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

Communication Aspects of IEC 61499 Architecture 55-15
FIGURE 55.12
IEC 61499.
The network topology of our system described as a part of distributed system configuration of
55.9 Internals of Communication FBs: Modbus
Modbus (described in Chapter 4.12 of this Handbook and in [20–24]) is one of the many communication
protocols that may be used for transferring events and data over a network. As this protocol is
based on a remote memory data model which is accessed and updated using the client–server interaction
model, it is naturally implemented in IEC 61499 as CLIENT and SERVER communication service
interface FBs illustrated in Figure 55.14.
The Modbus protocol includes three versions, namely RTU, ASCII, and TCP. The first two use a different
method of byte encoding on the same physical media (RS232 or RS485), while the last transmits
the messages over TCP/IP connections. Due to these variations of the protocol, one SERVER and one
CLIENT FB type are needed for each version.
A description of a typical data and event transfer sequence is as follows:
• The device containing the RCV FB (the server device) initializes this server (INIT+ event, i.e.,
INIT event with QI input set to TRUE) while simultaneously providing configuration parameters.
For the TCP version, these consist of the local IP addresses and port numbers on which it will listen
awaiting for the arrival of connection requests from clients. For the RTU and ASCII versions,
the parameters are the identification of the hardware interface (e.g., the serial port) to use as well
as the serial line parameters (baud rate, number of start and stop bits, number of bits, and parity).
All versions also require an additional parameter, the Modbus device ID.
• The device containing the SEND (client device) initializes this client (INIT+ event). Required
parameters include the serial line interface details (for RTU and ASCII versions) and the Modbus
device ID of the remote server with which it will communicate. For the TCP version, the IP address
© 2011 by Taylor and Francis Group, LLC