Glossary · POU
What is Program organization unit (POU)?
The unit every piece of PLC code lives in. The standard defines three: PROGRAM, FUNCTION and FUNCTION_BLOCK.
The real difference is memory: a FUNCTION remembers nothing between calls, a FUNCTION_BLOCK keeps memory per instance, and a PROGRAM is the top level bound to a task.
Splitting a project into small POUs with one clear responsibility each is the single biggest factor in how maintainable plant code turns out.
Related terms
- Function block (FB)
- A unit with inputs, outputs and internal memory that persists between scans, used by declaring an instance of it.
- PROGRAM
- The top program organization unit: bound to a task, executed cyclically, and keeping its variables between scans.
- IEC 61131-3
- The international standard that unified PLC programming: five languages, the data types, the program organization units and the standard function blocks.