SR — Set-dominant bistable

Pin layout

Direction	Name	Type	Description
Input	`S1`	`BOOL`	Set (dominant) — wins over R when both are TRUE
Input	`R`	`BOOL`	Reset
Output	`Q1`	`BOOL`	Latched state

SR vs. RS

FB	Set input	Reset input	Priority
SR (this page)	`S1`	`R`	Set wins
RS	`S`	`R1`	Reset wins

The “1” suffix marks the dominant input. Pick SR when an active set request must override a release (latching alarms, fault counters that should remember their trip until manual acknowledge); pick RS when an active reset must override a set (safety-stop systems).

Semantics

S1 = TRUE  : Q1 := TRUE  (set wins)
R = TRUE   : Q1 := FALSE (only when S1 = FALSE)
both FALSE : Q1 unchanged

Truth table:

S1	R	Q1 (next)
F	F	unchanged
T	F	TRUE
F	T	FALSE
T	T	TRUE (S1 wins)

Example

VAR
    alarmLatch : SR;
END_VAR

alarmLatch(S1 := xFaultCondition,
           R  := xOperatorAck);

IF alarmLatch.Q1 THEN
    xAlarmLamp := TRUE;
END_IF;

Latching alarms are the canonical SR use-case: a fault that self-clears (sensor recovers) should keep the alarm on until the operator acknowledges. SR encodes “set on fault, only release on ack” with set-priority by construction.

IEC reference

IEC 61131-3 third edition (2013), Annex F.4.1.

matiec conformance

Implemented per the standard.