PLCOPEN-CP17: Usage of parameters shall match their declaration mode

The way a parameter is used inside a POU body shall agree with how it was declared: inputs are read, outputs are written, and IN_OUT parameters are either read or written.

Why is it bad?

A parameter's declaration mode is a contract between the POU and its callers. Reading an output before it has been assigned, or writing to an input, produces behaviour that depends on the runtime's parameter-passing strategy — some implementations copy on entry, some pass by reference, some inline the call. Code that "works" under one PLC vendor silently breaks under another. Declaring a parameter you never use is also dead weight in the POU interface and forces every caller to supply a value that has no effect.

The rule applies four checks per POU:

• Each VAR_INPUT parameter shall be read at least once.
• Each VAR_INPUT parameter shall not be written.
• Each VAR_OUTPUT parameter shall be written at least once.
• Each VAR_IN_OUT parameter shall be either read or written.

Example

FUNCTION_BLOCK Compute
  VAR_INPUT
    threshold : INT;            (* never read     — PLCOPEN-CP17 *)
    factor    : INT;            (* written below  — PLCOPEN-CP17 *)
  END_VAR
  VAR_OUTPUT
    status : INT;               (* never written  — PLCOPEN-CP17 *)
  END_VAR
  VAR_IN_OUT
    counter : INT;              (* never used     — PLCOPEN-CP17 *)
  END_VAR

  factor := factor + 1;
END_FUNCTION_BLOCK

Use instead:

FUNCTION_BLOCK Compute
  VAR_INPUT
    threshold : INT;
    factor    : INT;
  END_VAR
  VAR_OUTPUT
    status : INT;
  END_VAR
  VAR_IN_OUT
    counter : INT;
  END_VAR

  IF factor > threshold THEN
    status := 1;
    counter := counter + 1;
  ELSE
    status := 0;
  END_IF;
END_FUNCTION_BLOCK

Resources

1. PLCOpen Software Construction Guidelines