Program Organization Unit (POU)

Functions, function blocks, and programs are called program organization units, or POU, in EN 61131-3.

There are three basic POU types:

• function
• function_block
• program

Note: Parameters of functions, e.g. when multiplying an input value by a constant at the MUL function, are declared as constants in the code. They can not be changed online. A constant can only be changed by editing the source program and uploading the project. If these parameters shall be changed online, the function inputs must be visible in the program, and connected to global variables. 

Every POU has two main parts: declarative and executive. In the declarative parts of a POU, variables necessary for the POU activity are defined. The executive part then contains the commands necessary for the execution of the required algorithm.