Introduction to Industrial Automation

Figure 7.21  Instructions in LAD language for the implementation of transitional contacts.

PLC

INPUTS

OUTPUTS

+24 V DC 

0 V

I 0.1

S

I0.1

I0.1

RLO= “0”

RLO= “1”

PLC

INPUTS

OUTPUTS

I 0.1

RLO= “0”

RLO= “1”

PLC

INPUTS

OUTPUTS

+24 V DC

I 0.1

S

I0.1

I0.1

RLO= “1”

RLO= “0”

PLC

INPUTS

OUTPUTS

0 V

I 0.1

RLO= “1”

RLO= “0”

(b)

(a)

PLC

Inputs

Outputs

I0.1

RLO = “0”

RLO = “1”

PLC

Inputs

Outputs

I0.1

RLO = “1”

RLO = “0”

Figure 7.22  The RLO in relation to the non-energized (a) or energized (b) status of a digital 

input.

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Basic Programming Principles of PLCs 

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or open input contact, the state “0” or “1” of the auxiliary bit is examined by the CPU in the cor-

responding memory location.

In a program with Boolean instructions, the creation of an RLO follows the same logical process. 

The only difference is the look of the programming where the RLO is created, because of a different 

language form. In Figure 7.24, a segment of a Boolean program is presented including for instructions 

A and AN among other instructions that precede or follow. Suppose that P.RLO

i

 is the RLO created 

by executing all the preceding instructions. The execution of the instruction A I1.0 in conjunction 

with P.RLO

i

 creates the new P.RLO

i+1

, and the same procedure continues with the execution of the 

following instructions. The S.RLO in each instruction depends on the state of the input (whether or 

not it is activated) and each time affects the newly created P.RLO. A similar procedure is followed if, 

instead of the instructions A and AN, instructions O and ON or combinations of them are utilized.

Subsequently, let’s consider the LAD program of one rung in Figure 7.25, where the execu-

tion of the program will give a different effect on each scan cycle. In particular, in odd scan cycles 

(first, third...) will give M1.0 = “1” and in even ones (second, fourth...) will give M1.0 = “0”. 

Memory of auxiliary bits

(logic coils)  

0

Μ4.2

Μ4.2

RLO= “0”

RLO= “1”

Μ4.2

Μ4.2

RLO= “1”

RLO= “0”

1

Μ4.2

Μ4.2

0

RLO = “0”

RLO = “1”

RLO = “1”

RLO = “0”

(b)

(a)

Memory of auxiliary bits

(logic coils)  

1