PMMI Programmable Logic Controllers (PLC) 1 Practice Test

What is the purpose of a timer TOF (Off-Delay Timer) and how does its operation differ from TON?

TOF keeps the output energized for a set time after the input goes false; TON energizes after a delay.

TOF energizes immediately; TON delays de-energization.

TOF counts events; TON stores data.

An Off-Delay Timer keeps the output on for a set period after the input has gone false. In operation, when the input is true, the timer isn’t counting and the output stays energized. When the input turns false, the timer starts counting down, and after the preset time the output turns off. This gives you a controlled delay before de-energizing the output.

This differs from an On-Delay Timer, where timing starts when the input goes true and the output only energizes after the preset delay. If the input drops before the delay elapses, the TON typically resets and the output remains off.

In short, TOF applies a delay after the input is de-asserted to keep the output on, while TON applies a delay after the input is asserted to turn the output on. Timers don’t count events or store data—that’s what counters handle.

TOF resets the coil on each cycle.