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

Communication Aspects of IEC 61499 Architecture 55-11
PUBLISHER
SUBSCRIBER
INIT(+)
PARAMS
normal_service_establishment
INITO(+)
~
INIT(+)
INITO(+)
PARAMS
normal_data_transfer
REQ(+)
SD_1, ..., SD_m
IND(+)
RD_1, ..., RD_m
~
CNF(+)
RSP(+)
FIGURE 55.8
Communication establishment and normal data transfer sequence.
Provisions of the compliance profile were implemented, for example, in FBDK [2] as the pairs of
PUBLISH_n/SUBSCRIBE_n and CLIENT_n_m/SERVER_n_m FBs. The PUBLISH/SUBSCRIBE communication
is implemented using services of the universal datagram protocol (UDP). The communication
is essentially unreliable and multicast. The CLIENT/SERVER communication is implemented via
the transfer control protocol (TCP). It is point-to-point and reliable.
Using this library of communication FBs, a user can develop distributed applications communicating
via local networks (Ethernet) and even via the Internet. The specifics of the implementation
imply the addressing schema. The communication FBs have input parameter IDs, which are set as
follows.
The PUBLISH/SUBSCRIBE communication pattern assumes that there could be many subscribers
for one (or even several) publisher(s) as illustrated in Figure 55.9. This communication pattern is implemented
in FBDK using the multicast services of the UDP.
The PUBLISH block has to have an ID that follows the rules for Internet multicast addresses. In general,
an Internet address consists of four numbers in the interval 0-255; for example, 124.11.2.1. (The
multicast addresses are in the interval 224.0.0.0 through 239.255.255.255, but the range of addresses
between 224.0.0.0 and 224.0.0.255, inclusive, is reserved for system purposes.) In the example, the ID of
the PUBLISHER (located in Device 1) is 225.0.0.1:1204 (where 225.0.0.1 is an IP address for that multicast
group and 1204 is a unique port number selected in Device 1). The ID of the publisher completely
identifies the multicast group.
Any SUBSCRIBE block located in a device on the same network segment as the PUBLISH can receive
the published data if it has the same ID as the PUBLISH. The published data will be received by both
subscribers located in Device 2 and Device 3.
The CLIENT_SERVER communication is point-to-point (i.e., only one client can send and receive
data to/from one server). The addressing is illustrated in Figure 55.10. The SERVER, which is located
in Device 2, has ID that refers to the address of the host device (localhost) and adds there a unique (for
that device) port number. The CLIENT, located in Device 1, has an ID that is composed of two parts: IP
address of the server’s device and the same port number as used in the SERVER’s ID.
© 2011 by Taylor and Francis Group, LLC