Introduction to Industrial Automation

tection from impulse voltages.

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  Introduction to Industrial Automation

6.10.7   Input Impedance Adjustment

Figure 6.51a presents the connection of a photocell, which is a solid-state device, to a digital 

input of the PLC. The fact that the leakage current I

leak

 of the photocell is constant falsely actu-

ates the input of the PLC, which means that the voltage V

Z

 = I

leak

 Z is greater than the one that 

the manufacturer of the PLC defines as the maximum voltage for the input to be considered 

logic “0”. The resistor R is connected in parallel to the input circuit of the PLC, i.e., to the 

Z

N

Digital inputs

module 

Ι

Leak

V

V L

I L

I

R

R

S

N

Ι Leak

V

I

R

V

PLC digital input with

high impedance Z

Digital outputs

module 

PLC digital output

with solid state

switching element

Output device with

impedance Z

(e.g., electronic driver)

S

Photocell

(solid state)

V

L

I

L

Ι

Leak

I

R

V

L

I

L

(a)

(b)

Z

Z

Figure 6.51  Resistive adaption of a digital input (a) or output (b) is applied when the leakage