Previously valid PLC standards focussing mainly on PLC programming were generally used in Europe at end of the seventies. This included non-networked PLC system, which primarily execute logic operations on binary signals. DIN 19 239, for example, specifies programming language which possess the corresponding language commands for these applications.
Since 1992, an international standard now exists for programmable logic controllers and associated peripheral devices like programming and diagnostic tools, testing equipment, man to machine interfaces.
In 1992, IEC 1131 standards were developed as an open framework for PLC architecture. The second edition of IEC 1131 ( Known as IEC 61131) was published in 2003.
The new IEC standard consists of five parts
- Part 1 : General information
- Part 2 : Equipment requirement and tests
- Part 3 : programming languages
- Part 4 : User guidelines in preparation with IEC
- Part 5: Messaging service specification
Parts 1 to 2 of this standard were adopted unamended as European standard EN 61 131, parts 1 to 2. As such, they also hold the status of a German standard. Part 3 of new IEC deals with programming languages and defines two graphical and two textural PLC programming language standards.
The standard also defines both graphical and textual sequential function chart elements to organise programs for sequential and parallel control processing. It is now possible to program PLC using following languages
- IL- Instruction list
- ST – structural text
- LD – Ladder diagram
- FBD - Functional block diagram
- SFC- Sequential function chart
The purpose of the new standard was to define and standardize the design and functionality of a PLC and the languages required for programming to the extent where users were able to operate using different PLC systems without any particular difficulties.
Large number o f major PLC suppliers are members of association called PLCopen which supports IEC 1131. Allen Bradley, Klockner–Moeller, Phillips Siemens or Mitsubishi to mention a few.