On-Delay and Off-Delay Timers: Identify Which Input Transition Starts the Delay

Name the transition being delayed

Schneider describes TON as delaying an on transition after a rising input and TOF as delaying an off transition after a falling input. Primary reference: on-delay; off-delay.

The same preset duration therefore does not give the same output history. Retain the actual block state, input changes and execution behavior. The following comparison uses ideal continuous timing to expose the difference, rather than claim exact scan-level timing.

A fictional timeline

Assume an input starts false, rises at 0 s, stays true until 5 s and then falls. Give each separate idealized timer a 2 s preset. The on-delay output becomes true at 2 s and returns false at 5 s. The off-delay output becomes true at 0 s and remains true until 7 s.

Both presets are 2 s, yet the true intervals are 2–5 s and 0–7 s. A real implementation’s cycle handling and repeated transitions need their own documented analysis. These times describe no installed machine delay or required acceptance condition.

Keep the event diagram with the requirement

Draw input and output histories on one axis. Include starting state, transition direction, preset and any interruption before the interval ends.

Ask which physical or logical event the application intends to delay. This worksheet turns a request for a two-second timer into a checkable timing definition before an actual program change is assessed. It does not prescribe a control setting or infer mechanical completion from the Boolean output.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Primary reference: on-delay
• off-delay