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PLCOPEN-N1: Avoid physical addresses

Hardcoded physical addresses should not appear in executable code.

Why is it bad?​

Hardwiring %I0.0, %MW10.2.4.1 or any other directly represented variable in the body of a POU makes the program fragile and unmaintainable. Whoever reworks the PLC topology has to hunt every literal address through the code, data flow analysis cannot follow the signal, and renaming the symbolic alias is useless because the real users bypass it. Physical addresses belong in VAR blocks via an AT %... clause; the body of the POU should only reference the symbolic name.

Example​

PROGRAM program0
  VAR
    head AT %MW10.2.4.1 : INT;
  END_VAR
  %MW10.2.4.1 := 42;   (* PLCOPEN-N1 *)
END_PROGRAM

Use instead:

PROGRAM program0
  VAR
    head AT %MW10.2.4.1 : INT;
  END_VAR
  head := 42;
END_PROGRAM

Resources​

  1. PLCOpen Software Construction Guidelines