Introduction to Industrial Automation

3

Figure 6.11  Thumbwheel switch for arithmetic data insertion in a PLC via digital inputs.

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Basic Operating Principles of PLCs 

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207

reed switches have a limited mechanical life. Some PLCs in the market are available with input 

modules that do not have optical or other type of isolation and should be avoided. As mentioned 

above, in a digital input module, the number of inputs varies depending on the type of unit. When 

the number of inputs in a module is relatively large, then providing a proper power supply should 

be considered. The inputs of a module can either have a separate power supply, as presented in 

Figure 6.13, or require a common one as shown in Figure 6.14. The separate power supply of the 

inputs has the advantage that we can connect to the same module various input devices supplied 

by different power sources. When the inputs have a common negative or low voltage node, as pre-

sented in Figure 6.14, then all the input devices should be powered from the same power source, 

e.g., from the same phase of the three-phase supply system. For this reason, in many PLCs, the 

inputs of a module are grouped into groups of 2, 4, or 8 inputs with a separate power supply. 

In Figure 6.15, two grouping cases with a DC power supply are presented.

C

1

2

3

4

Input module

4 DI

Input

device

L1

L2

Input

signal

Nominal voltage of input module

(b)

+   5 V

_

Logic signal to

CPU, 5 V DC

R

3

Z

1

R

4

R

R

1

Input signal

220 V ΑC

L1

C

D

1

Opto-isolator

0 V

LED

indicator

(a)

+  

_

+  

_

R

2

L2 (C)

Logic signal

Isolation

Conditioning 

Power signal