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PLCOPEN-CP13: POUs shall not call themselves directly or indirectly

Recursion is forbidden in IEC 61131-3 — rewrite as a loop.

Why is it bad?​

The IEC 61131-3 execution model has no call stack in the way a general-purpose language does: function activation records live in statically-allocated memory, sized at compile time. A recursive call would either run out of memory in seconds or — worse — silently scribble over neighbouring variables. PLCOpen forbids both direct and indirect recursion outright.

Any algorithm expressible as recursion can be expressed iteratively with an explicit accumulator and a FOR or WHILE loop.

Example​

FUNCTION Factorial : INT
  VAR_INPUT
    X : INT;
  END_VAR
  IF X > 1 THEN
    Factorial := Factorial(X - 1) * X;     (* PLCOPEN-CP13 *)
  ELSE
    Factorial := X;
  END_IF;
END_FUNCTION

Use instead:

FUNCTION Factorial : INT
  VAR_INPUT
    X : INT;
  END_VAR
  VAR
    Acc : INT := 1;
    I   : INT;
  END_VAR
  FOR I := 2 TO X DO
    Acc := Acc * I;
  END_FOR;
  Factorial := Acc;
END_FUNCTION

Resources​

  1. PLCOpen Software Construction Guidelines