For TON, when does the output Q become true?

• A. Immediately when IN becomes true.
• B. After IN remains true for the preset time.
• C. Only if ET equals PRE.
• D. When PRE is zero.

Answer: (B)

The main idea is how a TON timer works: it is an on-delay timer that delays the output becoming true until the input has been continuously true for the preset duration. When IN is true, ET starts counting up toward the preset time. The output Q becomes true when ET reaches or exceeds that preset time. If IN goes false before finishing the preset time, ET resets and Q stays off.

So the correct understanding is that Q turns on after IN has remained true for the preset time. ET tracks the elapsed time and is compared to the preset value; the trigger is ET >= PRE (or PT), not ET equaling PRE exactly. If the preset time were zero, some implementations would set Q immediately after IN goes true, but that’s an edge case dependent on the specific PLC.