Programmable Logic Controller (PLC)

What Is a Programmable Logic Controller (PLC)?

Answered September 16 2020

programmable logic controller

In this day and age, we’ve known computers to be high-tech devices capable of immensely complex computing power. Computers commonly refer to some of our most prized personal possessions, such as laptops, desktops, and handheld gadgets. However, for their most basic purpose, computers are fundamentally machines that operate using a precisely logical process. For industrial applications, computers more commonly come in the form of what are known as programmable logic controllers.

What Is a PLC?

A programmable logic controller, PLC for short, is the brain of a modern-day facility that depends on various machines and equipment. PLCs are small computers designed for heavy-duty industrial applications. Their rugged design allows them to thrive even in the harsh conditions of industrial operations. While relatively simple, PLCs are capable of applying programmable logical rules to automate processes and functions of plant equipment.

Before PLCs, relays were the only available way to control machines. Relays would energize and de-energize, in turn switching a component on or off. While this was a viable solution for one, two, or a few pieces of equipment, relay systems did not scale well with the size of operations. Control systems were a nightmare to design, maintain, and troubleshoot, as the number of components increased. PLCs revolutionized the way machines were controlled by offering computer-like capabilities while being competitively priced compared to the relay alternative at the time.

PLCs can generally fall into one of two general types:

1. Compact PLC

A compact PLC has a fixed number of input and output capabilities set by the manufacturer. These are generally appropriate for small-scale applications.

2. Modular PLC

A modular PLC, as the name implies, consists of individual modules or units that can more easily expand as needed. It offers additional flexibility in terms of capacity, which makes it advantageous for larger-scale operations.

How does a Programmable Logic Controller work?

A PLC takes in data through input modules, processes the information it gathers, and finally transmits its calculated outputs. Depending on the requirements of the plant, as well as the availability of inputs, PLCs can check for specific conditions to trigger corresponding actions.

The following are some key features of PLCs that allow them to perform the required tasks effectively.

Input Modules

A Programmable Logic Controller needs to gather information before it can even start processing any data. There are two main sources of inputs: those that come from equipment readings and those that are human-facilitated. Input modules connect the PLC with devices on equipment such as sensors, level detectors, and other measuring instruments. These input modules can also connect with human-facilitated inputs such as button pushes, switches, and dials.

Output Modules

After processing data, a PLC needs to translate its results to corresponding external actions for the linked equipment. Output modules connect the PLC to output devices such as relays, lights, valves, and other components.

Central Processing Units

Central processing units, or CPUs, act as the brain of a PLC. They read and interpret the input data to produce results and commands that come out of the output modules as signals.


In modern control systems, PLCs are components of a larger network of devices. Aside from the connections internal to the PLC itself, external connections to other components allow data to be more readily transmittable. For such applications, communication protocols must be in place to ensure that PLCs can send data to other systems.

Human-Machine Interface

In most cases, operators and facility staff require the information that is being processed by the PLCs. Human-machine interfaces (HMIs) should be in place to translate information within the processing units to an understandable form. HMIs can take the form of a display panel or simple status indicators.

These features of a PLC typically interact and operate in cycles. The user defines the required tasks through the programmable logic of the PLC. A typical step-by-step process may resemble the following:

  1. The PLC checks the status of the input modules and any connected devices.
  2. The CPU takes in the inputs then executes the logic defined by the user through the program.
  3. The output modules of the PLC send the signal to perform physical actions to the attached devices. Typical output commands would be to switch on or off, increase or decrease speed, and the like.
  4. The PLC runs an overall check to report the operating conditions of the attached devices. At this point, the PLC executes communication protocols to send out data to another system or some HMI.
  5. The PLC either ends or restarts the process as defined by the program.

What Is the Difference Between a PLC and SCADA?

SCADA stands for Supervisory Control and Data Acquisition. SCADA is generally a software system used to monitor various aspects of a plant facility and its components. It also allows the user to enter configured commands that translate to equipment actions. In simpler terms, SCADA acts as an overall control system that allows manipulation of both software and hardware elements.

In contrast, PLCs are generally hardware systems that directly link to power up machines, motors, and other equipment. With modified communication features, PLCs are also used to gather data from the equipment they control.

Together, PLC and SCADA systems allow users more control and flexibility over the whole facility. Imagine each machine to be controlled by a PLC, with potentially multiple PLCs across the facility. Further, you can suppose that all PLCs consolidate into a comprehensive view through SCADA. Not only does the user have full visibility of operations, but also total virtual control over the plant. With proper data communication in place, SCADA and PLC systems are constructive tools to implement a predictive approach in maintenance.


PLCs are some of the most adaptable products that have been in use for the past couple of decades across various industries. Apart from their practical and affordable design, PLCs are easily able to integrate with more recent innovations in technology such as SCADA.

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