TOF — Off-Delay Timer
TOF in rungs.dev is the off-delay timer — starts timing when the rung goes false and holds the .DN bit at 1 for the preset duration after the rung drops.
TOF
Timer
timer
Preset
preset
Accum
accum
Off-delay timer. Starts timing when the rung goes false; holds .DN at 1 for the preset duration after the rung drops.
Operands
| Name | Type |
|---|---|
timer | TIMER |
preset | DINT |
accum | DINT |
preset and accum are pseudo-operands: they hold the value of .PRE and .ACC on the timer tag.
How It Works
| Rung state | .EN | .TT | .ACC | .DN |
|---|---|---|---|---|
| True | 1 | 0 | 0 | 1 |
False, .ACC < .PRE | 0 | 1 | counting up | 1 |
False, .ACC ≥ .PRE | 0 | 0 | stops at its current value | 0 |
While the rung is true, the timer is "primed" — .DN is 1 with zero elapsed time. When the rung drops, .DN stays 1 and the accumulator counts up. The first execution where .ACC is at least .PRE turns .DN off. .ACC can pass .PRE before stopping; it is not forced back to .PRE.
Example — Exhaust Fan Run-On
Keep an exhaust fan running for 30 seconds after the machine stops.
0
MachineRunning
TOF
Timer
ExhaustTimer
Preset
30000
Accum
0
1
ExhaustTimer.DN
ExhaustFan
ExhaustFan stays on while the machine is running and for 30 seconds after it stops.
Practice
Try TOF in the Off Delay exercise — keep an output on for a set time after its input drops.
Common Mistakes
- Expecting
TOFto start timing when the rung goes true — it is the opposite.TONtimes on rising edge,TOFon falling edge.
Related
- Timers overview — members, units, shared behaviour
- TON — on-delay counterpart
- RTO — retentive timer
TON — On-Delay Timer
TON in rungs.dev is the on-delay timer — starts timing when the rung goes true, sets the .DN bit when .ACC reaches .PRE, and resets when the rung goes false.
RTO — Retentive Timer On
RTO in rungs.dev is the retentive on-delay timer — like TON, but the .ACC accumulator survives rung transitions and must be cleared explicitly with RES.