Introduction to Industrial Automation

Figure 7.41  The instructions S and R implement the conventional latching relay.

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Basic Programming Principles of PLCs 

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In quite a similar way, the S and R instructions can permanently activate/deactivate a logic 

coil. Subsequently, let’s consider the START-STOP classical automation circuit with a self-latch 

property, which can be utilized for the direct start and operation of an electric motor, as shown in 

Figure 7.42. The same figure also shows how the control buttons are connected to the PLC, as well 

as the Boolean language program, presented at the beginning of Section 7.5.2. The same operating 

logic can be achieved using the S and R instructions in the Boolean program that follows:

A

I1.3

S

Q0.2

AN I1.0

R

Q0.2

BE

At this point the two Boolean programs can be directly compared to derive any difference. For those 

who are familiar with classical automation, it would be interesting to consider whether via the program 

based on the S and R instructions, the non-reoperation feature of the classic automation circuit is lost.

Figure 7.43 shows a complex machine, which is an industrial mixer of raw materials. 

The machine includes a central motor and a number of sensors that are necessary for its operation. 

Program (BOOLE)

A(

O I1.3

O Q0.2

)

A I1.0

= Q0.2

BE

PLC

Inputs

Outputs

0 V

C

+24 V DC 

0 V

+24 V DC 

Q0.2

I1.0

STOP

START

I1.3

STOP

START

C

C

+24 V DC

0 V