wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

55-20 Industrial Communication Systems
with lower guarantees are sufficient. For example, when switching on a siren to warn of the occurrence
of an alarm, the “at least once” semantics would be sufficient.
Another issue is that of event ordering. If two events are generated inside one device in the order EV1,
EV2, these are not guaranteed to arrive at another device in the same order. If an application relies on
the ordering of events to correctly execute its algorithms, care should be taken to choose communication
service interface FBs whose implementation guarantees message delivery in the same order.
Some implementations of the communication service interface FBs may allow quality of service
(QoS) parameters to be specified. Others may have fixed QoS parameters. In conclusion, one should be
careful of the exact semantics implemented by the service interface FBs as these may affect the outcome
of the IEC 61499 control application.
55.12 Failures in Distributed Applications
Although distributed applications may have many advantages, they are not without their drawbacks.
One of these, as was discussed previously, is the fact that the communication network may exhibit
failures that will affect the outcome of the application execution. Another is the failure of the devices
executing the control logic itself.
When writing an industrial strength control application, usually all failure modes of the controlled
process must be taken into account, so that the physical machinery under control always remains in a
safe state. Likewise, the failure of the controlling devices, i.e., the PLCs executing the logic and the communication
networks linking them, must also be taken into account.
For applications that run on a single PLC, usually either the control application fails completely when
the executing PLC fails or it does not fail at all. Complete failures of the control application are usually
handled by external safety devices.
However, when writing a distributed application, one must be careful of how this application may fail,
as it may have partial failures when one or more, but not all, of the PLCs that are executing the distributed
control application fail. Even more likely is the possibility of failure of the communication network
that connects the distributed PLCs.
Now, to overcome partial failures of a distributed application, the application itself must be aware of
this scenario and be prepared to take appropriate action when it occurs. However, in order to do so, it
must first be able to identify the fact that a partial failure occurred (be it on the communication network
or another executing device). This identification may be done either at the application level or by the
communication service interface function blocks (SIFBs).
IEC 61499 communication SIFBs typically indicate the presence of failures by emitting local events
while their QO output is set to false. However, as was stated previously, the capacity of a communication
SIFB to detect the failure and consequently emit the event depends on the underlying communication protocol.
For example, some protocols include “keep alive” messages that are exchanged even when no data
need to be transmitted so that the lack of these messages may be safely assumed to be the result of a failure.
When communication SIFBs are not able to identify failures, then these will need to be identified at
the application level. One simple means of doing this is to configure the control applications to periodically
call the PUBLISH and/or CLIENT (or SEND, as in our example) SIFB, even when no new data
need to be transmitted. Another possibility is the implementation of handshakes and timeouts at the
application level.
55.13 Conclusion
The FBs of IEC 61499 are a powerful architectural model for abstract yet executable specification of
distributed control systems. It can be further extended by adding communication over any particular
network protocol. Interfaces to the protocol need to be implemented in libraries of CIFBs. Advanced
software tools can make adding the explicit communication seamless and hidden from the user.
© 2011 by Taylor and Francis Group, LLC