P4054

Code

P4054

Message

Function declares or invokes a function block instance

This error occurs when a function declares a function block instance, invokes one, or calls a method on one. IEC 61131-3 defines a call hierarchy: a program may invoke functions and function blocks, a function block may invoke functions and function blocks, and a function may invoke only functions. A function has no state between calls, and a function block instance is state.

Both the declaration and every invocation of the instance are reported, so the cause and each call site are marked.

Example

The following code will generate error P4054:

FUNCTION Delayed : BOOL
    VAR
        timer : TON;   (* Error: a function block instance in a function *)
    END_VAR
    timer(IN := TRUE, PT := T#1s);   (* Error: invoked in a function *)
    Delayed := timer.Q;
END_FUNCTION

Delayed would have to keep timer running between calls, which a function cannot do.

To fix this error, make the unit that needs the state a function block:

FUNCTION_BLOCK Delayed
    VAR_OUTPUT
        Q : BOOL;
    END_VAR
    VAR
        timer : TON;
    END_VAR
    timer(IN := TRUE, PT := T#1s);
    Q := timer.Q;
END_FUNCTION_BLOCK

or keep the function stateless by computing from its inputs alone.

Instances passed by reference

IEC 61131-3 Edition 3 allows a function to receive a function block instance through VAR_IN_OUT. The instance belongs to the caller, so the function stays stateless, and such a declaration is not reported. Note that IronPLC does not yet accept VAR_IN_OUT parameters on functions in calls, so this exemption matters only once that support lands. A VAR_INPUT instance is reported: passing an instance by value copies its state into the function.

See Also

  • FUNCTION — FUNCTION declaration syntax

  • FUNCTION_BLOCK — the stateful unit to use instead

  • P4012 — invocation of a variable that is not a function block instance

References

  • IEC 61131-3 §2.5.1 (Edition 2), §6.6.1 (Edition 3)

Think IronPLC is wrong about this?

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