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

Communication Aspects of IEC 61499 Architecture 55-17
Event
INIT INITO
REQ CNF
INIT INITO
IND
Data
ID
SEND_1
SD_1
ID
RCV_1
RD_1
FIGURE 55.14
Producer–consumer pair implemented in MODBUS.
and port number of the remote server is also required, and this INIT+ event will result in the client
establishing a TCP connection with the RCV FB. The SEND FB, upon receiving an REQ+ event,
sends the data (16-bit words) available on its SD_x input ports and simultaneously asks for the
return data, using the “read/write multiple registers” Modbus function (function code 23-17 hex).
• The RCV FB, upon receiving the message sent by the client, places the received data on its RD_x
data output ports and emits an IND+ event.
• The server device must then make the return data available on the RCV SD_x input ports and
emit the RSP+ event. Upon receiving this event, the RCV FB then replies to the “read/write multiple
registers” request. The SEND FB, upon receiving the reply message, places the received data
on its RD_x output ports and emits the CNF+ event.
If the RSP+ event is delayed beyond a certain timeout, the SEND FB will emit an RSP− event.
Since the IEC 61499 FBs do not use the same remote memory data model as the Modbus protocol,
with the mapping proposed above it is not necessary to specify the Modbus register addresses. These are
assumed to always start at 0, and the number of registers to read (write) is the same as the number of data
outputs (inputs) on the SEND FB. Note that the number of input words of the SEND FB must match the
number of output words in the RCV FB, and vice versa.
Similar SEND and RCV FBs are used for the transmission of Boolean data. However, since the
Modbus protocol does not include a function that allows simultaneous reading and writing of Boolean
data, these must only send data either from the SEND FB to the RCV FB (using the Modbus function
“Write Multiple Coils,” code 15-0F hex) or from the RCV FB to the SEND FB (using the Modbus function
“Read Coils,” code 01-01 hex).
The above communication assumes that the server device also supports IEC 61499. However, currently
many remote terminal units (RTUs) support the Modbus protocol, but do not implement IEC
61499. To allow a controller device to communicate with these legacy RTUs, SEND FBs are used that
connect directly to these legacy Modbus server devices. These RCV FBs require additional parameters
when the function is initialized in order to specify the Modbus function, data address, and, when reading
data, the number of registers to use when transferring data to/from the legacy Modbus server.
55.10 Communication via the CIP
In this section, we show how another communication network, based on CIP, can be used in our
example. CIP has originally been developed as application layer for Rockwell Automation’s CAN-based
DeviceNet and their controller network ControlNet. With the development for the industrial Ethernet
EtherNet/IP and the sensor actuator bus CompoNet, the CIP specification was further unified and provides
a common application layer for these four networks. CIP is a multimaster system and therefore
well-suited for the application in a distributed control system.
Interface of IEC FBs for CIP was defined in the compliance profile [25]. More implementation details
can be found in [26,27].
© 2011 by Taylor and Francis Group, LLC