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What is a PLC and How Does it Work?

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    Rosa Tiara

What is a PLC?

Programmable Logic Controller—also known as PLC—is an industrial digital computer that programmed to perform control functions in industries. PLC was founded initially to replace relay, as it includes complex hardwiring. However, PLC is capable not only performing relay switching tasks, but also performing other complex applications such as timing, counting, calculating, comparing, or even analog signal processing!

The Upsides of PLC

PLC offers over a wide range of benefits compared to conventional relay control. When using relay, you have to hardwire it to perform a specific function. It will get harder when you need to change your system's requirements, because that means you have to modify your relay wiring. Replacing hardwiring process that relay has is the main advantage of using PLC. Nonetheless, PLC also provides some other benefits:

✔️ Increased Flexibility

If you need to change or modify your application's configuration, you only need to change your code and skip the wiring process, because PLCS are program-based. PLCs work with the relationships between user inputs and outputs rather than how they are connected.

✔️ Lower Cost

If an application has more than about six control relays, it will probably cost lower to install a PLC.

💡 6 relays = 1 PLC

✔️ Capable to Communicate

PLC is capable to communicate with other controllers or computer equipment to perform functions such as data gathering, device monitoring, process parameters, and programs downloads and uploads. This kind of communication is performed by a PLC communication module shown at the following image.

A PLC communication module

✔️ Faster Response Time

PLCs are designed to serve high-speed and real-time applications, which means that an event or input by user will simultaneously executed by the PLC's quick-response capability.

✔️ Easy to Troubleshoot

Resident diagnostics and override functions that PLC has makes it easier for the maintainer to trace and fix the software and hardware issue. This can be done by displaying the control program on the PLC monitor and watch it in real time as it executes.

What's Inside the PLC?

There are four main parts that made up a typical PLC's architecture. These are the power supply, central processing unit (CPU), input section, and output section. Note that the term architecture refers to PLC hardware, PLC software, or even a combination of both.

plc architecture
  • Power supply: supplies DC power to the PLC.
  • CPU: the "brain" of the PLC, consists of a microprocessor to do the logic and controlling functions by the help of memory to store program's data.
  • Input/output section: interfaces to fetch input and produce output.

Three Kinds of Architecture in PLC

PLC architecture innovation has tremendously improved since its inception back in the 1960s. The three distinct types of PLC architecture available for industries are fixed, modular, and distributed. Let's see how each of them works.


Fixed PLCs are most commonly used for smaller sized and less complex application. This PLC system employs off-the-shelf components that adhere to accepted standards.

In a fixed PLC, all the hardware components are embedded into a sigle unit. The following is a diagram to visualize how a fixed PLC is structured.

fixed plc

Other names industries usually use to call a fixed PLC are integrated, micro, nano, compact, small, mini, basic, unitary, standard, and brick. If you're not sure whether you should use a fixed PLC, consider these ups and downs of it.

Small in sizeComplex tasks can be difficult to process as it has small memory
Low in costNot flexible (input, output, and communication interfaces are fixed)
Fast and easy to installOnly suitable for basic applications


Unlike fixed PLCs, each of its hardware components has a separate module. Those modules are connected using a common mounting system, thus makes it possible for a modular PLC to be specific for a certain application (more flexible than fixed PLCs). Modular PLCs are generally utilized for applications that require powerful processors and a large number of inputs and outputs.

modular plc

To sum up, these are some of the advantages and disadvantages of using modular PLCs.

Larger memory than fixed PLCsTakes up more space because of its large size
Higher performanceMore complex to install
Larger number of inputs and outputsHigher cost

Distributed Control System

Distributed Control Sytem (DCS) is a high-end system with modular architecture that allows hardware components to be connected from multiple places. This task is accomplished through the use of high-speed communication networks.

automation pyramid

The above image is called as Automation Pyramid. Automation Pyramid is a pictorial example of the different levels of automation in industries.

  • ERP -> Enterprise Resource Planning
  • MES -> Manufacturing Execution System
  • DCS -> Distributed Control System
  • SCADA -> Supervisory Control and Data Acquisition
  • HMI -> Human-Machine Interface
  • PLC -> Programmable Logic Controller

DCS is slightly different with PLC. In terms of automation pyramid levels, DCS is more advanced than PLC since it contains the supervision or supervisory aspect, whereas PLC doesn't.

DCS has such high-performance processors, enormous memories, and can handle significantly higher number of inputs and outputs than a PLC! To address those complicated duties, DCS also employs higher level programming languages.

Plant wide control network with multiple processors and remote I/OMuch more difficult to install because the mounting system is complex
Higher performance than modular PLCsRequires higher level programming skills
Can handle large data and complex tasksHigher cost

How PLC Works

To gain an understanding of how PLC works, take a look at the following cycle.

plc scan cycle

It is called as a scan cycle, a cycle that PLC follows when they're doing their jobs. At the very first cycle, PLC receives inputs or instructions from the maintainer. After reading them, the system will put the inputs through their paces to generate the desired outputs.

These outputs are used to perform diagnostics based on the requirements of the system. The input interface will receive these diagnostics and use them as input. A PLC system's subsequent cycles will automatically get better based on the diagnostics and updated outputs from the previous cycle.