What is a key characteristic of a retentive timer (RTO) in PLCs?

What is a key characteristic of a retentive timer (RTO) in PLCs?

• A. It loses elapsed time when the input goes off.
• B. It resets the elapsed time every time the input goes on.
• C. It only measures time while the output is energized.
• D. It preserves elapsed time across activations and can accumulate total time.

Answer: (D)

A retentive timer keeps the elapsed time value across input activations, so it can accumulate total time over multiple on/off cycles. When the input goes high, counting starts; when it goes low, counting pauses, but the elapsed time (ET) is retained. If the input goes high again, counting resumes from the previous ET, allowing the timer to accumulate more time. This retained ET is reset only by a separate reset condition or instruction, not automatically when the input toggles. That’s why the key characteristic is preserving elapsed time across activations and potentially summing it over multiple cycles. This differs from non-retentive timers, which reset their elapsed time each time the input goes off, or only measure time while the output is energized, which describe different timer behaviors.

A retentive timer keeps the elapsed time value across input activations, so it can accumulate total time over multiple on/off cycles. When the input goes high, counting starts; when it goes low, counting pauses, but the elapsed time (ET) is retained. If the input goes high again, counting resumes from the previous ET, allowing the timer to accumulate more time. This retained ET is reset only by a separate reset condition or instruction, not automatically when the input toggles. That’s why the key characteristic is preserving elapsed time across activations and potentially summing it over multiple cycles.

This differs from non-retentive timers, which reset their elapsed time each time the input goes off, or only measure time while the output is energized, which describe different timer behaviors.