Glossary · IEC 61131-3
What is IEC 61131-3?
The international standard that unified PLC programming: five languages, the data types, the program organization units and the standard function blocks.
Before the standard, every vendor had its own language and concepts. IEC 61131-3 settled on five languages: ladder, FBD, SFC, Structured Text and Instruction List (dropped in the third edition).
It also fixed the data types such as BOOL, INT, REAL and TIME, the standard blocks such as TON and CTU, and the idea of an instance that gives every timer its own memory. The third edition (2013) added object orientation: METHOD, INTERFACE and EXTENDS.
Related terms
- Structured Text (ST)
- A textual programming language in IEC 61131-3 resembling Pascal, used for complex PLC logic, calculations and state machines.
- Ladder logic (LD)
- A graphical PLC language drawn like relay circuits: rails, contacts and coils. The most widely used language in maintenance.
- Program organization unit (POU)
- The unit every piece of PLC code lives in. The standard defines three: PROGRAM, FUNCTION and FUNCTION_BLOCK.