Introduction to Industrial Automation

Figure 7.27  The relay C is energized when the contact S

1

 is closed. (a) Conventional circuit and 

(b) implementation of (a) with PLC.

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  Introduction to Industrial Automation

I1.2

Q0.2

AN    I1.2

=      Q0.2

AN    I1.2

=      Q0.2

Let’s now consider the classical automation of a START-STOP operation for a machine with a 

self-latch situation, as depicted in Figure 7.29. In this classical automation circuit, the STOP push 

button is typically a button with an NC contact, while the overall functionality cannot be imple-

mented without the use of an auxiliary relay, as in the case of the previous programming example. 

Instead, in PLC programming, the STOP function can be implemented with both an NC contact 

button and an NO contact button. Thus, depending on the type of STOP button (NC or NO) 

that will be utilized, the program in the LAD language should be written accordingly in order to 

achieve the desired functionality. The two cases are shown in Figure 7.29, where for every way of 

connecting the I/O devices to the PLC, there is a corresponding program in the LAD language, 

while programming in the Boolean language is similar. The two programs in Boolean language 

for both the NC and NO STOP buttons are, respectively, the following:

A(

O

I1.4

O

Q0.2

)

A

I1.0

=

Q0.2

BE

A(

O

I1.4

O

Q0.2

)

AN I1.0

=

Q0.2

BE

The same remarks are also valid for the START button, in the case that it is desired to be 

an NC contact button, where the need for developing a corresponding program is obvious.

PLC

INPUTS

OUTPUTS

0 V

C

+24 V DC

+24 V DC

(b)

0 V

Q 0.2

I 1.2

N

C

R 

(a)

d

d

S

1

PLC

Inputs

Outputs

Q0.2

I1.2

R 

Sensor

S

1

Sensor