CODESYS SR

SR is a standard function block that implements a bistable set-dominant latch. The output Q1 is set to TRUE by the input SET1 and reset to FALSE by the input RESET, with SET1 taking priority. The implementation is the Boolean expression Q1 := SET1 OR (NOT RESET AND Q1).

Source checked12/12 values · 2026-10-26

Specification

2 sources

Language	iec 61131 3
Code	SR
Group	Bistable Function Blocks
Syntax	SRInst(SET1 := VarBOOL1, RESET := VarBOOL2)
Dialect	CODESYS

Parameters

Name	Data type	Description
SET1	BOOL	Sets the output Q1 to TRUE (dominant)
RESET	BOOL	Resets the output Q1 to FALSE
Q1	BOOL	Output value

Study notes

Implementation in ST: Q1 := SET1 OR (NOT RESET AND Q1);
Example declaration: SRInst : SR;
Example call in ST: SRInst(SET1 := VarBOOL1, RESET := VarBOOL2); then read VarBOOL3 := SRInst.Q1;
Set-dominant means SET1 overrides RESET when both inputs are TRUE.
Q1 is an output, so it is read from the instance (SRInst.Q1).