What Are PLCs (Programmable Logic Controllers)?

A close-up shot of a circuit board with wires and electronic components, surrounded by various hairstyles and outfits.

Are you curious about the invisible gears that keep our industrial world running smoothly? One such crucial component is the Programmable Logic Controller, or PLC, a tiny computer device that plays a key role in automating manufacturing processes.

A PLC, or Programmable Logic Controller, is a small computer device used for industrial control systems. It receives information from sensors or input devices, processes the data, and triggers outputs based on pre-programmed parameters. PLCs are commonly used in manufacturing processes.

In this article, we will peel back the layers of complexity to better understand what PLCs are, how they function, and their significance in our everyday lives. Intriguing isn’t it? Let’s delve into this fascinating world of industrial control systems!

Key Takeaways

  • PLCs, or Programmable Logic Controllers, are tiny computer devices used to automate and control industrial processes.
  • They receive information from sensors or input devices and make decisions based on pre – programmed instructions.
  • PLCs help improve automation and efficiency in industries by reducing human error and performing complex tasks quickly.
  • They contribute to increased reliability, safety, and reduced downtime in industrial settings.

What is a PLC?

A PLC controller is connected to industrial machinery in a factory, showcasing industrial automation and technology.

A PLC, or programmable logic controller, is a digital computer used for automation and control of various industrial processes.

Definition and purpose of a PLC

A PLC is a tiny computer device. It helps control systems in factories. People call it Programmable Logic Controller or PLC for short. PLC uses data from sensors or input devices. Then, it starts certain actions based on rules set beforehand.

This makes work easier and faster in places like car making plants, food packaging units, and medicine factories. Because of this, the role of a PLC is very important in these fields.

It lets us control processes with great accuracy and speed.

History and development of PLCs

PLCs first came to life in the late 1960s. A car maker named General Motors wanted a better way for their machines to work. They did not want to change hard wires all the time. So, they asked for a machine that uses computer code instead of wires.

Richard Morley and his team decided to help. They made the first PLC named “Modicon 084”. This new device helped people use machines in a more efficient way.

Years passed, and PLCs grew smarter and smaller. Today, you can use them in many different ways like controlling lights or moving heavy objects in factories. Some even have touch-screen panels! Not only cars but food makers and drug companies also rely on these smart devices now.

The invention of PLCs made running big industries easier than ever before.

Components and architecture of a PLC

Let’s look at the parts of a PLC, called components. There are five main parts:

  1. CPU (Central Processing Unit): This is the brain. It makes all the decisions.
  2. Input and Output Modules: These modules get information from things like switches or sensors. They also send signals to control machines.
  3. Power Supply: This gives power to all parts of the PLC.
  4. Programming Device: This is where people can tell the PLC what to do.
  5. Rack or Mounting: This holds everything in place.

How do PLCs work?

The image showcases a control room with industrial devices and a PLC system, without any human presence.

PLCs work by receiving input signals from various devices, processing them according to a programmed logic, and generating output signals to control different industrial processes.

Operation and functioning of a PLC

A PLC is like a small computer that controls things in factories and other places. It takes information from sensors or input devices, thinks about it, and then tells things to happen based on what it’s been told.

It does this by following pre-programmed instructions that people write for it. PLCs are used in factories to make things work faster and more accurately. They help with different tasks like moving parts on an assembly line or controlling robotic machines.

PLCs are important because they can do many different jobs and make sure everything runs smoothly in industrial settings.

Input/Output devices and signals

PLCs use input devices to receive information and output devices to trigger actions. Here are some examples of input/output devices and signals used in PLCs:

  • Push buttons: These are physical buttons that workers can press to send a signal to the PLC.
  • Proximity sensors: These sensors detect the presence or absence of an object using infrared, ultrasonic, or magnetic fields.
  • Photoelectric sensors: They use light beams to detect the presence or absence of an object.
  • Pressure sensors: These measure pressure changes and can be used for controlling hydraulic or pneumatic systems.
  • Temperature sensors: They measure temperature changes and help control heating or cooling systems.
  • Motors and valves: These output devices are controlled by the PLC and perform actions based on the program’s instructions.
  • Digital signals: These signals have two states – on/off or high/low – and are used for simple binary control.
  • Analog signals: These signals have a continuous range of values and are used for more precise control, such as variable speed drives.

Programming languages used in PLCs

PLCs use different programming languages to control and automate industrial processes. Some common programming languages used in PLCs include:

  1. ladder logic
  2. function block diagram (FBD)
  3. structured text (ST)
  4. instruction list (IL)
  5. sequential function chart (SFC)

Benefits of PLCs

PLCs provide increased automation and efficiency, improved reliability and safety, and reduced downtime and maintenance. Read on to learn more about the advantages of utilizing PLCs in various industrial settings.

Increased automation and efficiency

PLCs, or Programmable Logic Controllers, play a crucial role in increasing automation and efficiency in industrial processes. They are small computer devices that receive information from sensors and input devices, process the data, and make decisions based on pre-programmed parameters.

By automating tasks that were previously done manually, PLCs can greatly improve productivity and reduce human error. They can also perform complex operations at high speeds, leading to faster production cycles and increased efficiency.

With their ability to monitor input devices and make quick decisions, PLCs ensure that processes run smoothly without any delays or bottlenecks. This not only saves time but also reduces costs by minimizing downtime and maximizing output.

Improved reliability and safety

PLCs play a crucial role in improving reliability and safety in industrial processes. With their advanced features and robust design, PLCs ensure that machines and systems operate smoothly without any breakdowns or failures.

They are designed to handle harsh industrial environments and can withstand extreme temperatures, vibrations, and electrical noise. This means that they are highly reliable and can continue working even under challenging conditions.

In terms of safety, PLCs have built-in safety features that prevent accidents and protect workers. For example, they can monitor the speed of rotating equipment to ensure that it doesn’t exceed safe limits.

They can also detect abnormal conditions such as leaks or excessive pressure in a system and take appropriate actions to prevent dangerous situations.

Moreover, PLCs allow for easy diagnosis of faults or malfunctions through their programming interface. This makes it easier for maintenance personnel to identify the source of a problem quickly and take corrective action promptly.

Reduced downtime and maintenance

PLCs offer several benefits in industrial settings, including reduced downtime and maintenance. With their reliable performance and easy fault diagnosis, PLCs help minimize system failures and breakdowns.

This means less time spent on repairs or troubleshooting, resulting in increased productivity and efficiency. Additionally, PLCs allow for remote monitoring and diagnostics, enabling technicians to identify issues quickly without the need for onsite visits.

By reducing downtime and maintenance requirements, PLCs contribute to overall cost savings and smoother operations in various industries.

Types of PLCs

There are two main types of PLCs – modular PLCs and compact PLCs, each with their own advantages and applications.

Modular PLCs vs. Compact PLCs

Modular PLCs and Compact PLCs are two common types of programmable logic controllers utilized in various industries today, and each one offers specific advantages. Let’s review these two types in a detailed manner.

Modular PLCsModular PLCs provide more flexibility due to their ability to be expanded.They can handle more complex tasks due to their higher processing power.A fault in one module doesn’t affect the entire system.They are more complex to program and setup.Modular PLCs are typically larger and more expensive than compact PLCs.
Compact PLCsCompact PLCs are easy to program and setup.They are smaller, cheaper, and ideal for applications with limited I/O points.Compact PLCs have limited expandability and may not be suitable for complex tasks.A fault in one part can impact the entire system.

Both types of PLCs are widely utilized in automotive, food processing, and pharmaceutical industries to increase automation and efficiency. The choice between a modular or compact PLC will depend on the specific requirements of each industrial application.

Allen-Bradley and GE Fanuc PLC modules

Allen-Bradley and GE Fanuc are two popular brands of PLC modules used in industrial automation. These modules are designed to work with the overall PLC system, providing specific functions and capabilities.

Allen-Bradley PLC modules offer a wide range of options, including input/output (I/O) modules, communication modules, and power supplies. They are known for their reliability and flexibility in various applications.

On the other hand, GE Fanuc PLC modules are also widely used and provide excellent performance in controlling different processes. They offer features like high-speed processing, extensive memory capacity, and advanced communication capabilities.

Distributed I/O systems

Distributed I/O systems are an important part of PLCs. These systems allow for the connection of input and output devices to the PLC through remote modules, which are placed closer to the actual devices being controlled or monitored.

This helps in reducing wiring costs and simplifying installation. The remote modules communicate with the main PLC unit using a network, such as Ethernet or fieldbus protocols. This enables flexibility in system design and allows for expansion and modification without major rewiring.

Distributed I/O systems are commonly used in large-scale industrial applications where there is a need for multiple input and output points spread across a wide area.

Advanced PLC Features

Advanced PLC features include embedded controllers and microcontrollers, single-board computers, and programmable logic relays (PLR).

Embedded controllers and microcontrollers

Embedded controllers and microcontrollers are advanced features found in some PLCs. These components enhance the functionality and capabilities of a programmable logic controller. An embedded controller is a small computer system that is integrated directly into the PLC, allowing it to perform more complex tasks.

Microcontrollers, on the other hand, are specialized chips within the PLC that handle specific functions or processes. These advanced features enable PLCs to control intricate operations with speed and precision.

They also offer greater flexibility in programming options, making it easier to customize the PLC for specific applications. With embedded controllers and microcontrollers, industrial automation becomes even more efficient and reliable.

Single-board computers

Single-board computers are a type of computer that is built on a single circuit board. They have all the essential components like the processor, memory, storage, and input/output ports integrated onto one board, making them compact and efficient.

These computers are commonly used in industrial applications and can be programmed to perform specific tasks. A single-board computer is similar to a regular computer but has fewer features and is designed for embedded systems or specialized applications.

They are often used in conjunction with PLCs to provide additional processing power and capabilities. Single-board computers play an important role in automation by enhancing the control system’s functionality and enabling more complex operations.

Programmable logic relays (PLR)

Programmable logic relays (PLR) are another important feature of PLCs. These relays are used to control and monitor electrical circuits in industrial settings. They can be programmed to perform specific functions such as turning on or off motors, opening or closing valves, and controlling the speed of machines.

PLRs are like electronic switches that can be easily customized according to the needs of a particular process. They play a crucial role in ensuring the smooth operation and coordination of various components within an industrial control system.


In conclusion, PLCs have revolutionized industrial control systems with their small computer devices. They automate processes, monitor input devices, and make decisions based on pre-programmed instructions.

With their reliability and efficiency, PLCs play a crucial role in improving productivity and accuracy in various industries today.


1. What is PLC in simple words?

PLC, or Programmable Logic Controller, is a device that gives instructions to machines and tools.

2. Can you tell me the basics of programmable logic controllers known as PLCs?

The basics of PLCs are that they take inputs like buttons or sensors, use a program to decide what to do, then order outputs like motors or lights.

3. What does an introduction of PLC controllers cover?

An intro of PLC controllers covers how they work, their parts and how we can use them in many jobs.

4. Can you name the five key parts of a programmable logic controller (PLC)?

Yes! The five big parts of a PLC are input units, output units, central processing unit (CPU), power supply unit (PSU) and programming device.

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