Introduction to Industrial Automation

Figure 7.48  The basic possible time functions of a timer, each one selected by a corresponding 

instruction.

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Basic Programming Principles of PLCs 

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has also been defined via the corresponding activation. Therefore, the output Q4.0 through the 

instruction pair {A T8, = Q4.0} expresses nothing else but the behavior of the timer’s output. 

Before each triggering instruction of a timer, the timer should be loaded with the time to be mea-

sured through the time instruction (L “T sec”). The way of declaring the number and type of time 

units in the program will not concern us further because it is a secondary issue and varies from 

manufacturer to manufacturer.

The pulse timer has an output that is activated and remains activated (ON) for a time T from 

the instant that the timer’s input is activated with a positive rising edge (the input I0.0 changes 

from “0” to “1”). If the input of the timer is deactivated before the time T expires, then the output 

of the timer is deactivated at the same time instant. The timer output is also deactivated as soon 

as the reset input (I1.0) is activated. In this case, the output of the timer cannot be activated again 

unless a new positive rising edge pulse in the input is preceded.

The self-holding pulse timer has a behavior similar to the simple pulse timer, except that if the 

input of the timer (I0.0) is switched off before time T expires, the output remains energized until 

the T time measurement has been completed, thus this timer has the term “self-holding”. If one or 

more positive rising pulse edges occur to the timer’s input, while the timer output is activated, the 

T-time measurement is updated to each applied pulse.

In the case of the ON-delay timer, its output is activated after a time T has elapsed from the 

moment of the positive rising pulse edge at the input (I0.0), and remains activated for as long 

as the input is activated (ON). If the activation of the input stops before the T time expires, 

then obviously the timer’s output is not activated, and the restarting of the time T measurement 

requires a new positive rising pulse edge to occur at the input. Activation of the reset input (I1.0) 

immediately deactivates the output and inhibits the T time measurement.

The self-holding ON-delay timer has a similar behavior to the previous timer, except that the 

time T countdown continues even if the timer input (I0.0) is deactivated. Therefore, the output 

of the timer will necessarily be activated after time T, and the only way to deactivate this is to use 

the reset input (I1.0). Successive deactivations and activations of the setting input cause the T time 

measurement to be updated.

The OFF-delay timer activates its output at the same time when the input is activated and it 

remains activated for as long as the setting input is activated. If the timer’s input is deactivated, then 

the deactivation of the output will be delayed for a period of T time (thus the term “OFF-delay”). On 

successive activations and deactivations of the input, there is also a renewal of the time T measurement.

In the LAD language, the graphic symbol of the ON-delay timer instruction is shown in 

Figure 7.49, while the graphic symbol is similar for all the other types of timers. In particular, the 

graphic symbol of a T

xx

 ON-delay (SD) timer activation instruction accepts five connections or 

definitions. The input S is the setting input (logic “0” or “1”) of the timer. The numerical value 

SD

TV

Τ

xx

R

S

Q

BI

SD

TV

R

S

Q

BI