Semantic Error: Read-only variables cannot be assigned to VAR_IN_OUT parameters.

In a code worksheet, a variable has been assigned to a VAR_IN_OUT parameter ('Usage = InOut') of a function block which is

a read-only system variable
or assigned to an input terminal.

This is not allowed. Replace this read-only variable by a variable which can be written by the FB.

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. Writing a read-only variable, however, is not possible. Thus, only writeable variables are allowed at in/output parameters.

Further Info
Please refer to the user manual of the controller for further information on system variables.