What Is a Programmable Logic Controller (PLC)?

January 31, 2023

Modern businesses rely on IIoT (Industrial Internet of Things) and PLC (Programmable Logic Controllers) devices to automate the control of industrial processes and systems. While companies primarily use IoT to collect data from various connected devices, PLC provides the logic and control to these devices. IoT can provide real-time data to a PLC, which a company can use to make decisions and control the system. 

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Furthermore, IoT devices can monitor the performance of a programmable logic controller and detect errors or anomalies in the system. Companies can create a powerful and efficient automation system by combining the two technologies.

What Is a PLC (Programmable Logic Controller)?

A programmable logic controller is an industrial computer that uses a set of rules to determine the output for a given input. PLC is one of the various components required to implement SCADA systems.

These systems automate decision-making by receiving input from a machine or device. Then the system makes a decision based on preconfigured rules. Finally, the PLC sends a signal to the output module. 

“We find that companies typically use automation to address a number of opportunities, including increasing throughput and productivity, eliminating variation and improving quality, improving agility and ensuring flexibility, and improving safety and ergonomics.”


PLC History

Over time, PLCs have evolved to help manufacturers with more than just automation. Modern PLCs include functionalities like motion control, storage, and networking. The system also collects and shares equipment data. This data can provide deeper insights into your manufacturing processes. In addition to automating processes, manufacturers use PLCs as a basis for predictive maintenance, which helps minimize equipment downtime.


PLC systems are more advanced and less prone to failure than relay-based control systems. That’s why these systems are now considered one of the most critical components of an industrial control system.

How a PLC Works

PLCs are special-purpose computers. Unlike desktop computers, they don’t have peripherals like mice, keyboards, or even a screen. Instead, a programmable logic controller is a device installed on the manufacturing floor to perform the same functions as relays in a traditional factory.

While running machines using relays as logic can be inefficient, they are a more efficient alternative to running machines based on logic.

PLC Components

PLCs consist of a CPU module and an input-output (I/O) module. You can set up the CPU and I/O modules in two ways. 

  1. Rack-mount modular PLCs use a backplate that provides space for the CPU and I/O modules, and power supply. A common example of a PLC system includes a CPU, DC and analog outputs, and analog inputs.
  2. Using data cables, you can connect the CPU module to I/O modules placed elsewhere. High-speed serial interface (HSSI) connections can connect modules within 50 feet. If you want to place the module further away, use an Ethernet connection.

PLC Input Types

In addition to digital inputs, the CPU module can also handle analog and numerical input and output. Because the CPU system can work with numeric inputs and outputs, you can also use PLCs for calculation. For example, you can calculate statistical process control values with the system

Of course, you’ll need a desktop computer to program the programmable logic controller. Depending on the manufacturer, there may be a few differences in the programming process. The American National Standards Institute (ANSI) has introduced IEC 61131-3, specifying the syntax and semantics of a unified suite of programming languages for programmable controllers (PCs). However, multiple PLC programming languages continue to be in use.

Traditional PLCs were programmed using ladder logic, a PLC programming language. Ladder logic uses a GUI similar to an electrical diagram and includes symbols for contacts, relays, counters, timers, and more. You can create rules (called “rungs”) in ladder logic. The ladder logic is then presented graphically as a ladder diagram where each rung has one output.

However, other PLC programming languages, like structured text (ST) and sequential function chart (SFC), offer a more sophisticated toolkit.

How a Programmable Logic Controller Receives the Input

Once you’ve set up the PLC, the system is ready to receive inputs from an input device or equipment. When the PLC receives input from a machine or human-machine interface (HMI) connected with the input module, the CPU module runs the program and determines the output. Then, the PLC sends the appropriate signal to the output module in real time.

Applications of PLC Systems

As you can imagine, many industries use PLC systems for industrial automation. Examples of industries that use PLC systems include the following:

  • Cement: Cement manufacturing requires using a predefined proportion of raw materials. A PLC system helps monitor and control these proportions.
  • HVAC: Almost all manufacturers need to control heating, ventilating, and air conditioning (HVAC). PLC systems can help control these variables and alert you when needed. For example, the PLC system can monitor the temperature, send a signal to decrease or increase the temperature, and alert you if the temperature cannot go back to an acceptable range.
  • Glass: Manufacturing products like window glass or windshields involve maintaining an accurate proportion of materials and carefully monitoring the production processes. Collective data and implementing quality control are essential, given the complexity of glass manufacturing processes.
  • Other applications: Other industries like oil and gas, automotive, and healthcare also use PLC systems.

How to Maintain a PLC System

If you are running a PLC system, you need to maintain it. Proper maintenance ensures your PLC systems last longer and helps minimize downtime. Consider including the following maintenance activities in your preventive maintenance schedule:

  • Clean debris: Debris can contain particles that corrode or otherwise damage the PLC system. Perform visual inspection for debris and clean the PLC system as and when needed.
  • Change filters: Debris and dust also collect on ventilation filters. Check all enclosures and regularly change filters (according to manufacturer guidelines) to prevent damage.
  • Check modules for wear and tear: The I/O modules are susceptible to general wear and tear like any other equipment. Check the modules regularly to determine if they need to be replaced.
  • Data backup: A system failure can result in data loss. Create a backup of critical information to ensure you save the information needed for decision-making.
  • Monitor the PLC system’s environment: Overheating or high humidity can damage your systems. Consider installing sensors to monitor the PLC system. If you find any objects that can block airflow, like instruction manuals too close to the PLC system, remove them to prevent ventilation issues.
  • Review PLC placement: The closer the PLC system is to the equipment, the better. However, you should keep PLC systems away from equipment that generates heat or noise.

How a CMMS Helps Maintain PLC Systems

The PLC maintenance frequency depends on your facility’s environment, the team’s capacity, and your current maintenance schedule. First, consider the type of maintenance activity. For example, you should clean the PLC system weekly or bi-monthly, but you can schedule data backups at a six-month interval.

Managing these schedules can be tricky, given the varying frequency of maintenance tasks. That’s where a CMMS (computerized maintenance management system) like MaintainX can help. Using a CMMS helps streamline your entire maintenance program, but selecting the right platform is critical.

PLC on MaintainX

MaintainX: A CMMS for Comprehensive Maintenance Programs

MaintainX allows you to create and automate maintenance checklists and procedures. This allows your team to access these resources. MaintainX is a cloud-based, mobile-friendly CMMS with a built-in chat option. Your team can view their tasks on their mobile devices and request help from their managers when needed.

If this sounds interesting, try MaintainX today!

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Caroline Eisner
Caroline Eisner
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