An off-delay timer, written TOF in most ladder-logic dialects, keeps its output ON for a preset length of time after its input has already gone OFF. It is the mirror image of the more familiar on-delay timer: instead of making you wait before something starts, it makes something keep running after the trigger is removed. Field engineers reach for a TOF whenever a device needs to coast, cool, or purge after its command drops out. Because its bits behave opposite to a TON, the TOF trips up programmers who assume every timer counts up the moment its input goes true.
Off-Delay Timer (TOF) in one line: An off-delay timer (TOF) energizes its output the instant its input turns true and keeps that output energized for a preset time after the input goes false. The timer only starts accumulating when the input drops out, so the output stays ON during the delay and turns OFF when the accumulator reaches the preset.
A TOF instruction has the same building blocks as any ladder timer: a preset value that sets how long the delay is, an accumulator that counts elapsed time, an enable bit, a timing bit, and a done bit. What confuses people coming from an on-delay timer is that these bits do the opposite of what habit expects. When a TOF's input is true, the done bit is on and the accumulator sits at zero. The timer does nothing while the input stays true - it is simply armed and waiting.
The action begins the moment the input goes false. At that transition the accumulator starts counting up from zero, the timing bit goes true, and the done bit stays on. The done bit remains true, holding the output energized, right up until the accumulator reaches the preset. Only then does the done bit finally drop, the output de-energizes, and the timing bit clears. If the input goes true again before the delay expires, the accumulator resets and the done bit simply stays on, so brief interruptions do not restart the whole run-on period.
The practical takeaway is that you usually wire the process output to the TOF's done bit, and that done bit is true both while the input is on and during the timed run-on after the input turns off. Compared with an on-delay timer, where the done bit only comes on after the delay while the input is held true, the TOF gives you the inverse behavior: instant on, delayed off. Getting this straight on paper before you write the rung prevents the classic bug where a coder expects the output to drop the instant the input does.
The clearest field use of a TOF is the run-on timer. Consider a purge fan on a gas-fired heater or a burner enclosure. When the burner shuts down, you do not want the fan to stop at the same instant, because unburned or residual gas needs to be swept out of the firebox first. Feeding the burner-run signal into a TOF and driving the fan from the done bit keeps the fan spinning for the preset purge time after the burner drops out, then stops it automatically. The same pattern covers an engine or compressor cooldown, where lube-oil and cooling circuits need to run on for a set period after the driver is commanded off so heat can carry away before everything sits still.
A TOF also handles post-lubrication and coast-down needs on rotating equipment. A pre-lube pump or a seal-flush pump can be told to run for a fixed time after the main machine stops, protecting bearings and seals during the vulnerable minutes just after shutdown. Because the timer holds the auxiliary running independent of the main command, an operator or a higher-level sequence can trip the machine cleanly and trust that the support systems will finish their job before they too fall silent.
The third common role is preventing short-cycling. Rapid stop-start cycling wears contactors, stresses motors, and on refrigeration or compression duty can even damage the machine. A TOF used as a minimum-off dwell keeps a restart inhibited until a preset time has elapsed since the last stop, and a TOF used as a minimum-run guard keeps a pump running for at least a set period once it has started. Combined, these dwell timers smooth out the hunting behavior that a jittery level switch or a noisy pressure signal would otherwise cause at the final control element.
On a remote wellsite or compressor package, the TOF logic runs entirely inside the local PLC or RTU, but the states it produces are exactly the kind of thing a cloud SCADA platform brings back for visibility. Exposing a run-on timer's done bit and accumulated value as points lets an operator watching a Merobix dashboard see not just that a burner has shut down, but that the purge fan is still in its timed run-on and how many seconds remain. That distinction matters during a callout, because a fan that is deliberately running on looks identical to a fan stuck on unless the timer state is visible.
Trending these timer states also turns a silent sequence into an auditable record. If an engine is supposed to hold its cooldown pump for a fixed period after every shutdown, historizing the pump command and the timer done bit shows whether that cooldown actually happened on every stop, or whether a wiring fault or logic change quietly skipped it. Over a fleet of unmanned sites, that historical trail is often the only way to confirm protective run-on and dwell logic is behaving as designed without driving out to each location.
Because a stuck or mis-preset TOF can either cut a purge short or leave equipment running far longer than intended, alarming on the timer's behavior is worthwhile. A cloud monitoring layer can flag a run-on that never completes, a cooldown that ends suspiciously early, or a minimum-off dwell that is being repeatedly re-triggered, all of which point at either a mechanical problem in the field or a logic value that needs review. The timer keeps the local sequence safe; the SCADA layer keeps humans informed that it is doing so.
A TON (on-delay timer) waits a preset time after its input goes true before turning its output on, so it delays the start of an action. A TOF (off-delay timer) turns its output on immediately when the input goes true and delays the turn-off, keeping the output on for a preset time after the input goes false. In short, a TON delays on and a TOF delays off, and their done bits behave in opposite ways.
By design, a TOF energizes its output the moment the input becomes true and keeps it energized while the input stays true, so the done bit is on the whole time the input is held. The timer only starts counting when the input drops to false, and the done bit remains on through that count until the accumulator reaches the preset. This is the opposite of a TON, whose done bit only comes on after its delay while the input is still true.
A run-on timer keeps a device operating for a fixed period after its trigger is removed, which is exactly the job of a TOF. Typical uses are keeping a purge fan running after a burner shuts to sweep out residual gas, holding a cooling or lube pump on after an engine stops so heat can dissipate, or maintaining a minimum run to stop a pump from short-cycling. Driving the device from the TOF done bit gives instant start and a clean timed stop.
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