What is a Programmable Logic Controller (PLC)?

Since the dawn of the Industrial Era, people have looked for ways to streamline production and save valuable time. The very invention of the factory process has transformed the world of commerce and ultimately defined how work happens in the modern age.

As automation has improved and more processes can happen without human input, one device in particular has become crucial to modern industry: the Programmable Logic Controller (PLC).

The History of the PLC

A PLC is a computer that monitors other machines or devices in order to manage relays and logic-based triggers for industrial processes. By constantly tracking inputs, a PLC can make decisions that control the overall state of the process. For example, if a certain temperature is reached in one device, the PLC can determine whether or not other devices need to be shut down for safety.

PLCs began in the automotive industry, where inventor Richard Morley created them as an alternative to standard relays. Switching among relays often led to arcing that welded contacts shut, so PLCs were a much safer option. Also, each relay needed its own hardware for logic configuration, while the PLC could centralize its logical operations.

Unlike other controllers, the PLC could be extended and easily altered, and engineers could “mix and match” various devices and input types. Best of all, the PLC used a simple programming language that made it highly adaptable for process management. Due to this versatility, the PLC quickly earned its place in many industrial processes and continues to be used to this day.

Modern PLCs are sophisticated enough to fully automate various machinery and overall inputs. They can identify gaps and triggers at which to start or cease processes. Then, they streamline the overall production, allowing for shorter timelines and better error prevention. Plus, they generate data that can be analyzed to further improve the process’s efficiency.

How Does a PLC Work?

A PLC system comprises a central processing unit, which performs arithmetic and executes logic-based decisions, and the various inputs and outputs of the connected devices. These I/Os can be analog or digital and may include sensors, switches, valves, etc. A PLC’s flexibility permits it to efficiently manage any industrial process.

The core program is downloaded directly into the PLC’s CPU and stored in memory, so the PLU can immediately return to action if the power goes out. A basic program includes four stages:

  • Input scan: the PLC evaluates the state of all connected devices and their inputs (sensors, meters, switches, etc.)
  • Program scan: the CPU executes any pre-programmed logical operations based on the inputs
  • Output scan: the PLC alters or maintains the state of the output devices (valves, relays, drives, light indicators, etc.)
  • Housekeeping: all data and diagnostics are exported to the HMI, IoT devices, etc.

Unlike standard relay systems, where an error could cause a domino effect leading to device failure, a PLC centralizes process management. Its CPU draws data from the various inputs and outputs, then performs operations or diagnostics depending on its programming. Most PLCs use Ladder Logic, also called “C,” which allows programmers to write simple logic-based commands (e.g. “if–then”).

Operating a PLC

PLCs are designed to run themselves, using their internal logic and pre-programmed commands to start, stop, mediate, and report on their connected devices. Thanks to their combination of simple yet flexible programming and overall customizability, they remove the need for constant monitoring or user manipulation.

That said, there is always a situation in which humans need to adjust, examine, or intervene in a process. A PLC can execute or stop processes on its own, but in real-time, it’s most effective to deploy an HMI device. As relatively simple computers, PLCs usually don’t have monitors or detailed read-outs. An HMI, short for Human–Machine Interface, offers a display in which a user can manipulate the device and receive diagnostic reports, process status, and other data.

Now that modern technology has enabled the Internet of Things (Iot), PLCs can also export data to Internet-based applications or files. An HMI may connect a PLC’s information to a web browser, SQL database, or cloud-based application so that users can easily track and report on an industrial process.


A PLC is a simple yet powerful solution for industrial process management. It is highly customizable and can consistently and concurrently manage a range of devices. Once its core programming is installed and its housekeeping scan connected to external data monitors, a PLC is an extremely effective way to automate and monitor complex processes. This ensures the overall safety and productivity of your industrial devices and their associated tasks.

For expert assistance in choosing, developing, and configuring a PLC and its HMI/IoT system, reach out to Browning Electric Co. We’re proud to help companies streamline their industrial processes and collect valuable data from their devices. Contact us for help with designing, building, and programming your PLC system.