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PLCOPEN-N5: Local names shall not shadow global names

A local variable declaration must not reuse a name that is already declared at global scope.

Why is it bad?​

When a local variable in a POU has the exact same name as a VAR_GLOBAL entry, every reader has to work out which one a statement touches — and every maintainer risks picking the wrong one. The symptoms are quiet: the program compiles and the wrong value simply propagates. Rename either side; the two scopes should not fight over the name.

Example​

CONFIGURATION config
  VAR_GLOBAL
    MyCalculation : REAL;
  END_VAR
  ...
END_CONFIGURATION

FUNCTION_BLOCK Example
  VAR
    MyCalculation : REAL;  (* PLCOPEN-N5 — shadows the global *)
  END_VAR
  MyCalculation := 0.0;
END_FUNCTION_BLOCK

Use instead:

FUNCTION_BLOCK Example
  VAR
    localResult : REAL;
  END_VAR
  localResult := 0.0;
END_FUNCTION_BLOCK

Resources​

  1. PLCOpen Software Construction Guidelines