General information
According to IEC 61131-3, program organization units or POUs are the language elements of a controller program. They are small, independent software units containing the program code. The name of a POU must be unique within the project.
In EcoStruxure Machine Expert - Safety, two types of POUs are available: One program and a configured number of user-defined function blocks (FBs).
Each POU consists of a declaration part (local variables worksheet) containing the declarations of the local variables (used only within the POU) and a code part containing the instructions to be processed.
Further Information
Information about opening and managing the different worksheets can be found in the topic "Project tree - overview".
Function block POUs
Function blocks are POUs with multiple input/output parameters and internal memory. The value a function block returns depends on the value of its internal memory. Within a function block it is possible to call other function blocks or functions but not programs. Recursive calls are not allowed. The abbreviation for function blocks is FB.
Like IEC 61131-defined FBs and Safety Logic Controller-specific FBs (contained in a library), user-defined FB POUs are available in the Edit Wizard after editing, saving, and compiling the corresponding worksheets. This way, the call of a user-defined FB can be easily inserted into the code of the calling POU via drag & drop and declaring an instance name. Refer to the topic "Inserting function blocks using the Edit Wizard" for details.
Calling an FB in another POU is designated as instantiation.
Program POUs
A program POU usually contains a logical combination of function/function block calls. The behavior and use of programs are similar to function blocks. Programs have an internal memory. Programs cannot be called from other POUs. In EcoStruxure Machine Expert - Safety only one program is allowed in a project. This program is automatically inserted while creating a new project. Its predefined name 'Main' cannot be modified and the program can neither be copied nor deleted.
Function POUs
According to IEC 61131 standard, a function is a POU with multiple input parameters and exactly one output parameter (return value). As functions do not have an internal memory, calling a function with the same values always returns the same result. In EcoStruxure Machine Expert - Safety, some basic functions are provided in libraries. However, no user-defined function POUs can be created.
