Introduction to Industrial Automation

6.7   Special Purpose Input/Output Modules

The majority of the requirements of digital and analog I/Os for industrial real-life applications 

have been covered in the previous subsections. However, in industrial automation applications, 

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Analog output module

Analog output module

Figure 6.27  Typical connections of analog devices in an analog output module (a), and four-

wire connection of an analog device (b).

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Basic Operating Principles of PLCs 

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there are times when specific signals are needed that a PLC is not able to process by utilizing the 

previous modules. Some classical examples from this category are the following:

 

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Applications that require a fast response of the PLC input: In these cases, there are devices 

in industrial automation that are producing signals that are varying faster than the scanning 

capabilities of the PLC. This means that the PLC, based on the conventional I/O modules, 

is no longer able to detect rapid changes of the input signal and act on them.

 

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Applications related to the control of rotation of special motors, e.g., the case of stepper 

motors: These motors, and especially their driving circuits, require the supply of special 

pulse trains, where their frequency affects the rotation speed and number of pulses affecting 

their angle of rotation. In this case, conventional I/O modules are not capable of providing 

such signals.

 

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Applications with complicated control problems, such as analog PID control with specific 

response requirements.

 

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Applications where the transmission of data related to human machine interfaces (HMIs), 

such as data for monitors, printers, personal computers, etc., is needed: In addition to this, 

special equipment is needed for the communication of a PLC with another PLC, or for their 

connection to an industrial communication network.

In all these cases, and in other similar ones, special input/output modules are required for 

special manufacturing that is also provided by the majority of PLC manufacturers. A typical 

characteristic of these modules is the fact that they are utilizing their own microprocessor that 

operate independently in order to perform the required task, but are simultaneously connected to 

the main processing unit of the PLC to exchange data and control signals.