Which statement about retentive timers is true?

Which statement about retentive timers is true?

• A. They accumulate and retain the total energized time and can start/stop without losing accumulated time
• B. They reset to zero every cycle
• C. They require no hardware to implement
• D. They do not have a Done bit

Answer: (A)

The key idea being tested is how a retentive timer behaves across cycles: it keeps track of elapsed time even when its input toggles off, and only clears to zero when a reset is explicitly issued. This is what "retentive" means in ladder logic: the accumulated energized time is preserved between energizations, so you can pause and resume timing without losing what has already counted. The statement that retentive timers accumulate and retain the total energized time and can start/stop without losing accumulated time describes this behavior precisely. While the input is energized, the timer counts up toward its preset. If the input goes off, counting stops, but the already accumulated time stays stored. You can later re-energize and it will continue from that stored value, rather than starting over. To clear the stored value, you send a reset, which resets the timer to zero. It also typically provides a Done bit that goes true when the accumulated time reaches the preset value, signaling that the timer has completed its required interval. The other options don’t fit retentive timing: resetting to zero every cycle describes a non-retentive timer, which forgets elapsed time as soon as the input de-energizes. Claiming no hardware is needed ignores the fact that a timer function block is a hardware or software timer component in the PLC. Saying there is no Done bit is incorrect because retentive timers usually expose a Done output when the preset is reached.

The key idea being tested is how a retentive timer behaves across cycles: it keeps track of elapsed time even when its input toggles off, and only clears to zero when a reset is explicitly issued. This is what "retentive" means in ladder logic: the accumulated energized time is preserved between energizations, so you can pause and resume timing without losing what has already counted.

The statement that retentive timers accumulate and retain the total energized time and can start/stop without losing accumulated time describes this behavior precisely. While the input is energized, the timer counts up toward its preset. If the input goes off, counting stops, but the already accumulated time stays stored. You can later re-energize and it will continue from that stored value, rather than starting over. To clear the stored value, you send a reset, which resets the timer to zero. It also typically provides a Done bit that goes true when the accumulated time reaches the preset value, signaling that the timer has completed its required interval.

The other options don’t fit retentive timing: resetting to zero every cycle describes a non-retentive timer, which forgets elapsed time as soon as the input de-energizes. Claiming no hardware is needed ignores the fact that a timer function block is a hardware or software timer component in the PLC. Saying there is no Done bit is incorrect because retentive timers usually expose a Done output when the preset is reached.