Edge detection: R_TRIG, F_TRIG and the "it fired every scan" bug

The bug everyone writes once

You want to count how many times the operator pressed start, so you write:

IF Start THEN
  Starts := Starts + 1;
END_IF;

One short press and the counter reads 14. A short press is about 300 ms, the scan is 20 ms, so the condition was true for 15 consecutive scans. The program never sees a press. It sees the state of the button, once per scan.

The edge

What you want is the edge: the moment the value changed, not the value itself.

Q stays TRUE for exactly one scan, which is why this pattern is called a one-shot.

Using it

The block has memory, so declare one instance in VAR per signal you watch:

VAR
  StartEdge : R_TRIG;
  StopEdge  : F_TRIG;
END_VAR

StartEdge(CLK := Start);

IF StartEdge.Q THEN
  Starts := Starts + 1;   (* once per press *)
END_IF;

Never share one instance between two signals. An instance stores the previous value of one signal.

What is inside

There is no magic in it:

Q    := CLK AND NOT Last;
Last := CLK;

You can write that by hand with a single BOOL, but the standard block reads clearer in review and travels between vendors.

When you actually need an edge

• Counting: batches, parts, number of starts.
• One step of a sequence: move state on the press, not for the whole press.
• Acknowledge and reset: the ack button clears the alarm once.
• Logging an event: writing a record or sending a message. Without an edge you send 50 messages a second.
• Falling edge: start a run-on delay when the pump stops, or log the run duration when it ends.

The rule

A level asks "is it on now?"; an edge asks "did it just start?" Anything that counts, logs, or advances a step needs the second question.