Timers: TON, TOF and TP

Three blocks, four pins

The standard gives you three timers and they share the same interface:

The whole difference is when Q goes true and when it drops.

TON · on-delay

Q goes TRUE after IN has stayed true for the whole of PT, and drops immediately when IN drops. Any break in IN before the time is up resets ET to zero and the count starts again.

IN  ___----------______----___
ET  ___/\/\/\/\/\__0___/\__0__
Q   _______-------______________   (PT = 3s)

TOF · off-delay

Q goes TRUE immediately when IN rises, and stays TRUE for PT after IN drops. The timer counts after the signal goes away.

IN  ___-------_____________
Q   ___-------------________    (PT = 5s after the fall)

TP · pulse

Q goes TRUE on the rise of IN and stays true for exactly PT, however long IN stays on. Holding the button for ten minutes does not stretch the pulse.

IN  ___-----------------____
Q   ___------_______________   (PT = 500ms)

Calling one

VAR
  AlmDly : TON;
END_VAR

AlmDly(IN := LSH, PT := T#3s);
HiAlm := AlmDly.Q;

The call must run every scan. Put it inside an IF that is sometimes skipped and the timer freezes mid-count. Keep the call at the top level and control it through IN.

Use one: alarm de-bounce

The alarm standard ISA-18.2 recommends an on-delay on every process alarm. A level hovering at 90% produces 40 alarms a minute, floods the operator, and the whole screen gets ignored. Three seconds turns that into one real alarm:

AlmDly(IN := Level > 90.0, PT := T#3s);
HiAlm := AlmDly.Q;

Add a deadband on the return and the chatter is gone for good.

Use two: a run-on fan

A hot motor needs cooling after it stops, so the fan keeps turning for five minutes. That is literally a TOF:

RunOn(IN := Pump, PT := T#5s);
Valve := RunOn.Q;

The same shape covers a lamp that stays lit after the event and a valve that closes late.

Writing time

T#500ms, T#3s, T#2m30s, T#1h. Never count your own scans into a REAL: a timer measures real time and stays correct when the scan time changes.