P4082¶

Code: P4082
Message: VAR_EXTERNAL declaration matches no global variable

This error occurs when a VAR_EXTERNAL declaration does not match a global variable. A VAR_EXTERNAL declaration gives a program or a function block access to a global variable. It declares no variable of its own, so it must name a global variable and declare it with that variable’s type.

The compiler reports this error in two cases:

No global variable has the name the declaration uses.
The type the declaration uses is not the type of the global variable.

Two declarations have the same type when they name the same type. A type and an alias of that type are different types. Two arrays written out in place have the same type when they have the same bounds and the same element type.

The compiler reports this error on every dialect.

Example¶

The following code will generate error P4082:

Try it

Source

CONFIGURATION config
  VAR_GLOBAL
    MaxSpeed : INT := 100;
  END_VAR
  RESOURCE res ON PLC
    TASK plc_task(INTERVAL := T#100ms, PRIORITY := 1);
    PROGRAM plc_task_instance WITH plc_task : main;
  END_RESOURCE
END_CONFIGURATION

PROGRAM main
  VAR_EXTERNAL
    MaxSpeed : DINT;  (* Error: the global variable is an INT *)
    MinSpeed : INT;   (* Error: no global variable is named MinSpeed *)
  END_VAR
  VAR
    currentSpeed : DINT;
  END_VAR
  currentSpeed := MaxSpeed;
END_PROGRAM

MaxSpeed is declared INT as a global variable and DINT in the program. No VAR_GLOBAL block declares MinSpeed.

To fix this error, declare each VAR_EXTERNAL with the name and the type of its global variable, and declare the global variable when it is missing:

Try it

Source

CONFIGURATION config
  VAR_GLOBAL
    MaxSpeed : INT := 100;
    MinSpeed : INT := 10;
  END_VAR
  RESOURCE res ON PLC
    TASK plc_task(INTERVAL := T#100ms, PRIORITY := 1);
    PROGRAM plc_task_instance WITH plc_task : main;
  END_RESOURCE
END_CONFIGURATION

PROGRAM main
  VAR_EXTERNAL
    MaxSpeed : INT;
    MinSpeed : INT;
  END_VAR
  VAR
    currentSpeed : DINT;
  END_VAR
  currentSpeed := MaxSpeed;
END_PROGRAM

Checking One File of Several¶

The compiler checks the files you give it as one unit. A file that declares a function block with a VAR_EXTERNAL reports this error when you check it without the file that declares the global variable. Check the files together:

ironplcc check counter.st configuration.st

Think IronPLC is wrong about this?

If you believe this diagnostic is incorrect, open an issue on GitHub with a small sample that demonstrates the problem.