SR and RS flip-flops: which one wins

Memory with two signals

The seal-in from lesson four:

Pump := (Start OR Pump) AND NOT Stop;

works fine, but it mixes memory and logic into one line. The standard gives you two blocks that separate the intent: set here, reset there.

SR · set dominant

VAR
  Run : SR;
END_VAR

Run(S1 := Start, R := Stop);
Pump := Run.Q1;

S1 sets, R resets. If both arrive TRUE in the same scan, Q1 stays true: set wins. Note the digit in S1; the standard puts the digit on the dominant pin.

RS · reset dominant

VAR
  Alarm : RS;
END_VAR

Alarm(S := LSH, R1 := Ack);
HiAlm := Alarm.Q1;

Here R1 is dominant. Event and acknowledge together, and the acknowledge wins: Q1 drops.

The table that settles it

That last row is the entire difference. In the other three the blocks are identical.

Which one to use

The rule comes from safety: pick the block whose dominant side is the safe side.

• A motor or pump: use RS. Reset means stop, and stop is safe. Do not use SR to run a motor, or a start command outranks the stop.
• An alarm that must be seen: use SR. Set means the alarm appears, and a momentary event must not be lost because the acknowledge landed on the same scan.
• A fault latch: SR exactly. The fault is captured and holds until an acknowledge arrives on a later scan.

When to use a block instead of a seal-in

• When set and reset conditions get long. (A OR B OR C OR Out) AND NOT (D OR E) stops being readable; the block splits it into two lines.
• When the conflict is real and you want the winner declared in the code rather than in your head.
• When the state is retained across a restart, since the memory lives in one named instance.

Latched alarms

A latched alarm stays visible after its cause has gone, until the operator acknowledges it. Every safety-relevant event needs this: a transient that clears before anyone looks at the screen must not vanish from the record.

Alarm(S1 := Level > 95.0, R := AckEdge.Q);

Put an R_TRIG on the acknowledge button, or a held-down button keeps the alarm from ever appearing.