EXTENSION the A Technical Supplement to Control Network

EXTENSION the A Technical Supplement to Control Network
EXTENSION
the
A Technical Supplement to Control Network
JULY–AUGUST
Volume 5 Issue 4
© 2004 Contemporary Control Systems, Inc.
Introduction to Real-Time Ethernet II
By Paula Doyle, a doctoral researcher with the Circuits and Systems Research Centre at the University of Limerick in Ireland
INTRODUCTION
In “Real-Time Ethernet I”, we introduced the basic
concepts of Ethernet’s capacity to deliver a real-time
(RT) communication system. “Real-Time Ethernet II”
introduces some of the RT solutions available to
industry today*: PROFInet, EtherCAT and ETHERNET
Powerlink. It also provides an introduction to a single
standard, IEEE 1588 that is growing in popularity
amongst RT Ethernet developers to provide submicrosecond synchronization accuracy of distributed
clocks over Ethernet.
* EtherNet/IP is included in the full article available at
http://www.ccontrols.com/pdf/volume5n4.pdf
IEEE 1588
IEEE 1588 [1] specifies “A protocol to synchronize
independent clocks running on separate nodes of a
distributed measurement or control system to a high
accuracy and precision.” IEEE 1588 is, or will be,
incorporated into EtherNet/IP, ETHERNET Powerlink,
EtherCAT and PROFInet—making it a popular standard
for delivering RT over Ethernet.
In IEEE 1588, all network nodes down to the
transducer level contain an IEEE 1588 clock,
synchronized with all network peers (see Figure 1) using
Precision Time Protocol (PTP). At device level, sensors
can timestamp their data locally and actuators can
operate at a precise time, avoiding stack and
application delays between transducer and controller.
The accuracy of the system depends on the
synchronization of local RT clocks.
M
Node
Rules:
1 Grandmaster / System
1 Master / Subnet
1 Slave / Switching Device
1588
GMC
S
Node
OC
Ethernet
S
1588
BC
Switching Device
S
Node
1588
BC
OC
M
1588
BC
M
S
Ethernet
S
Node
OC
Node
OC
Ethernet
S
1588
BC
Switching Device M
M
1588
BC
1588
BC
Figure 1—IEEE 1588 Configuration
IEEE 1588 defines two separate types of clocks:
ordinary and boundary. Boundary clocks (BC) are
employed in devices such as hubs or switches—where
more than one PTP communication path (port) exists.
Ordinary clocks exist in devices having a single port—
e.g., normal network devices. Each BC port can act as
a master or ordinary clock in its own segment.
PTP is for networks that support multicasting but
keep multicasts within a subnet and where each local
clock fulfills exacting requirements. The grandmaster
clock (GMC) is the best clock in the system—with the
best inherent stability, accuracy, resolution, etc.
defined by the standard [2]. The Best Master Clock
Algorithm (BMC), run by every live node, determines
clock quality. Within each subnet, the BMC determines
the master clock; in a single-subnet system the master
is the GMC.
The GMC determines system synchronization;
system clocks synchronize their subnet clocks to the
system. There is only one GMC per system, and only
one master clock per subnet.
Synchronization is performed as follows. All
masters periodically broadcast “Sync” messages
containing an estimate of the time the message will
physically leave the master. The precise receipt time of
these messages is noted at the slaves. The precise
sending time of the message is noted at the
grandmaster. All precise timing measurements are
performed as close to the physical layer as possible—to
eliminate the delays from the network stack and
operating system—while the estimated times are
calculated by the IEEE 1588 code at the Application
Layer (see Figure 2). Following the Sync message, the
master transmits a related “Follow_Up” message
containing the precise sending time of the Sync
message. A slave uses the transmission and reception
times to calculate its offset and can initiate
synchronization with the delay measurement, which is
not periodic and not performed as often as the
protocol synchronization. Sync messages do not
propagate beyond their originating subnet.
The resolution of the system clock is the resolution
of the GMC. If required, the GMC can be synchronized
to an external source such as GPS.
IEEE 1588 is a highly precise system for
synchronizing distributed nodes for applications such as
motion control and robotics. It was designed for
multicasting networks but with the popularity of
Industrial Ethernet, Annex D was included for an
(No part of the Extension may be reproduced without the written consent of Contemporary Controls.)
1
Ethernet implementation of PTP. Although IEEE 1588
does not alter Ethernet or make it more deterministic or
reliable, it does provide a method for other protocols
to do so. A highly synchronized system of distributed
nodes—coupled with an application for handling
resolution and controlling traffic—could deliver hard,
deterministic RT over Ethernet.
Estimate Times
Calculated
Application Layer
Network Protocol
Stack & OS
Precise Times
Calculated
ms delay
Sync Detector &
Timestamp
Creator
Physical Layer
ns delay
Figure 2—IEEE 1588 node timing
PROFInet
PROFInet [3] is a plant-wide fieldbus standard for
distributed automation systems. It uses objectorientation and available IT standards (TCP/IP, Ethernet,
XML, COM). PROFInet is also built on IEEE 802.3 and
is interoperable with TCP/IP—allowing it to be
implemented on existing Ethernets. It is compatible
with PROFIBUS-DP.
PROFInet V1, has a response time of 10-100 ms.
PROFInet-SRT (Soft Real-Time) allowed PROFInet to
work with a factory automation cycle time of 5-10 ms,
achieving RT solely in software. It uses TCP/IP and a
dedicated software channel for RT communications.
PROFInet-IRT brings a hard-RT element to the
PROFInet protocols. The three PROFInet protocols
allow differing degrees of RT. PROFInet for hard RT
is PROFInet-IRT.
PROFInet-IRT
PROFInet IRT (Isochronous RT) was developed for
systems requiring sub-microsecond synchronization,
typically high-performance motion control systems.
The benchmark for such a system is 1 ms cycle time,
1 µs jitter accuracy, and guaranteed determinism [4]—
which IRT fulfills.
Since software introduces jitter above 1 µs, IRT
(unlike SRT) is a hardware solution with highly
synchronized Ethernet nodes. Using full-duplex
switched Fast Ethernet, it divides the communication
cycle into a standard TCP/IP open channel and a
deterministic RT channel. The channel ratio is systemdependent and is chosen by the systems engineer.
Each PROFInet-IRT device has a special ASIC
(Application-Specific Integrated Circuit) for handling
node synchronization and cycle subdivision and
incorporates an intelligent 2 or 4 port switch.
The PROFInet switch in every node is highly
synchronized, contains a schedule of bus access and
2
can deal with RT and non-RT traffic. It prioritizes
RT traffic and provides full-duplex links for all ports.
Contemporary switches (even cut-through) add jitter
that would impact on determinism. PROFInet switches
minimize jitter to where it has a negligible effect. The
PROFInet communication model allows both RT and
non-RT traffic to co-exist on one network without
additional precautions.
By 2005, PROFInet-IRT and SRT will incorporate
PROFISafe, the PROFIbus safety solution for
manufacturing and processing industries.
PROFInet, of all the solutions discussed here offers
the greatest determinism—and since this is built into
the PROFInet-IRT device, the systems engineer is
spared from the burden of configuration to guarantee
RT communication.
EtherCAT
EtherCAT (Ethernet for Control Automation
Technology) is the motion-control RT solution from
Beckhoff. It can process 1000 I/Os in 30 µs [5], but
requires full-duplex. It can use copper or fiber optic
cables. EtherCAT is based on the master/slave principal
and can interoperate with normal TCP/IP-based
networks and other Ethernet-based solutions such as
PROFInet. It also supports any Ethernet topology,
including the bus.
The EtherCAT master processes RT data via
dedicated hardware and software (Beckhoff currently
use their PC-based TwinCAT OS and TwinCAT Y
driver). In the future, further variations will be
introduced that will also provide the same guarantees.
The current master prioritizes EtherCAT frames over
normal Ethernet traffic, which is transmitted in gaps.
The master controls traffic by initiating all transmissions.
The telegrams are standard Ethernet, and the data
field encapsulates the EtherCAT frame (an EtherCAT
header and one or more EtherCAT commands). Each
command contains a header, data and Working Counter
(WC) field. Each Ethernet telegram can contain many
EtherCAT commands—realizing a higher bandwidth
and more efficient use of the large Ethernet data field
size and header (see Figure 3). The standard Ethernet
CRC is used to verify message correctness.
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4
Ethernet Header
Data
Ethernet CRC
2
EtherCAT
Header
EtherCAT Command
2
10
Header
EtherCAT Command
Data
Working
Counter
Figure 3
EtherCAT Encapsulation
The EtherCAT master fully controls its slaves. Its
commands only elicit responses; slaves do not initiate
transmissions. The two EtherCAT communication
methods used are “Ether Type” or UDP/IP encapsulation.
The “Ether Type” uses the type field (defined in
Ethernet II), which is more commonly known as the
length field in IEEE 802.3. The Ether Type implementation
does not use IP, thus limiting EtherCAT traffic to the
originating subnet. Encapsulating commands using
UDP/IP allows EtherCAT frames to traverse subnets,
but has drawbacks. The UDP/IP header adds 28 (20:
IP, 8: UDP) bytes to the Ethernet frame and undermines RT performance through its non-deterministic
stack. EtherCAT slaves range from intelligent nodes to
2-bit I/O modules and are networked via 100Base-TX,
fiber optic cable or E-bus (depending on distance
requirements). E-bus is an EtherCAT physical layer for
Ethernet offering a LVDS (Low Voltage Differential
Signal) scheme. Slaves are hot pluggable in any
topology of branches or stubs. Multiple “slave rings”
can exist on a single network if connected by a switch.
EtherCAT slaves have integrated memory from
2 bits to 64 Kbytes. They appear to the Ethernet as a
single device though actually comprising up to 65,535
devices. They are configured in an open-ring topology,
with the Ethernet interface at the open end. Masters
transmit commands to the MAC address of the first
device. When the signal reaches the Ethernet/slave
interface, it is converted to E-bus specifications (if
E-bus is employed) and forwarded.
A slave receives a telegram, processes it (in
hardware) then forwards it to the next slave on the
ring. This processing delays the telegram by an order
of nanoseconds. The last slave returns the completed
telegram, via the ring, to the master. On the return
route, each slave amplifies and regenerates the signal.
Each slave has two Tx & Rx interfaces, so bi-directional
communication occurs without contention.
In each EtherCAT command, the WC
increments when a slave processes a
command addressed to it, allowing the
Start
master to determine if each addressed slave
Period
is exchanging data, although correct data is
not guaranteed.
The FMMU (Field Memory Management
Unit) of each configurable slave converts a logical
address to a physical one, and that information is
available to the master at initialization. Thus, each
slave needs a special ASIC. On telegram reception, a
slave determines if it is addressed and then passes
data to/from the telegram—incurring a delay of some
nanoseconds. EtherCAT is also internally synchronized
by a distributed clock algorithm (a simplified version
of IEEE 1588) although external synchronization is
achievable with IEEE 1588.
EtherCAT is a fast RT Ethernet solution and
deterministic if not used with UDP/IP or intermediate
switches or routers between master and slaves.
ETHERNET Powerlink (EPL)
EPL [6] is a hard-RT protocol based on Fast
Ethernet. Like EtherCAT, it uses the Ethernet II Frame
type field. EPL devices use standard Ethernet hardware
with no special ASICs. EPL can deliver a cycle time of
200 µs with jitter under 1 µs. Its frame is encapsulated
as illustrated in Figure 4.
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4
Ethernet Header
Data
Ethernet CRC
Ethernet II Frame
1
1
1
Service ID
Destination
Address
Source
Address
Data
Powerlink Frame
Figure 4—PowerLink Encapsulation
EPL uses cyclic communication with time-slot
division and the master/slave model. One master (manager) is allowed per network. The master schedules all
transmissions and is the only active station—slaves
transmitting on request.
The four EPL cycle subdivisions are illustrated in
Figure 5.
During the Start Period, the EPL master broadcasts
the “Start-of-Cyclic” (SoC) frame which synchronizes
the slaves. The timing of this frame provides the only
time base for the network synchronization: all other
frames are purely event-driven.
After transmitting the SoC frame, the Cyclic Period
occurs as the manager polls each station with a “Poll
Request” frame. Only then does the slave respond with
a “Poll Response” frame containing data— hence,
collisions are avoided. The slave broadcasts its
response to all devices; thus, inter-slave communication
can occur.
Powerlink Cycle
Cyclic (Isochronous) Period
Asynchronous Period
Idle Period
Figure 5—PowerLink Cycle
After successful polling of all slaves, the master
broadcasts the “End-of-Cyclic” (EoC) frame, informing
each slave that the cyclic traffic progressed correctly.
The Asynchronous Period allows non-cyclic data
transfers under master control. To transmit during this
period, a slave must have informed the master in its
“Poll Response” during the Cyclic Period. The
master builds a list of waiting slaves and employs a
scheduler to guarantee that no send request will be
delayed indefinitely. During the Asynchronous Period,
standard IP datagrams can be transferred.
Unlike PROFInet, EPL does not employ switches
to avoid collisions or to provide the network
synchronization; the master controls this. EPL networks
can be built with standard hubs. It is proposed that
each device incorporate a hub for ease of bus
implementation. Switches, although not prohibited, are
not recommended for EPL because they add jitter and
3
reduce determinism. Since the EPL network avoids
collisions via time-controlled bus access, up to 10 hubs
can be cascaded (an allowable exception to the 5-4-3
Ethernet rule).
Currently, EPL devices demanding RT communication
cannot co-exist on the same segment as non-RT
Ethernet devices. However, EPL devices can operate as
normal Ethernet devices. In Protected Mode, the RT
segment must be separated from normal traffic by a
bridge or router. In Open Mode, RT traffic shares the
segment with normal traffic, but RT communication is
compromised. In the next Powerlink version (V3), IEEE
1588 will be used to synchronize traffic across multiple
RT segments—providing a more distributed EPL
implementation, but true RT segments will still contain
only EPL devices. Unlike PROFInet where normal
Ethernet and RT devices can co-exist and not affect RT
traffic, EPL must be protected from non-RT communication through bridges or routers. Unlike PROFInet or
EtherCAT, which need special ASICs, EPL employs
standard Ethernet hardware.
REFERENCES
1 IEEE Std. 1588 -2002, “IEEE Standard for a Precision
Clock Synchronization Protocol for Networked
Measurement and Control Systems.”
2 Eidson, J. et al. “Synchronizing Measurement and
Control Systems.” Sensors, November 2002.
3 PROFInet available at http://www.profibus.com/
4 Baumann, G. “Solving the Compatible Real-Time
Ethernet Conundrum,” The OnLine Industrial Ethernet
Book, Issue 16, September 2003.
5 Beckhoff EtherCAT available at
http://www.beckhoff.com/english/default.htm?
ethercat/default.htm.
CONCLUSION
Real-Time Ethernet is a fast-growing, exciting
development of the Ethernet protocol. The ability to
have RT control segments running on the same
network as office applications brings many new
possibilities for industrial applications. With different
protocols offering different levels of RT service, it is
vital to understand the RT requirement of the system
before choosing a solution. Sub-microsecond
synchronization accuracy, with IEEE 1588, along with
an RT protocol can provide an Ethernet capable of
delivering hard, fast and deterministic RT for
applications such as motion control, while other
solutions cater for softer applications. Therefore, when
choosing RT Ethernet, it is vital to consider the
real-time, interoperability and flexibility requirements
of your system along with all possible solutions before
making a commitment.
www.ccontrols.com
Past issues of the Extension are available. If you would like a
copy, please send your request to [email protected]
4
6 ETHERNET Powerlink available at
http://www.ethernet-powerlink.org
Look for Paula Doyle’s complete article as Elective
IE402 in the Curriculum of the virtual Industrial
Ethernet University at www.industrialethernetu.com.
If you wish to contact Paula, her e-mail address is:
[email protected]
Paula Doyle is a doctoral
researcher with the Circuits
and Systems Research Centre
at the University of Limerick
in Ireland. She has a first
class honors B. Engineering
degree in Computer
Engineering from the
University of Limerick. Her
research interests include
embedded real-time systems, with emphasis on control
networks for smart transducers with applications in the
field of industrial automation. Paula is currently in the
final stages of her Ph.D. research, based on the timetriggered transducer clusters in an Ethernet
networking environment.
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