What is On-Delay and Off Delay timer in PLC?


#1

Study about different Timers in Programmable Logic Controller (PLC). Commonly used timers are on-delay, off-delay timers.

ON Delay Timer

Timer%20ON%20Delay

When the timer input bit (IN) receives power, the TON instruction adds the preset time (PT), in milliseconds, and the timer output bit (Q) turns ON.

OFF Delay Timer

Timer%20OFF%20Delay

When the timer input bit (IN) stops receiving power, the TOF instruction adds the preset time (PT), in milliseconds, and the timer output bit (Q) turns OFF.

Pulse Timer

Timer%20Pulse

When the timer input bit (IN) receives power one time, the TP instruction turns ON the output bit (Q) for the duration of the preset time (PT), in milliseconds.


#2

There are 3 types of timers: On- delay timer, Off-delay timer, and retentive timer.

On delay timer

  • Use this instruction to program a time delay after instructions become true.

  • On – delay timers are used when an action is to begin a specified time after the input becomes true. For example, a certain step in the manufacturing is to begin 45 seconds after a signal is received from a limit switch. The 45- seconds delay is the on-delay timers preset value.

Off delay timer

  • Off- delay timer instructions is used to program a time delay to begin after rung input goes false.

  • As an example, when an external cooling fan on a motor is provided, the fan has to run all the time the motor is running and also for certain time (say 10min) after the motor is turned off. This is a ten minute off- delay timer. The ten-minute timing period begins as soon as the motor is turned off.

Retentive timer

  • Retentive timer is a timer which retains the accumulated value in case of power loss, change of processor mode or rung state going from true to false (rung state transition).

  • Retentive timer can be used to track the running time of a motor for its maintenance purpose. Each time the motor is turned off, the timer will remember the motor’s elapsed running time. The next time the motor is turned on, the time will increase from there. This timer can be reset by using a reset instruction.