Overview

So far we have only considered part 3 of the IEC 1131 PLC standard. As we have seen, IEC 1131-3 addresses all aspects related to programming PLC based control systems. However, it does not address communications between PLCs.

This chapter reviews part 5 of the IEC 1131 standard, which focuses solely on PLC communications. In this chapter we will review:

• The main scope of the IEC 1131-5 standard;

• The communications model;

• The PLC functional model;

• Accessing PLC status information;

• Communications function blocks;

• Mapping of communications function blocks on to MMS.

12.1 Background to IEC 1131 part 5

The only feature defined in the IEC 1131-3 standard that goes any way towards providing inter-PLC communications is the VAR_ACCESS construct. This simply defines access paths to named variables within the PLC application software. IEC 1131-3 does not define how the communications facilities are provided or controlled. The PLC communications model showing the use of the VAR_ACCESS construct is discussed in Chapter 2, Section 2.

Part 5 of the IEC 1131-3 standard is concerned with facilities that can allow PLCs that are connected via a communications network to exchange status and control information and initiate executive commands, such as re-starting resources and downloading configurations.

IEC 1131-5 is concerned with all the external communications...