Introduction to Industrial Automation

Figure 6.12  Simplified electronic circuit for an AC voltage signal of a digital input module (a) 

and an external electric connection of an input device (b).

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208

 

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  Introduction to Industrial Automation

+ 5 V

Logic signals to

CPU/memory

– 0 V

R

3

Z

1

R

4

R

R

1

R

2

Input 1

L1

L2 

C

D

1

R

3

Z

1

R

4

R

R

1

R

2

Input 2

L1

L2

C

D

1

R

3

Z

1

R

4

R

R

1

R

2

Input 3

L1

L2

C

D

1

R

3

Z

1

R

4

R

R

1

R

2

Input 4

L1

L2

C

D

1

bit 1

bit 2

bit 3

bit 4

Figure 6.13  Simplified electronic circuit of a digital input module with four isolated-supply 

inputs.

Z

1

R

1

Input 1

L1

+ 5 V

Logic signals to

CPU/memory

– 0 V

R

3

R

4

R

R

2

C

D

1

R

3

Z

1

R

4

R

R

1

R

2

Input 2

C

D

1

R

3

Z

1

R

4

R

R

1

R

2

C

D

1

R

3

Z

1

R

4

R

R

1

R

2

L2

C

D

1

bit 1

bit 2

bit 3

bit 4

L1

L1

L1

Input 3

Input 4

Figure 6.14  Simplified electronic circuit of a digital input module with four non-isolated-

supply inputs.

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Basic Operating Principles of PLCs 

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209

6.5   Digital Output Modules

In the introductory description of the PLC’s operation, it has been indicated that the results of 

the logical processing of an automation program are stored in the output image table and are 

transferred subsequently, through the digital output module, to the output devices. The results of 

the program execution are logic signals that should be altered into power signals inside the digital 

output module, capable of energizing the corresponding output devices. In general, it can be stated 

that the digital output module processes the low voltage signals coming from the microprocessor 

in a completely opposite way than the digital input module. In Figure 6.16, all the operations that 

are carried inside the digital output module are indicated. The majority of these electrical circuits 

that perform these functionalities are similar to the ones in the digital input modules and are not 

going to be described again; however, the major differences will be highlighted.

When the logic signals from the output image table are transferred serially to the out-

put module, then in the demultiplexing unit these signals are decoded and transmitted to 

the respective individual outputs of the output module. Since the logical signals are sent 

Digital

inputs

8Χ24 V DC

Digital

inputs

16Χ24 V DC

4

5

6

7

1

2

3

C

C

4

5

6

7

0

0

1

2

3

C

4

5

6

7

0

1

2

3

C

Figure 6.15  Modules of digital inputs with DC power supply in groups of four and eight inputs.