Functions

Functions are program organization units (POUs) having one or more input parameters and one output parameter. A function can be called by a program, a function or a function block. A function has no instance meaning that local data is not stored and is usually lost from one call to the other.

The execution of a function is driven by its parent program. Therefore, the execution of the parent program is suspended until the function ends:

Any POU of any section can call one or more functions. A function can have local variables.

Recursivity is not supported during function calls. When a function of the Functions section is called by itself or one of its called functions, a run-time error occurs. Furthermore, functions do not store the local values of their local variables. Since functions are not instantiated, these cannot call function blocks.

The interface of a function must be explicitly defined with a type and a unique name for each of its calling (input) parameters or return (output) parameter. A function has one return parameter. The value of the return parameter for a function differs for the various programming languages.

Functions within a project and the global library must have unique names within a scope. Function names and function parameter names are limited to 128 characters beginning with a letter followed by letters, digits, and single underscores. The last character for a function name must be a letter or digit; function names cannot end with an underscore character. Reserved words, defined words, or data types, such as elementary, structures, or arrays are not valid names.