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PLCOPEN-CP28: Time and physical measures comparisons shall not be equality or inequality

Use range comparisons instead of = / <> when comparing TIME values.

Why is it bad?​

TIME values come from PLC scan-cycle counters, network timestamps and operator inputs — sources where the exact value almost never lands on a precise literal like T#100MS. Comparing for equality means the TRUE branch fires only when the cycle happens to land on that exact tick, which on a real PLC is roughly never.

The fix is to use a range comparison (>= / <= / < / >) that captures the intent ("after 100 ms have elapsed") instead of an exact match.

Example​

PROGRAM demo
  VAR x : TIME; END_VAR
  IF x = T#100MS THEN          (* PLCOPEN-CP28 *)
    x := x + 1;
  END_IF;
  IF x <> T#100MS THEN         (* PLCOPEN-CP28 *)
    x := x + 1;
  END_IF;
END_PROGRAM

Use instead:

PROGRAM demo
  VAR x : TIME; END_VAR
  IF x >= T#100MS THEN
    x := x + 1;
  END_IF;
END_PROGRAM

Resources​

  1. PLCOpen Software Construction Guidelines