Semantic Error: VAR_IN_OUT parameter '...' must not be unassigned.
In an ST code worksheet, a VAR_IN_OUT parameter ('Usage = InOut') of a function block is not assigned to any variable. This is not allowed. Assign a variable to correct the error.
A formal parameter declared with 'Usage = InOut' is not a "real variable" but a reference to another variable. It can be compared to a pointer that points to the address of another variable.
This referenced variable is read and written by the FB. Thus, in/output parameters must not be unassigned.