Bistable function blocks (latches)

Both SR and RS are set/reset latches with one Boolean output Q1. They differ in which input wins when set and reset are asserted at the same time.

SR — set-dominant

Direction	Name	Type	Meaning
Input	S1	BOOL	Set (dominant)
Input	R	BOOL	Reset
Output	Q1	BOOL	Latch output

When both S1 and R are TRUE, set wins and Q1 stays TRUE.

FUNCTION_BLOCK SR
VAR_INPUT
    S1 : BOOL;
    R  : BOOL;
END_VAR
VAR_OUTPUT
    Q1 : BOOL;
END_VAR
END_FUNCTION_BLOCK

VAR
    latch : SR;
END_VAR

latch(S1 := startCmd, R := stopCmd);
running := latch.Q1;

RS — reset-dominant

Direction	Name	Type	Meaning
Input	S	BOOL	Set
Input	R1	BOOL	Reset (dominant)
Output	Q1	BOOL	Latch output

When both S and R1 are TRUE, reset wins and Q1 becomes FALSE.

FUNCTION_BLOCK RS
VAR_INPUT
    S  : BOOL;
    R1 : BOOL;
END_VAR
VAR_OUTPUT
    Q1 : BOOL;
END_VAR
END_FUNCTION_BLOCK

VAR
    safeLatch : RS;
END_VAR

safeLatch(S := startCmd, R1 := emergencyStop);
running := safeLatch.Q1;   // emergency stop always wins