The Ultimate Guide to Programmable Logic Controllers (PLCs)

Worker supervising PLC

A programmable logic controller (PLC) works to control a computer system in an industrial organization. PLCs monitor the inputs to the system and then make decisions about related outputs. Typically used to monitor motors or machines, PLCs are often the basis of a predictive maintenance system, which can warn businesses of potential problems before they cause major breakdowns.

In this guide, we'll cover:

  • The history of the PLC
  • Types and components of PLCs
  • PLC maintenance best practices and checklist
  • Real-life PLC applications
  • PLC risks and benefits

Be sure to click the hyperlinks in the headings for more information on a specific topic.

History of PLC

Dick Morley explains that he built the first PLC in his garage without realizing that he was building it. Morley’s garage-based business developed a simple relay design with four outputs, which was attractive to automotive companies in 1969.

By 1973, Michael Greenburg had created a PLC that was commercially available, and it was refined by Modicon to be used by General Motors and Landis. Physical relays, wired connections, and timers benefited from the PLC ladder logic, which allowed increased functionality without a lot of new wiring and hardware.

Over the next seven years, PC-based software came into the picture, which meant that PLCs could be programmed, processing speeds could be increased, and new features could be more easily developed.

By the turn of the century, an advanced version of PLC was renamed a programmable automation controller (PAC). PAC continued to use PC-based software but added human-machine interface (HMI) as well as asset management. Although larger organizations began using PAC, small- to medium-sized businesses still found the less advanced PLC technology extremely beneficial to their companies.

PLC Basics

Manufacturers use PLCs to monitor selected inputs on a continuous basis. They then run these inputs through a computer system to generate appropriate outputs. The end result is that industrial equipment, production lines, and manufacturing processes become more efficient. PLCs help adjust or repeat specific operations while collecting and sharing data that can help manufacturers make better decisions.

The beauty of PLC systems is that they can be customized to use particular input and output devices. By taking advantage of PLC flexibility as well as understanding related technologies, you can choose and build the best solution to meet your organization’s needs.

PAC: As mentioned before, PLC is typically used for machine control while PACs are designed for more complex automation systems that include HMI, advanced process control, or asset management.

SCADA: This stands for supervisory control and data acquisition and often works as the software partner of a PLC hardware system. When implemented together, SCADA and PLC can form the backbone of a predictive maintenance system.

DCS: A distributed control system (DCS) is frequently made up of multiple PLCs. Each PLC is assigned to one process or machine; a DCS are several PLCs that handle the needs of a larger manufacturing plant. They are connected via a complex communication system, giving a manager a better bird's-eye view of the entire facility.

DDC: Known as direct digital control, DDC collects sensor information, assigns it to the proper domain, and manages actuators, often for building automation applications.

HMI: HMIs is the component of a PLC that allows an employee to interact with a machine directly. A technician may review data, adjust inputs, manage outputs, or make related decisions using the HMI.

Types and Components of PLC

As we know, PLC is sort of the brain of the manufacturing plant that allows management and technicians to maintain better control of various processes and equipment. These small yet powerful computer systems are designed for industrial applications, and they are often able to function in harsh environmental conditions. 

PLCs are typically either compact or modular. Compact PLCs have a set number of input and output capabilities and work well for smaller applications. Modular PLCs allow easier expansion of an overall system. Larger or growing companies may opt for modular systems, which can be customized for a current need and added onto later.

Key Components

Although PLCs are customizable, each one includes common key components.

Inputs: The information gathered by PLCs come through different input devices. They may include sensor-generated data from specific equipment or come from individuals entering data manually through dials or buttons.

Outputs: Output devices include things like valves, lights, and relays that respond to the interpreted results of input data.

CPUs: Also known as the brain of the PLC, the central processing units evaluate the data based on pre-established rules to generate actions for the output devices.

Communications: A network of communication equipment and associated protocols help the data flow throughout the organization as needed.

HMI: This interface allows managers and technicians to extract the data from a PLC system. Ultimately, the information needs to be used to make facility-related and business decisions.

Typical PLC Operational Steps

The PLC monitors connected inputs of its specific devices, and then measures them against any rules or parameters that have been established. The CPU considers whether any outputs should be triggered based on the programmed logic requirements. If so, the output components tell the devices to perform an action such as turning something on or off or changing the speed of a process.

During this time, the PLC can be accumulating overall checks and communicating results through an HMI. This process may begin and end as directed by a technician or repeat continuously.

PLC Maintenance

Like any other major system in an industrial setting, it’s important to keep your PLC system well-maintained to ensure it is running as optimally as possible.

PLC Maintenance Checklist

The following tasks should be part of any general PLC maintenance program. Others may need to be added, depending on your particular facility and equipment needs. Be sure to schedule these preventive maintenance tasks regularly.

Clean Dust: Any industrial setting will generate dust over time, and this dust can wreak havoc on a computer system as complex as a PLC. Be sure to remove dust on all input and output devices as well as the hardware itself regularly. Although you may want to use a low-dust enclosure to help, be sure ventilation remains adequate.

Change Filters: Along the same lines, be sure to change ventilation filters in any PLC enclosures to help with dust control. The frequency should be determined by environmental conditions and specific needs.

Tidy Area: If you find that things like papers, books, and manuals tend to accumulate around your PLC area, this can cause ventilation issues. Be sure to tidy the area to ensure proper air flow.

Inspect Connections: A PLC relies on solid connections to do its job properly. Be sure to periodically check things like plugs, sockets, and terminals to make sure all connections are secure. Areas with high vibrations are more prone to loose connections so be sure those are checked more frequently, especially things like screws and bolts.

Replace Modules: From time to time, you may need to replace input or output modules from general wear. Be sure to follow instructions when replacing modules in terms of turning off power as needed.

Increase Awareness: Pay attention to unusual activity. For example, if you find that input and output devices are frequently burning out, you may want to check for power spikes or shorts. You may also want to employ a back-up power source, just in case. Be sure to keep your backup maintained as well.

Backup Data: Be sure to periodically back up your data in case something causes your system to lose critical information. Most facilities choose to do this at least twice a year.

Check Environmental Conditions: Be sure to monitor things like humidity levels and temperature as they can hurt your PLC components. Sensors can help you monitor these conditions around the clock.

Calibrate Devices: If any of your input or output devices require calibration, be sure to include those tasks in your preventive maintenance program. Be sure circuit cards are calibrated every six months as well.

Conduct Visual Inspection: Take a regular look at your components for discoloration and wear as well as burning smells.

Check LED Lights: If your system has LED indicators, be sure to check them regularly. They will alert you to change your RAM module battery or other such requirements.

Review Error History: Take a look at regular reports to see if your PLC system has noted any scanning or error flags. Be sure to find the cause of these problems early.

Inspect Sensors: If sensors are part of your PLC system, be sure they are maintained per manufacturers’ recommendations.

Source EMI: Check your local wiring to see if you have any issues with electromagnetic interference. Be sure your lower-level components are far away from high-current wires to avoid static electricity problems.

Review Proximity of Equipment. Although your PLC should be near the machine it's controlling, be sure you keep other equipment, particularly those that generate noise or heat, away from your PLC.

Keep Current: If your system has recalls, patches, product notices, or upgrades, be sure to incorporate them into your preventive maintenance program to stay current.

Frequency of Maintenance

The frequency of maintenance should be determined on a facility-by-facility basis. Things like the surrounding environment, how often machines are run, and the size, priorities, and capacity of the maintenance staff must be considered.

Basic tasks such as dusting and tidying should be performed daily to ensure the PLC can be working optimally most of the time. Things like backing up data should be done every six months regardless of conditions.

However, other preventive maintenance tasks will need to be scheduled on either a time or usage basis. If you have a computerized maintenance management system (CMMS), you can easily collect historic data to help you set the ideal preventive maintenance schedule. For instance, if you see that a particular component tends to break every 8 or 9 months, you may want to schedule it to be changed twice a year to prevent that breakdown.

Maintenance Best Practices

Besides these common maintenance tips, you may want to consider the following best practices to help you refine your preventive maintenance program.

Inventory PLCs: Since manufacturers can have one PLC for each process or piece of equipment, it’s easy to lose track of them over time. Be sure you record how many you have, identifying information such as brand or model number, which machines they control, and their maintenance history.

Train Employees: Be sure at least one person per shift knows how to use, fix, and maintain your PLCs. 

Commit to Single Brand: By sticking to the same brand of PLC, you can more easily streamline training, maintenance, spare parts, and usage.

Safeguard Equipment: Consider investing in low-dust enclosures, filters, and power protection to lengthen the lifespan of your PLC.

Stock Spares: Be sure you have a reasonable number of parts to maintain your PLC system and a way to track their location, usage, and replenishment.

Benefits of PLC Maintenance

When you invest in a solid preventive maintenance plan, you can expect to generate cost savings, improve efficiency, boost production, and experience other benefits. Here are some common advantages of a PLC maintenance system.

Extend PLC Lifespan: Although PLCs are designed to operate effectively in industrial environments, you can extend the lifespan of your equipment even more with a solid maintenance program.

Boost Uptime. Keep your equipment running more consistently and efficiently when you have a well-maintained PLC system. Your PLC, after all, is often responsible for keeping your processes up and running.

Save Money: There are obvious cost savings to less downtime, but a well-maintained PLC system can also help you better things like your inventory of replacement parts. Avoid overnight shipping or lost labor costs associated with not having the right parts in stock or well-organized.

Real-Life PLC Applications

PLC systems are flexible and applicable to a wide variety of industries. Here are some real-life applications of PLC technology.

Oil & Gas

Companies operating in the oil and gas industry often add well pad sites as part of their expansion plans. These sites let businesses drill more than one well at each location, which means more production in a smaller surface area. 

Each of these well pads, which may have one to six wells, must have an individual PLC. These systems use pumps, valves, and sensors to work effectively and efficiently. By developing a single, scalable PLC system that incorporates HMI, oil and gas companies can get a handle on all its well pads quickly and easily.

PLCs on well pads allow the computer to accurately read inputs and share outputs for specific locations. Technicians can be efficiently sent out to the well pads that require repair, maintenance, or additional inspection without manually sorting through well pad data. Adding well pads is simple as the infrastructure and training already exists.

Glass Companies

Manufacturers that make glass products use PLCs along with bus technology to manage material ratios and production processes. Glass companies must employ sophisticated, complicated processes, which require data collection and precise quality control. PLC technology can assist with both.

Paper Manufacturing

Within this industry, PLCs can help control the machines that frequently run at very high speeds like newspaper and book printing.

Cement Companies

Cement kilns require specific proportions of materials in order to produce high quality cement. A PLC system can control ball milling, shaft kilns, and coal kilns.


All businesses have to manage a heating, ventilating, and air conditioning (HVAC) system to maintain comfortable, safe, and healthy climates for employees. Within certain industries, HVAC specifications may have to fall within tight tolerances. For example, certain electronics facilities or medical labs may have very narrow temperature or humidity ranges that are acceptable for sensitive equipment.

Often these applications use sensors that can send an alert as soon as acceptable limits are exceeded. A PLC system can then notify the maintenance team that an immediate repair is in order. This can prevent expensive damage of electronic equipment or spoilage of food, beverage, or medical products.

Even in basic climate control applications, multiple PLCs can be tied together for easier building management in a centralized location.

Other Industries

These are only a few industries that often use PLC systems. Others include health care, textiles, aerospace, automotive, food production, and many others.

PLC Risks & Benefits

As with any system, PLCs carry risks and benefits with implementation. This may be multiplied as the PLC system grows within a large organization. Companies should weigh both sides and determine the best solution for their organization.

PLC Risks

Many of the risks associated with PLCs are connected to one another and may be greater as the size of your system grows. Here are the top issues to watch for:

I/O Device Defect: When an input or output device goes down, the entire PLC system is at risk. This can be caused by a power outage and may lead to a sudden stop of the entire system. Usually, the PLC is waiting for a signal to begin its next sequence of activities. In order to resolve this problem, an engineer typically must find the cause of the stop and trace it back to the specific device. When many devices are regularly experiencing problems, there may be a power fluctuation, internal error, or power outage.

Power Problems: Although this is somewhat out of a company’s control, power outages and fluctuations can lead to PLC risks. Be sure to monitor those things that are within your control like replacing frayed extension cords or employing a backup power system, which also needs to be checked and maintained regularly.

Grounding Issues: Somewhat related to power issues, grounding of PLC equipment is critical to the safety of those working at your facility. Consider an electrical white noise barrier that can prevent distractions during faults.

Interference: External interference is common within environments that have many electrical components. Both radio and electromagnetic interference create the largest risk for manufacturing operations. They can result from handheld radio systems, large motors starting, lightning, or nearby radio antennae.

Temperature Problems: Like all electrical components, PLCs are sensitive to heat. If other heat-generating equipment is nearby, it can raise the surrounding temperature to dangerous levels.

Costly Downtime: All of the above issues may lead to expensive downtime. After all, a PLC is relied upon to keep your critical equipment running smoothly. When controls break down, equipment fails, and processes can come to a screeching halt.

PLC Advantages

Although there are risks with PLC systems, many advantages exist as well. PLC systems are easy to integrate, require little space, and result in productivity increases, among others.

Integration Ease: PLCs are by nature easy to integrate because they are networked devices. The bottom line is that changes and expansions can be programmed into the system itself, freeing the time of managers and technicians to focus on other priorities.

Increased Productivity: Many of those other priorities may result in significant productivity gains. For example, project times can be reduced and downtime decreased. In addition, you may have fewer systems to maintain and a better system of organizing and retrieving data.

Small Space Footprint: PLCs take up very little physical space when you consider the breadth of work they accomplish around the clock. You can eliminate server rooms as well as other types of hardware and software as a result. 

Project Flexibility: Because data is stored in a set of devices instead of throughout a disparate system, you have more flexibility for each and every project. It also allows easier expansion of the entire system through adding devices and PLC equipment as needed.

Troubleshooting Ease: The fact that PLCs are composed of various input and output devices makes it easy to identify and repair problems. There are only so many things to check on each component, which means a simple process for identifying trouble spots.

Security: Most PLC systems have high levels of safety and security controls to protect the data within. In addition, companies are able to access sensitive information more safely than on more conventional, broader systems.


PLCs play a critical role in today’s industrial companies, particularly for small and medium-sized manufacturers. They are an excellent solution that allows close monitoring for important equipment and machinery around the clock. They are easy to maintain and expand as needed. Although some risks are associated with PLCs, they offer many advantages as well. PLCs provide a flexible, reliable, and secure solution to form a foundation for a predictive maintenance program.

Want to keep reading?

Programmable Logic Controller (PLC)

A programmable logic controller, PLC for short, is the brain of a modern-day facility that depends on various machines and equipment.
View Article

Predictive Maintenance

Predictive maintenance is a type of condition-based maintenance that monitors the condition of assets through sensor devices.
View Article

What is the difference between PLC and SCADA?

The primary difference between a PLC (or Programmable Logic Controller) and SCADA is the fact that a PLC is hardware and SCADA is generally software
View Article


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