Introduction to Industrial Automation

6.10.6   Input Devices with Leakage Current—

Impedance Adjustment of I/Os

During the installation, interconnection, and operation of PLCs, special care should be pro-

vided to the issue of resistance adaptation of I/O circuits. Today, many of the input sensors and, 

in general, the input devices that cooperate with the PLCs, are made up of solid-state electronic 

circuits. Every input device containing a solid-state switching element (e.g., a triac, a transis-

tor, etc.) that is connected in series with the high impedance digital input of a PLC, may cause 

an erroneous (i.e., no real signal) activation of the input. The same problem also occurs when 

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  Introduction to Industrial Automation

PLC 1

PS

C

P

PS

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U

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PLC 2

PLC n

PLC 1

PLC 2 

PLC n

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(a)

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(b)

Figure 6.49  Protective devices against impulse voltage in a communication bus at the ends of