Introduction to Industrial Automation

7.5.2   Programming with Boolean Logic and Activation Instructions

Let’s consider the classic automation circuit of Figure 7.26a, where the sensor S

1

 controls the oper-

ation of the power relay C. It is preferable to operate a machine that is powered by a relay C 

when the NO switching contact of the sensors S

1

 close. Before proceeding with the programming, 

M1.0

M1.0

New state

Figure 7.25  Program for RLO alternation per scan cycle.

Sensor

N

C

S

1

Sensor

S

1

R

 

(a)

(b)

PLC

INPUTS

OUTPUTS

0 V

C

+24 V DC 

0 V

+24 V DC

Q 0.2

I 1.2

 

PLC

INPUTS

OUTPUTS

Q 0.2

I 1.2

 

PLC

Inputs

Outputs

Q0.2

I1.2

Figure 7.26  The relay C is energized when the contact S

1

 closes. (a) Conventional circuit and 

(b) implementation of (a) with PLC.

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Basic Programming Principles of PLCs 

◾

 

301

it should be determined how the input and output devices are connected to the PLC, a step that 

should always be done initially regardless of the application. A PLC program has no meaning to be 

created, without or before the proper connection of the I/O devices and the corresponding defini-

tion of the addresses. These specifications, for the currently examined application, are presented in 

Figure 7.26b, where the NO switching contact S

1

 is connected to the input I1.2 and the relay C to 

the output Q0.2. In this case, the required Boolean program is:

A I1.2

= Q0.2

Subsequently, the classical automation circuit shown in Figure 7.27a will be considered, where 

the relay C is activated when the NC switching contact of sensor S

1

 remains closed. For the imple-

mentation of the programmable automation and the connection of the I/O devices, as depicted 

in Figure 7.27b, the required Boolean program will be:

A I1.2

= Q0.2

which has the same form as in the previous case, but with a different state of the switching contact. 

The explanation can be derived by examining the specifications of operation in each case and what 

they do in the PLC.

The automation circuit of Figure 7.28a permits a machine, powered by a relay C, to operate 

when the sensor contact S

1

 is open (the machine stops working when the contact S

1

 is closed). Such 

an operation is impossible to be implemented with a conventional automation circuit without the 

use of an auxiliary relay d. In contrast, in the programmable automation, this can easily be imple-

mented by using the AN instruction. For the connection of the I/O devices and their addresses 

shown in Figure 7.28b, the required program in Boolean and LAD languages is the following:

PLC

INPUTS

OUTPUTS

0 V

C

+24 V DC 

0 V 

+24 V DC

Q 0.2

I 1.2

Sensor

N

C

S

1

Sensor

S

1

R

 

(a)

(b)

PLC

Inputs

Outputs

Q0.2

I1.2