Introduction to Industrial Automation

(Allen Bradley).

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Basic Operating Principles of PLCs 

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205

As was mentioned previously, each input signal connected to a digital input module is a dis-

crete signal of two ON and OFF states. More specifically, the ON state corresponds to the nomi-

nal input voltage and the OFF state to zero. Because the input voltages may be of different range 

or type, the digital input modules are offered by the manufacturers in various AC and DC nomi-

nal voltages that are compatible with the corresponding standard operating voltages of the input 

devices. These are the 24, 48, 110, and 230 V DC or AC, the most popular being 24 V DC, since 

it is selected by the majority of engineers when designing new automation systems, mainly because 

of the protection from electrical shock that it offers. Each digital input module is characterized by 

the nominal operating voltage and the maximum number of input signals that it can sample. The 

usual standard number of inputs per module is 4, 6, 8, 10, 16, and 32.

The term “input devices” characterizes the devices that are able to carry commands or infor-

mation from the industrial automation system or the operator of the control system, which in this 

case is the PLC. As an example, the pressing of a button from the operator of a machine is “com-

mand” to the control system (PLC), while the activation of a limit switch from a moving part of 

a machine informs the PLC that the component has received the desired end position. Typical 

input devices are:

 

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Limit switches

 

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Selector switches

 

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Buttons of all kinds

 

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Rotary switches

 

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Switching relay contacts

 

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Photoelectric switches

 

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Proximity switches

 

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Keyboard contacts

 

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Any kind of converters

 

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Arithmetic switches

Input devices have been described extensively in Chapter 2, where the major types of sensors, 

actuators, and power switching devices were analyzed. However, in this chapter, we will only men-

tion “numerical switches”, which are able to insert numbers in the PLC in a simplified approach 

without using a computer or other digital programming device. The numeric switches carry a 

numbered thumbwheel switch, through which the numerical data to be transferred to the PLC by 

using a limited number of digital inputs can be selected. A typical kind of this numerical switch 

Multiplexing

electronics

Optical

isolation

Input signals 24, 48, 110, 230 V DC/AC

230 V AC

Input

device

5 V DC

To CPU/

inputs’

memory

Block of terminals

Conditioning

and level

conversion

Status

indication

of inputs