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PLCOPEN-CP1: Access to a member shall be by name

Direct addressing should not be used when a symbolic name exists.

Why is it bad?​

Directly addressing a memory location like %I0.0 or %MW12 when a symbolic name has already been declared for it makes the code unmaintainable: the symbolic name is lost at the use site, any rewiring of the underlying PLC hardware requires hunting through the source for hardcoded addresses, and static analysis tools cannot follow the data flow.

This rule fires when a directly-addressed location is also declared with a symbolic name in a VAR block — the symbolic name should be used instead.

Example​

PROGRAM program0
  VAR
    head AT %B0 : INT;
  END_VAR
  %B0 := 42;
END_PROGRAM

Use instead:

PROGRAM program0
  VAR
    head AT %B0 : INT;
  END_VAR
  head := 42;
END_PROGRAM

Resources​

  1. PLCOpen Software Construction Guidelines