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

55-10 Industrial Communication Systems
INIT
REQ
INITO
INITO
CNF
INIT
RSP
IND
PUBLISH_m
SUBSCRIBE_m
QI QO
QI QO
PARAMS STATUS
PARAMS STATUS
SD_1
RD_1
...
...
SD_m
RD_m
Network
FIGURE 55.7
Publish and subscribe CIFBs.
between blocks which are mapped to the same device are preserved. The connections crossing the device
boundaries are appended by communication FBs.
55.6 Communication FBs
In IEC 61499, communication is implemented via a particular case of service interface FBs called CIFBs.
The standard explicitly refers to two generic communication patterns: PUBLISH/SUBSCRIBE for unidirectional
transactions and CLIENT/SERVER for bidirectional communication. These patterns can be
adjusted to a communication mechanism of a particular implementation. Otherwise, providers of communication
hardware/software can specify their own patterns if they differ from the above-mentioned
ones. Figure 55.7 illustrates the generic PUBLISH and SUBSCRIBE FBs performing unidirectional data
transfer via a network.
The PUBLISHER serves for publishing data SD_1, …, SD_m that come from one or more FBs in
the application. It is, therefore, initialized/terminated by the application. Upon a request event from the
application at the REQ event input, the data that need to be published are sent by the PUBLISHER via an
implementation-dependent network. When this is done, the PUBLISHER informs the publishing application
via the event output CNF.
The SUBSCRIBER FB is initiated by the application. The application reads the data RD_1, …, RD_m.
Normal data transfer is initiated by the sending application via the REQ input event to the PUBLISHER.
This is illustrated in Figure 55.8 by means of time-sequence diagrams. The PUBLISHER sends the data
and triggers the IND event at the outputs of the SUBSCRIBER to notify the reading applications that
new values of data are available at the RD_1, …, RD_m outputs of the SUBSCRIBER. The reading application
can notify the SUBSCRIBER by the RSP event that the data have been received.
55.7 Communication Using Services of Internet Protocol Suite
The generic communication patterns of the standard were further specified in the “IEC 61499
Compliance Profile for Feasibility Demonstrations” [1] for implementations using services of the
Internet protocol suite.
© 2011 by Taylor and Francis Group, LLC