E037: Invalid assignment

This error is reported for assignments or accesses that are not allowed, for example when the types of the left- and right-hand side are incompatible, when a VOID function result is assigned to a variable, or when a variable is written or read from a place where it must not be accessed.

Example - incompatible types

FUNCTION main
VAR
    x : INT;
    s : STRING;
END_VAR
    x := s; (* cannot assign 'STRING' to 'INT' *)
END_FUNCTION

Example - VAR_OUTPUT assigned outside of its scope

VAR_OUTPUT variables of a function block may only be written by the function block itself. From the outside they can only be read.

FUNCTION_BLOCK FB
VAR_OUTPUT
    out : BOOL;
END_VAR
END_FUNCTION_BLOCK

PROGRAM main
VAR
    fb : FB;
END_VAR
    fb.out := TRUE; (* VAR_OUTPUT variables cannot be assigned outside of their scope *)
END_PROGRAM

Example - VAR_IN_OUT accessed outside of its scope

VAR_IN_OUT variables of a function block or program may not be read or written from outside. They are references that are only bound to a target while the function block is being called, so any outside access - reading, writing or taking the address - would dereference an unbound reference. Pass the variable in the call instead.

FUNCTION_BLOCK FB
VAR_IN_OUT
    inOut : LREAL;
END_VAR
END_FUNCTION_BLOCK

PROGRAM main
VAR
    fb : FB;
    value : LREAL;
END_VAR
    fb.inOut := value;  (* VAR_IN_OUT variables cannot be accessed outside of their scope *)
    value := fb.inOut;  (* VAR_IN_OUT variables cannot be accessed outside of their scope *)
    fb(inOut := value); (* correct: the variable is passed in the call *)
END_PROGRAM