SEMA

VAR_INPUT

VAR_INPUT
    CLAIM   : BOOL; 
    RELEASE : BOOL;
END_VAR

VAR_OUTPUT

VAR_OUTPUT
    BUSY    : BOOL;
END_VAR

If BUSY is TRUE when SEMA is called up, this means that a value has already been assigned to SEMA (SEMA was called up with CLAIM = TRUE). If BUSY is FALSE, SEMA has not yet been called up or it has been released (called up with RELEASE = TRUE).

Internal implementation of fb:

BUSY := X;

IF CLAIM THEN
    X:=TRUE;
ELSIF RELEASE
    THEN
        BUSY := FALSE;
        X:= FALSE;
END_IF

Requirements