#12 The ON-Delay Timer (TON)

In this article will be explained about the on-delay timer (TON), before we know about TON, first it is better if we know about the timers. Timers are used to activate or de-activate a device after preset interval of time. PLC timers provide the same function as mechanical and electronic timing relays. There are three different timers, which are the on-delay timer (TON), the off-delay timer (TOF), and the retentive timer on (RTO). And there are two methods used to represent a timer instruction within a PLC’s logic ladder program, which are relay coil and block format.

The on-delay timer (TON) is the most commonly used timer. The on-delay timer operates so that when the rung containing the timer is true, the timer time-out period commences. At the end of the timer time-out period, an output is made active. The timed output becomes active sometime after the timer becomes active, hence the timer is said to have an ON delay.

The timer instruction consits of three data table words, such as the control word, the preset word, and the accumulated word.

1. Control word

The control word uses three bits, which are :

a. The Enable Bit (EN)

The enable bit is true (has status of 1) whenever the timer insstruction is true. When the timer instruction is false, the enable bit is false (has status 0).

b. The Timer-Timing Bit (TT)

The timer-timing bit is true whenever the accumulated value of the timer is changing, which means the timer is timing. When the timer is not timing, the accumulated value is not changing, so the timer timing bit is false.

c. The Done Bit (DN)

The done bit changes state whenever the accumulated value reaches the preset value.

2. Preset word

The preset value is the set point of the timer, that is, the value up to which the timer will time.

3. Accumulated word

The accumulated value is the value that increments as the timer is timing. The accumulated value will stop incrementing when its value reaches the preset value.

The timer instruction requires you to enter a time base, which is either 1.0 or 0.01 for long short time delays. The actual preset time interval is the timer’s preset word. The actual accumulated time interval is the time base multiplied by the value stored in the timer’s accumulated word.

The timer is activated by closing the switch. We can see in below figure, the preset time for this timer is 10 s, at which time output D will be energized. When the switch is closed, the timer begins counting, and counts until the accumulated time is automatically reset to zero. This timer configuration is termed nonretentive since loss of power flow to the timer causes the timer instruction to reset. This timing operation is that of an ON-DELAY timer, since output D is switched on 10 s after switch has been actuated from the OFF to the ON position.

The timing diagram first shows the timer timing to 4 s and then going FALSE. The timer resets, and both the timer-timing bit and the enable bit go FALSE. The accumulated value also resets to 0. Input A then goes TRUE again and remains TRUE in excess of 10 s. When the accumulated value reaches 10 s, the done bit (DN) goes from FALSE to TRUE and the timer-timing bit (TT) goes from TRUE to FALSE. When input A goes FALSE, the timer instruction goes FALSE and also resets, at which time the control bits are all reset and the accumulated value resets to 0.

Ladder diagram in TON

Ladder diagram in TON

Timing Diagram in TON

Timing Diagram in TON

Source :

–        Petruzella, Frank D. Industrial Electronics. 1996. McGraw-Hill International Editions. pg. 354-356.


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