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Understanding What PLC Means: Your Essential Guide to Programmable Logic Controllers

Todd Huber
Posted by Todd Huber on Jun 25, 2024 11:23:32 AM
Understanding What PLC Means: Your Essential Guide to Programmable Logic Controllers

Welcome to our comprehensive guide on programmable logic controllers (PLCs). If you are new to industrial automation or have heard the term but aren't exactly sure what it means, you've come to the right place. PLCs are a vital component in modern manufacturing processes, allowing for efficient and adaptable control of machinery and equipment. Despite their importance, many people still have misconceptions about PLCs or don't fully understand how they function. Fear not, as I'll break down everything you need to know about this powerful yet often misunderstood technology. From its history and evolution to its various applications and benefits in different industries, we'll cover it all. So buckle up and get ready to dive into the world of PLCs with us!


Jump to Section: 

PLC Basics | PLC Evolution | How PLCs Function | Programming | Integration | Selection 
Top PLCs | PLC Impact | Future Trends | Contact Us & Resources | PLC FAQ



Key Takeaways from This Article

✔️ PLCs (Programmable Logic Controllers) are vital for industrial automation, with the ability to adapt to various inputs and conditions due to their microprocessor-based design and robust components including a CPU, I/O devices, and programming devices.

✔️ Originally developed in the 1960s to replace hard-wired relay systems, PLC technology has significantly evolved with enhancements like internet connectivity and integration with PC interfaces, which have made real-time monitoring and data processing more efficient.

✔️ PLCs function in a cycle of reading inputs, executing control logic, updating outputs and performing diagnostics in milliseconds, using a variety of programming languages standardized by IEC 61131-3, and can be selected based on specific industrial needs and environmental conditions for optimal performance.




Exploring the Basics of PLC Systems

Machine Learning@3xConsider the PLC as the brain of industrial automation. Tailored to control manufacturing processes, their flexibility, reliability, and ease of maintenance have made them indispensable in our industrial facilities.

They monitor the conditions of their environment and execute control logic to manage various industrial processes, ensuring efficiency and cost-effectiveness. PLCs also play a critical role in monitoring and managing safety systems in the manufacturing environment. PLCs' robust design enables them to perform a diverse range of automation tasks, even in the harshest industrial environments.


Defining a PLC

PLCs are essentially industrial microprocessor-based controllers. Their programmable memory stores instructions for various tasks, tailored to the needs of the industrial control systems they govern. This ability to adapt and respond to different inputs and conditions makes them remarkably versatile devices.


The Core Components

At the heart of a PLC system, you’ll find:

The central processing unit (CPU), which acts as the brain of the system, executes control instructions

Input modules, which use optocouplers for circuit isolation and LED indicators to show the status

A power supply unit plays a crucial role in converting AC voltage to DC voltage. It is responsible for powering the PLC.

A programming device

Output modules, which control external devices


The Evolution of PLC Technology

PLCs have a rich history, originating in the late 1960s in the automotive industry in the US. They brought numerous benefits over previous automation systems, being more reliable, compact, requiring less maintenance and easily scalable with additional I/O modules. Technological advancements in the late 1970s and 1980s further enhanced their capabilities, paving the way for direct interfacing with personal computers and enhancing diagnostics and machine monitoring.
By the end of the 1990s, PLC advancements brought internet connectivity to the factory floor, dramatically expanding their functionality.
Source: Wikipedia - Programmable Logic Controller


From Relay Logic to Digital Control

PLCs were developed to replace hard-wired relay logic systems, a major leap in automation technology. These devices emulate the functions of electromechanical relays, assigning discrete inputs a unique address, testing if the input state is on or off, and performing logical operations such as AND and OR. This development was significantly influenced by the advent of single-board computers.


Pioneers in PLC Development

2024-06-12 12_44_33-Richard E. Dick MorleyThe journey to modern PLCs would not have been possible without the pioneering work of Richard E. Dick Morley, considered the father of the Programmable Logic Controller (PLC). His company, Modicon, was the first to develop a PLC, marking a significant milestone in the field of automation.

Source: Control Engineering - Dick Morley remembered as 'Father of the PLC'



Unpacking How PLCs Function

To truly appreciate the genius that is a PLC, it’s essential to understand how they function. All PLCs function through four fundamental steps:

1. Input Scan

2. Program Scan

3. Output Scan

4. Housekeeping

The above steps are essential in determining the operation of the programmable logic controller. The processor evaluates all instructions and updates the status of outputs typically within tens of milliseconds, demonstrating these devices' real-time processing capabilities. Special-purpose I/O modules, such as a modular digital controller, are employed when the standard PLC scan time is insufficient for predictable performance.


The PLC Operation Cycle

During the input scan, the PLC performs the following steps:

1. Reads the state of its inputs, such as sensors and switches, to detect any changes.

2. Executes the user’s program, processing the inputs.

3. Determines the logic and actions required for the output devices.

Once the program is executed, the PLC adjusts the output signals accordingly, concluding the cycle with a housekeeping step for internal diagnostics.

Real-Time Processing and Control

PLCs handle real-time control by rapidly performing the following tasks in a scan cycle:

1. Reading inputs

2. Executing user-defined logic

3. Updating outputs

4. Performing internal diagnostics

All of these tasks are completed within milliseconds, making PLCs ideal for real-time processing.

Note: The size of a PLC’s memory directly affects its ability to run complex programs and manage large volumes of process data. 


Programming the Brain of Industrial Automation

Process Simulation@2xWith an understanding of how PLCs function, we can now delve into how we instruct them to perform certain tasks. This is achieved through PLC programming. The IEC 61131-3 specification, published in 1993, created standardization in PLC software development.

This standard includes graphical and textual programming languages such as:

✔ Ladder Diagram

✔ Function Block Diagram

✔ Structured Text

✔ Sequential Function Chart

✔ Instruction List


8tjhx2Kids can program PLCs too!

Read La Salle Academy's story about a group of 7th graders who learned PLC programming through a functional traffic intersection with our PLC trainer!



Commonly Used PLC Programming Languages

Ladder logic, a graphical programming language inspired by relay logic diagrams, is the most commonly used PLC programming language due to its intuitive interface for I/O operations. Its modular design allows for straightforward modifications by adding or removing individual rungs.

< Example of a ladder logic diagram

When choosing a programming language for a PLC, it’s important to consider the application’s complexity, the team’s expertise with the language, and the control system’s efficiency needs.

Writing and Implementing a PLC Program

The magic of PLCs comes to life when we write and implement a PLC program using PLC programming software. Software features such as online editing, simulation modes, and diagnostic tools enhance productivity and ease of PLC programming. PLC programs are commonly created with a programming device, such as a desktop console, software on a personal computer, or a handheld device. These tools provide different options for writing and editing the programs.

The logic of a PLC control system, which utilizes programmable logic controllers, is defined by a program that can be transferred to the PLC, allowing changes in control logic without reconfiguring connected electrical components.

Interested in PLC Programming Courses? Learn more here.


Integrating PLCs with External Systems

PLCs do not operate in isolation. They are designed to communicate with external devices and systems using ports such as:


✔ Ethernet

✔ RS-232

✔ RS-485/422

✔ Fiber-Optic and Wireless technologies

Furthermore, PLCs enable the remote monitoring and control of industrial systems, a capability not feasible with older electromechanical relay systems.


Communication Networks and Protocols

Programming Language@2xIndustrial Ethernet and the Industrial Internet of Things (IoT) have significantly increased the ability of PLCs to transfer large amounts of data quickly, facilitating efficient real-time monitoring. PLCs utilize various protocols like Modbus, PROFINET and EtherNet/IP and are equipped with communication interfaces such as Ethernet, serial, and USB ports to meet connectivity requirements.


Interfacing with Human Machine Interfaces (HMIs)

HMI@2xHMIs, or the human-machine interface, also play a crucial role in PLC systems as input and output devices. Input devices, along with output devices, are used for key functions such as configuration, alarm reporting, and everyday control.

The communication between PLCs and machines facilitates data collection, which is then made visible to operators through HMIs, providing a critical link for data visualization in SCADA systems.



Selecting the Right Programmable Logic Controller for Your Needs

With a myriad of PLCs available in the market, selecting the right PLC for your needs can be a daunting task. It requires considering integration capabilities with current systems, compatibility with existing hardware, software, and communication protocols.

The flexibility of PLCs to adapt to changes in production requirements constitutes a significant factor in their selection.


Evaluating Input/Output Requirements

One of the key factors to consider when selecting a PLC is the type and quantity of I/O ports required for your specific application. PLCs can be monolithic with built-in I/O capabilities, or modular with replaceable I/O cards, which can be customized for specific applications and replaced in case of failure.


Assessing Environmental Conditions

Environmental conditions play a significant role in the selection of a PLC. It’s crucial to choose a PLC that can withstand specific environmental challenges such as:

✔ Temperature

✔ Vibration

✔ Humidity

✔ Dust

✔ Corrosive atmospheres

5138250For environments with electromagnetic interference or explosive atmospheres, PLCs must have appropriate shielding, grounding, and certifications to ensure safe and reliable operation.


Our Top Control Systems Worth Investigating

To help you navigate the landscape of PLCs, here are a few popular models and manufacturers to consider.


PLCnext by Phoenix Contact


PLCnext is an open-source, modular PLC platform that combines traditional IEC 61131-3 programming languages with high-level languages like C++, Python, and many more. It offers seamless integration with external systems through various communication protocols.

> Learn more: Top 4 Questions Answered About PLCnext


SYSMAC by Omron

SYSMAC is a PLC platform that caters to various industrial automation applications and boasts a wide range of I/O options, making it highly versatile. It supports advanced programming languages like C++ and offers robust communication capabilities.

> Learn more: SYSMAC


ctrlX PLC by Bosch Rexroth

ctrlX PLC is a flexible, open-source PLC platform that supports various programming languages and communication protocols. Its modular design allows for easy customization and expansion to meet specific application needs.

> Learn more: ctrlX PLC


Learn more about these systems and how to select in our article: How to Choose the Right Automation Software for you



The Impact of PLCs in Different Sectors

The impact of PLCs extends far beyond the factory floor. They have replaced many manually executed processes in industrial plants globally, revolutionizing the way operations are carried out. Their application spans various sectors, significantly enhancing efficiency, productivity, and safety in industrial and commercial operations.


Manufacturing and Production Lines

PLCs have become an essential component in modern manufacturing, enabling the automation of various processes from simple tasks like controlling conveyor belts to complex tasks such as coordinating robotic arm movements.

For instance, in the food industry, PLCs play a crucial role in quality control by detecting burnt potato chips on a production line and automatically removing them.


Beyond Manufacturing: Diverse Applications

PLCs are not limited to manufacturing. They aid in transportation systems like escalators and elevators, managing motor speed, door mechanisms, and safety features through sensor inputs. They also control automated drainage water pump monitoring and controlling systems, managing water levels in reservoirs and sewage systems through sensor activation and pump operation.


Future Trends in PLC Technology

As we look to the future, PLC technology is advancing with new capabilities like edge computing, allowing for more efficient data processing and enabling PLCs to operate reliably in harsh environments. Companies are focusing on specialized PLC applications, developing expertise in areas like position sensing interfaces and motion control modules, which expands the versatility of PLCs in various industrial settings.


Advances in PLC Hardware and Software

Recent advancements in PLC hardware include the creation of faster processors and increased memory capacity to accommodate more complex tasks and larger data sets. Open-source software solutions, such as CODESYS, are becoming influential in the development of PLC software, contributing to more accessible and flexible control system programming.


The Convergence of PLCs and Modern Technologies

The Internet of Things (IoT) is expanding the capabilities of PLCs, allowing them to:

✔ Manage extensive data collection and processing tasks in real-time manufacturing settings

✔ Incorporate characteristics of Programmable Automation Controllers (PACs)

✔ Provide users with a broad range of features once specific to either device

✔ Providing a connection to databases via SQL and broadcasting data via MQTT.

This fusion of technologies is revolutionizing the field of industrial automation.


In this exploration of PLCs, we’ve traveled through time, learned about their core components, seen how they function, explored programming languages, and looked at how they interact with external systems. We’ve seen how PLCs have revolutionized industries, and taken a glimpse into the future of PLC technology. Whether you’re selecting a programmable logic controller for your needs or just curious about industrial automation, understanding PLCs equips you with valuable knowledge in our increasingly automated world.



If you’re feeling overwhelmed, don’t worry! We have application engineers ready to provide in-the-field support as well as in-house expertise to help specify and order to your exact needs.

Whether you need assistance with PLC programming, selection, or integration, our team is here to help. We offer PLC programming classes and specify, stock, and support PLC technologies from industry-leading manufacturers.

📝 Contact Us




Frequently Asked Questions on PLCs

What is a PLC?

A PLC, or Programmable Logic Controller, is an industrial computer that controls manufacturing processes, especially in machine automation.


How does a PLC function?

A PLC functions through four basic steps: Input Scan, Program Scan, Output Scan, and Housekeeping. These steps are essential in the operation of all programmable logic controllers.

What are the most commonly used PLC programming languages?

The most commonly used PLC programming languages are Ladder Logic, Function Block Diagram, and Structured Text. These languages are widely utilized for programming PLCs in various industries.

How do PLCs interact with external systems?

PLCs interact with external systems by communicating through various ports such as USB, Ethernet, RS-232, and RS-485/422.. These ports allow for the exchange of data and control signals between the PLC and external devices or systems.

What PLC is best for my application?

The best programmable logic controller for your application depends on factors like input/output needs, environmental conditions, and integration with existing systems. Consult experts and evaluate options before deciding. Ultimately, the best PLC will meet your requirements and enable efficient process automation.

What are some current trends in PLC technology?

Programmable logic controllers are currently seeing trends such as the integration of edge computing and the merging of PLCs with technologies like the Internet of Things (IoT). This reflects an increasing emphasis on real-time data processing and connectivity in industrial automation.


Additional Resources

More Industrial Automation Articles

Explore PLCs

Explore HMIs

PLC Programming Courses

Top 4 Questions Answered About PLCnext

SYSMAC by Omron Automation

ctrlX PLC by Bosch Rexroth

La Salle Academy's PLC Programming story

Topics: Automation, Explainers

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