Introduction to Industrial Automation

6.3   PLC I/O Components

For most PLCs, I/O modules are one of the most common and simple parts of equipment, but 

also the most important after the microprocessor unit. These I/O modules constitute the interface 

between the various I/O devices of the industrial automation system and the microprocessor that 

executes the desired program, and hence perform the control logic. Although the form of the I/O 

modules differs from manufacturer to manufacturer, without exception, all of them require a 

power source, carry the necessary electronic circuits for converting the I/O signals and to commu-

nicate with the microprocessor, have a terminal block for the cable connection and, finally, carry 

the necessary mechanism for mounting in the case of a modular structure. The standard and most 

common I/O modules are the following:

  1. Digital input modules

  2. Digital output modules

  3. Analog input modules

  4. Analog output modules

In addition to these modules, there are many others for special purposes. A module can 

contain only inputs, only outputs, or a combination of inputs and outputs. Digital I/O modules 

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Digital inputs

Digital inputs

Digital Outputs

Digital Outputs

Analog inputs

Digital inputs

Digital outputs

Analog inputs

Analog outputs

Central unit

Communication

modules

Expansion cable

Expansion unit 

Expansion base for

mounting I/O modules

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Digital inputs

Digital inputs

Digital outputs

Digital outputs

Analog inputs

Digital inputs

Digital outputs

Analog inputs

Analog outputs