A TON on-delay timer has a 10 s preset. Its input goes TRUE, holds for 8 s, then drops. What did the output do?

Stayed FALSE the whole time — 8 s never reached the preset, and ET cleared to zero the moment the input dropped.. A TON is the sentence "the input must stay true this long before I believe it." Eight seconds is short of the 10 s preset, so Q never fired — and the moment the input dropped, ET zeroed. As far as anything downstream knows, that 8 s pulse never happened: the TON is a minimum-duration filter that deletes every TRUE shorter than its preset.

A fan status signal feeds a TOF off-delay timer with a 10 s preset. The status drops out for 6 s, then comes back. What did the TOF's output do?

Stayed TRUE throughout — the dropout was shorter than the preset, so the countdown was cancelled and ET cleared when the status returned.. A TOF holds its output TRUE for the preset after the input drops. The input came back at 6 s — inside the window — so the countdown was cancelled, not paused: ET zeroed, the output never blinked, and the next dropout gets a fresh, full-length 10 s delay. The TOF is the mirror of the TON: it deletes every FALSE shorter than its preset.

If a TON's input drops before the preset is reached, the elapsed time holds where it was and resumes counting from there when the input comes back.

False. False — ET clears to zero the instant the input drops, and the next attempt starts the count from scratch. A standard TON keeps no credit across attempts. Timers that do accumulate across interruptions exist in some palettes — usually called retentive or accumulating timers — but that's a different block; don't assume you have one.

A TOF's output goes TRUE the instant its input does — the preset delays only the turn-off.

True. True — that's what off-delay means. There is no delay on the way up: Q rises with the input. The preset only matters on the drop, when Q hangs on while ET counts up toward it. If the input returns before ET gets there, the countdown is cancelled and the output never blinked. A block that delays the way up is the TON.

Heating just shut off, and the sequence should keep the supply fan running another 90 s to purge heat from the exchanger. Which timer does the job?

A TOF on the run call driving the fan — the fan follows the call up instantly and holds on for 90 s after it drops.. Say the sentence and the block picks itself: "keep saying run for 90 s after the call drops" — that's the TOF's sentence exactly. The fan starts with the call (no delay up) and outlives it by the preset, carrying the exchanger's residual heat away instead of letting it soak. A TON would do the opposite — delay the start and drop the fan the instant heating stopped, exactly when the purge is needed.

A fail-to-start alarm pages on nearly every start of a big return fan — which always proves within about 20 s. The proof TON's preset is 10 s. What's the right fix?

Lengthen the preset past the slowest honest start — about 25 s here.. The window is shorter than an honest start, so the alarm cries wolf on every one — and alarms that cry wolf get ignored or disabled. But the 5-minute "fix" just trades nuisance for a blind spot: a start that genuinely failed runs unreported for the whole preset, and everything interlocked on "proven" inherits the blind spot. The rule: as long as the slowest honest start plus a little margin, and no longer.

In the proof-of-flow alarm sheet (command BI + proof BI → NOT → AND → TON 15 s → SR latch → alarm BO), the TON's output already goes TRUE on a confirmed failure. What does the SR latch add?

It holds the alarm after the failure condition goes away — the TON only reports while command-with-no-proof stands, so switching the fan off would otherwise erase the alarm.. The timer detects; the latch remembers. The TON's Q is only TRUE while the failure condition stands — the moment an operator kills the fan command, the AND drops and the TON releases. Without the latch, that natural response would erase the alarm, evidence and all. Latched, the trip stands until a deliberate reset — a person acknowledging — no matter what the inputs do in the meantime.

A DP status point flutters on and off at marginal airflow, spamming alarms. You want the status believed only after it has held made for 3 s. Which block goes between the raw point and the logic?

A 3 s TON — the status must stay TRUE for the full preset before it passes.. The sentence is "hold made for 3 s before I believe it" — a TON. The flutter never survives the filter, because no single TRUE lasts the preset. A short TOF is the mirror trick — riding through brief dropouts of a signal you already believe. And remember the timer treats the symptom: if the switch flutters because its setpoint has no margin against the operating pressure, the honest fix is the setpoint, not a longer filter.

The sequence narrative says: "alarm only if the filter DP switch stays made for 30 s." A tech wired it as below — and now the alarm fires the instant the switch closes, then lingers for 30 s after it opens. What's wrong? FILTER-DP (BI) ──► TOF (PT 30 s) ──► ALARM (BO) switch closes → alarm ON immediately switch opens → alarm holds another 30 s

A TOF passes a rising input straight through and delays only the drop. "Stays made this long before I believe it" is the TON's sentence — swap the block.. The symptom is the block's signature read back to you: instant rise, delayed fall — that's an off-delay doing exactly what it does. The narrative wanted the mirror image: ignore the switch until it has held made for 30 s, which is an on-delay. Same preset, same wiring, opposite sentence. When a timer misbehaves, check which edge the delay is on before touching anything else.

A compressor's nameplate allows a maximum of 6 starts per hour. If anti-short-cycle timing spaces the starts evenly, what is the minimum time between one start and the next, in minutes?

10 min. 60 minutes ÷ 6 starts = 10 minutes between starts. That whole gap is what the minimum-run and minimum-off timers enforce together: however eagerly the deadband decision flips, the equipment cycles no faster than one start per 10 minutes. Starts are the expensive part — inrush heat in the windings, oil pumped out on short runs, pressures that need the off-time to equalize — which is why the number comes from the manufacturer, not from taste.

Practice · Timers & Delays

Timers & Delays Quiz Logic

Read the Timers & Delays lesson first, then test yourself — ten questions on gating decisions through time: which timer matches which sentence, what actually clears the elapsed time, sizing a proof window between nuisance trips and blind spots, debouncing a fluttering status, and the minimum run and off times that keep a compressor alive. Each reveal links back to the section it came from.

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