Introduction to Industrial Automation

Figure 7.49  Graphical symbol of a timer in LAD language including instructions for activation, 

resetting, outputting, time setting, etc.

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  Introduction to Industrial Automation

of the time and the time unit that T

xx

 will measure are declared in the TV input. The input R is 

linked to or denoted by the variable that is required to perform the reset action of T

xx

. At the Q 

output of T

xx

, it is connected or declared the variable that needs to be activated when T

xx

 termi-

nated the measurement of the defined time. Finally, the output BI is provided by the CPU, and the 

remaining time to be measured in integer form becomes zero when Q = “1”.

In the conventional automation circuit shown in Figure 7.50a, the usual industrial need for start-

ing up a machine (relay C) with a time delay is implemented. The initiation of the machine with a 

delay time T is performed with respect to an event expressed by a discrete signal. When the S

1

 sensor 

is activated and its switching contact closes, the timer T is activated, which measures time T, closes 

the switching contact T after the elapse of the time T, and finally the relay C is activated. In Figure 

7.50b, the required I/O connections for the implementation of the above operation to a PLC are 

presented. In this case, the reset button allows the reset of the current time measurement. Whether 

the timer will restart the time measurement depends on the state of the sensor S

1

 and the time func-

tion ON-delay of Figure 7.48. The required program in the LAD language is shown in Figure 7.50c, 

where the time T has been defined as 30 s. From the graphical symbol of the timer T12, the output 

BI has been omitted, while a rung has been added to the program to activate the logic coil M3.7 with 

a delay as well at the output. The corresponding Boolean language program is the following:

A

I1.0

L

‘30 sec’

SD   T12

A

I1.3

R

T12

A

T12

=

Q0.2

=

M3.7

BE

((

Sensor

N

C

S

1

R

(a)

(b)

(c)

T

T

PLC

Inputs

Outputs

0 V

C

+24 V DC

0V

+24 V DC 

Q0.2

I1.0

Reset

I1.3

LAD

I1.0

Q0.2

I1.3

T12

S

30 sec

R

T12

M3.7

TV

SD

Sensor

S

1