Method Summary
int m_MODBUSConfirmation(TItemConnexion * lnkObject)
Method for Receiving incoming Response, calling the Client : could be by reference, by
Message Queue, Remote procedure Call, ...
int m_MODBUSIndication(TItemConnexion * lnkObject)
Method for reading incoming MODBUS Query and calling the Server : could be by
reference, by Message Queue, Remote procedure Call, ...
5.4.3.2 Interface Response Class
Class CInterfaceResponseMsg
Direct Known Subclasses:
CMODBUSClient, CMODBUSServer
class CInterfaceResponseMsg
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Class for sending response or sending query to TCP_Management from Client or Server
Stereotype interface
Method Summary
TitemConnexion m_GetMemoryConnexion(unsigned long IPDest)
* Get an object ITemConnexion from memory pool Return -1 if not enough memory
int m_MODBUSRequest(TItemConnexion * lnkCMODBUS)
Method for Writing incoming MODBUS Query Client to ConnexionMngt : could be by
reference, by Message Queue, Remote procedure Call, ...
int m_MODBUSResponse(TItemConnexion * lnkObject)
Method for writing Response from MODBUS Server to ConnexionMngt could be by
reference, by Message Queue, Remote procedure Call, ...
5.4.4
Connexion Management class
Class CConnexionMngt
class CConnexionMngt
Class that manages all TCP Connections
Stereotype implementationClass
Field Summary
protected char GlobalState
Global State of the Component ConnexionMngt
Int NbConnectionSupported
Global number of connections
Int NbLocalConnection
Number of connections opened by the local Client to a remote Server
Int NbRemoteConnection
Number of connections opened by a remote Client to the local Server
Constructor Summary
CconnexionMngt(TConfigureObject * lnkConfigureObject)
Constructor : Create internal object , initialize to 0 variables.
Method Summary
int m_EventOnSocket(void )
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wake-up
bool m_IsConnectionAuthorized (unsigned long IPAdress)
Return true if new connection is authorized
int m_ReceiveData(TItemConnexion * lnkConnexion)
Interface with CTCPConnexion::write method for reading data from network return
negative value if problem
bool m_Reset(void )
Method for Resetting ConnectionMngt component return true if Reset
int m_SendData(TItemConnexion * lnkConnexion)
Interface with CTCPConnexion::read method for sending data to the network Return
negative value if problem
bool m_Start(void )
Method for Starting ConnectionMngt component return true if Started
bool m_Stop(void )
Method for Stopping component return true if Stopped
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