An AHU enable is wired OCCUPIED AND OAT-OK AND NOT FREEZE-TRIP. Which set of live values enables the unit?

OCCUPIED TRUE, OAT-OK TRUE, FREEZE-TRIP FALSE.. Read the chain as a sentence: run only when occupied AND the outdoor air is OK AND the freeze stat is NOT tripped. The AND needs every input TRUE, and the NOT flips the trip signal to OK-sense — so FREEZE-TRIP must be FALSE for its leg to read TRUE. Only the first combination satisfies all three; each of the others has one leg FALSE, and one FALSE anywhere drops the permit.

Three trouble contacts — filter, condensate overflow, smoke — should light one common alarm when any of them trips. Which block combines them?

OR — TRUE when at least one input is TRUE.. The sentence is "any one of these is enough" — that's OR, which outputs TRUE when at least one input is TRUE. An AND would stay silent until every trouble tripped at once. The permissive chain is the mirror image: AND for "every proof required," OR for "any trip counts."

Hardwired safeties — freeze stats, high-static switches — are almost always wired normally closed, so the binary input reads TRUE while the device is healthy. Why?

A broken wire or dead device then reads FALSE — the same as a trip — so a failed safety circuit stops the equipment instead of silently promising protection.. It's a fail-safe choice: healthy = circuit closed = TRUE, so the safety actively proves itself every scan. If the wire breaks or the device dies, the circuit opens and the point reads FALSE — indistinguishable from a trip — and the permissive chain drops the equipment. Silence is treated as a trip, which is exactly what you want from a circuit whose job is protection.

Every device on this unit is healthy, but the fan will not start. The freeze stat's trip signal is wired straight into the run-permit AND. What's wrong? FREEZE-TRIP (BI): FALSE (stat healthy) PROOF OK (BI): TRUE │ both wired directly into ▼ AND → RUN PERMIT: FALSE (fan never starts)

FREEZE-TRIP is a trip-sense signal (TRUE = tripped), so its healthy FALSE reads as a standing trip. It needed a NOT to flip it to OK-sense before the AND.. The AND wants OK-sense inputs — TRUE when healthy. FREEZE-TRIP arrives in the opposite polarity (TRUE = tripped), so on a perfectly healthy day it feeds the AND a FALSE and the permit can never rise. One NOT between the trip signal and the AND fixes it. Swapping the AND for an OR would "fix" the symptom while wrecking the logic — the fan would then run on proof alone, freeze trip or not. Signal sense is a classic day-one commissioning bug: everything healthy, nothing runs.

A freeze stat trips and stops the fan. With no airflow, the coil warms up and the stat clears itself. Why is the trip wired through an SR latch instead of plain combinational logic?

Plain logic would restart the fan the moment the stat cleared — cold air hits the coil, the stat trips again, and the unit short-cycles through its own safety with nobody ever finding the cause. The latch holds the trip until a person acknowledges it.. The fault clearing is not the same thing as the problem being fixed. Combinational logic has no memory: stat clears, permit returns, fan restarts, coil freezes, stat trips — around and around, invisibly. Latching the trip keeps the unit off and the alarm standing until someone resets it deliberately and goes looking for why the coil was freezing in the first place.

A freeze stat trips for 30 seconds, then clears. Sheet A runs the trip through combinational logic straight into the fan permit; sheet B sets an SR latch that drops the fan. After the trip clears, what's the difference?

Sheet A restores the fan by itself; sheet B keeps it off until a reset arrives.. Combinational logic is a pure function of the inputs right now — the trip clears and sheet A's permit comes straight back. The latch remembers: sheet B's Q stays TRUE with both inputs FALSE, holding the fan off until R is pulsed. That flat stretch of Q after the fault clears is the memory, and it's the entire difference between a nuisance trip that self-heals and a safety that stands until acknowledged.

On a set-dominant SR latch, if S and R are both TRUE on the same scan, Q is TRUE — the set wins.

True. True — that's what set-dominant means, and it's the right behavior for a safety: while the fault is still standing (S TRUE), holding the reset does nothing. You cannot reset your way past a live fault; the reset only lands once the fault has cleared. Palettes vary — reset-dominant flavors exist, where R wins — so check which one your palette gives you before trusting it with a safety.

After a latched freeze trip, the coil warms back up and the freeze stat closes again. In the latch + manual-reset idiom, what actually restores the fan?

A deliberate reset on the latch's R input — a button on a graphic, a software point, a physical switch. The stat clearing changes nothing on its own.. That's the idiom: S comes from the safety, R comes from a person. Once latched, Q holds TRUE with both inputs FALSE — the stat clearing is invisible to the output. Only a pulse on R drops the latch and restores the fan, which is the point: a human acknowledges the trip and looks for the cause before the equipment gets its permit back.

An operator holds RESET (R TRUE) while the freeze stat is still tripped (S TRUE) on a set-dominant latch — and keeps holding it as the stat finally clears. What does Q do?

Q stays TRUE while S is TRUE, then clears on the first scan after S drops with R still held.. Walk it scan by scan with set-dominance: while S is TRUE, S wins and Q holds TRUE no matter what R says. On the first scan where S has dropped and R is still TRUE, the reset finally lands and Q clears. The held reset isn't queued or remembered — it simply keeps losing to S until the fault is gone, then takes effect.

A fan's command (BO) and its proof contact (BI) are wired into one block whose output should go TRUE exactly when the two don't match — commanded with no proof, or proved with no command. Which block?

XOR — TRUE when the inputs disagree.. XOR is the disagreement detector: TRUE only when its inputs differ. Command TRUE with status FALSE catches a broken belt or tripped overload; status TRUE with no command catches a welded contactor or a hand switch left in HAND. The same shape checks any pair of states that must never read alike. In practice the mismatch alarm waits a few seconds before sounding so the equipment has time to actually move — delays are their own lesson.

Practice · Boolean Logic

Boolean Logic & Latches Quiz Logic

Read the Boolean Logic & Latches lesson first, then test yourself — ten questions on the logic that makes equipment decisions: reading a permissive chain, fail-safe normally-closed wiring and the NOT that fixes a trip-sense signal, why a safety latches instead of self-clearing, set-dominant SR behavior when set and reset collide, and the XOR command/status mismatch check. Each reveal links back to the section it came from.

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