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What Is a Programmable Logic Controller?

December 20, 2022 Brian Diver

If you've ever wondered, "What is a programmable logic controller?" we have the answer you're looking for. Programmable logic controllers have revolutionized automation in the manufacturing industry.

This article will review the definition of a programmable logic controller, detail the construction of a controller, and answer the question, "How does a PLC work?"

what is a programmable logic controller

What is a PLC?

Many people have asked, "What does PLC stand for?" The answer is simple. PLC is the abbreviation for programmable logic controller.

A PLC — programmable logic controller — is a small industrial computer that can be adapted to control a wide range of manufacturing activities. PLCs are made of a few simple components:

  • A central processing unit
  • Input and output modules
  • A memory unit
  • A communications interface
  • A power supply

Programming controllers are solid-state devices, meaning they have no moving parts.

How Does a PLC Work?

A PLC works in a very straightforward manner. It takes inputs from a variety of sources, including:

  • Limit switches
  • Photoelectric sensors
  • Proximity sensors
  • Human-machine interfaces
  • Pressure switches
  • Level switches
  • Temperature switches
  • Vacuum switches
  • Float switches

It then processes the inputs based on its predetermined programming and creates outputs. Depending on the programming, a PLC can control the processes of a wide range of devices, such as:

  • Motors
  • Solenoid valves
  • Switchgear
  • Warning lights
  • Emergency shut-offs
  • Relays
  • Drives
  • Temperature controls
  • Counters

They can also monitor and record conditions like productivity or operating temperature.

Regardless of the input and output devices, PLCs generally operate on a loop, continuously checking for inputs, processing their programs, and issuing outputs. They can automate a single process or an entire production line.

Some PLCs operate independently with a handful of inputs and outputs, while others can be networked in large racks, managing thousands of inputs and outputs.

plc programming

Find the PLCs You Need

Industrial Automation Co. offers a wide range of PLCs, and Allen-Bradley and Mitsubishi parts. We provide durable, dependable PLCs that you can count on. Leading manufacturers rely on us for the products they need to keep their operations running.

Types of PLC Programming

Several kinds of PLC programming languages are outlined in the IEC 61131-3 international standard for programmable logic controllers. These include:

  • Ladder diagrams — A schematic language that uses symbols to represent various actions
  • Functional block diagrams — Diagrams that utilize blocks to describe functions between input and output variables
  • A high-level language that uses sentence statements like "if/then/else" or "repeat/until
  • Sequential function charts — These allow complex control systems to be programmed using basic building blocks that run specific sub-routines

The Benefits of PLC Systems

PLC systems offer numerous advantages, especially compared to the old relay logic systems, which were hard-wired and difficult to adapt and maintain. On the other hand, PLCs are easy to alter, since you need to load a new program into the central processing unit. Other benefits include:


PLCs are designed to handle often harsh manufacturing environments and are unaffected by dust, moisture, extreme temperatures, and debris.

Simpler Programming

Unlike relay systems, which require complex logic sequences, PLCs can be controlled with languages that are easier to learn and execute.


Most PLCs are capable of handling a wide range of functions and systems.

Ease of Troubleshooting

Because PLCs have so few components, they're simple to troubleshoot when problems arise.


PLCs consume little electric power and can help conserve energy.

Faster Responses

PLCs operate in real-time and can respond immediately to inputs.

What to Consider When Selecting a PLC

Many factors go into the selection of a PLC, meaning you need to choose carefully to ensure the right fit for your operation. For instance, does your manufacturing process use discrete functions, like "on/off" switches, or does it require analog functions that can manage variables? Some PLCs offer each, while others can simultaneously handle discrete and analog functions.

Other factors to consider include:

  • System compatibility
  • Processing speed
  • Number of ports
  • Memory
  • Durability

Buy the PLCs You Want at Industrial Automation Co.

Now that we've answered questions like "What is a programmable logic controller?" and "How does a PLC work?" you're ready for the next step. Whether you need Allen-Bradley replacement parts, Mitsubishi products, or other PLCs, Industrial Automation Co. has what you want. We stock major brands, so you can find the PLCs that will save your operation time and money.

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