SR¶

Set-dominant bistable function block. A flip-flop where the set input takes priority: if both S1 and R are TRUE, the output Q1 is TRUE.

IEC 61131-3	Section 2.5.2.3.1
Support	Supported

Inputs

Name	Type	Description
`S1`	`BOOL`	Set input. Sets Q1 to TRUE while TRUE; takes priority over R.
`R`	`BOOL`	Reset input. Clears Q1 to FALSE while TRUE, unless S1 is also TRUE.

Outputs

Name	Type	Description
`Q1`	`BOOL`	Latched output state. Retains its value between scans.

Behavior¶

The output is computed as Q1 := S1 OR (NOT R AND Q1). When S1 is TRUE, the output is set regardless of R. When only R is TRUE, the output is cleared. The output retains its value between scans (latching).

Example¶

This example shows that set dominates: both S1 and R are TRUE, yet output is TRUE.

Try it

Source

PROGRAM main
   VAR
      latch : SR;
      output : BOOL;
   END_VAR

   latch(S1 := TRUE, R := TRUE, Q1 => output);
   (* output is TRUE because set dominates *)
END_PROGRAM

SR¶

Behavior¶

Example¶

See Also¶