The IEC-1131 Standard for Programmable Controllers is divided into five parts, namely,
Part 1 General Information (IS)
Part 2 Equipment and Test Requirements (IS)
Part 3 Programming Languages (IS)
Part 4 User Guidelines (CD)
Part 5 Messaging Services (CD)
The Part 3 document was prepared by Task Force 3 (Programming Languages) of Working Group 7 (Programmable Controllers) of IEC Subcommittee 65B (formerly SC65A/WG6)
The programming methods of PLCs are application-oriented and thus by uniform programming in accordance to the IEC 1131-3, it enables shorter retraining and familiarisation times, clearer interface definitions and fewer errors as a result of standard function blocks. The standard also makes the structuring of a particular system environment easier for programmers by breaking down the system into smaller manageable sub-units.
The entire control system can be formulated into a configuration, or a number of configurations, which is linked by defined interfaces. A configuration describes a PLC system that can communicate with other PLC systems and may itself be made up of a number of resources, with each resources administering several tasks. These tasks can be assigned different priorities or repetition intervals. These individual tasks in turn consist of programs which are also structured with functions and function blocks.