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Summary- With a mixture of pictures ad words there is so much to uncover, the the history of PLCs to the Language the computer speak. they also go over the logic to understand PLCs. This article is also the first to show a diagram depicting the inside of a common PLCs. as well as describing how to know which PLC to use for different jobs.

Reflection- This is another fantastic article for beginners who are getting into PLCs. as all the information is a need to know before starting work on PLCs. the most need to know information would be knowing and understanding the logic behind the PLCs and their computers

Unitronics- What is a PLC, January 2018

https://www.unitronicsplc.com/what-is-plc-programmable-logic-controller/ Accessed on December 13th, 2023

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There are a few key features that set PLCs apart from industrial PCs, microcontrollers, and other industrial control solutions:
* I/O - The PLC's CPU stores and processes program data, but input and output modules connect the PLC to the rest of the machine; these I/O modules are what provide information to the CPU and trigger specific results. I/O can be either analog or digital; input devices might include sensors, switches, and meters, while outputs might include relays, lights, valves, and drives. Users can mix and match a PLC's I/O in order to get the right configuration for their application.
* Communications - In addition to input and output devices, a PLC might also need to connect with other kinds of systems; for example, users might want to export application data recorded by the PLC to a supervisory control and data acquisition (SCADA) system, which monitors multiple connected devices. PLCs offer a range of ports and communication protocols to ensure that the PLC can communicate with these other systems.
* HMI - In order to interact with the PLC in real time, users need an HMI, or Human Machine Interface. These operator interfaces can be simple displays, with a text-readout and keypad, or large touchscreen panels more similar to consumer electronics, but either way, they enable users to review and input information to the PLC in real time.

In today's world of the Industrial Internet of Things (iIoT), and Industry 4.0 programmable controllers are called upon to communicate data via Web browser, connect to databases via SQL, and even to the cloud data via MQTT.

An All in One PLC integrates the controller with the HMI panel, creating a compact, easy-to-use automation solution. Users no longer need to establish PLC to panel communications and can program both the Ladder Logic and HMI design in a single software environment. An all-in-one approach saves time, reduces wiring, and cuts the cost of purchasing multiple devices.

A PLC program is usually written on a computer and then is downloaded to the controller
Most PLC programming software offers programming in Ladder Logic, or "C". Ladder Logic is the traditional programming language. It mimics circuit diagrams with "rungs" of logic read left to right. Each rung represents a specific action controlled by the PLC, starting with an input or series of inputs (contacts) that result in an output (coil). Because of its visual nature, Ladder Logic can be easier to implement than many other programming languages.
"C" programming is a more recent innovation.
Some PLC manufacturers supply control programming software.

In addition to the traditional PLC described above, there are variations, including PLC + HMI controllers.

Unitronics now offers several rugged PLC product lines to support a broad range of system requirements. Compactly designed, these game-changing controllers create immediate savings by eliminating time-intensive tasks such as PLC-Panel wiring and communication setup. Unitronics supports the All-in-One concept with revolutionary software that enabled Ladder control, HMI design, and all hardware and communication configuration to be programmed in a single, simple software environment.

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What is PLC ? Programmable Logic Controller - Unitronics

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Learn the basic structure of the PLC programmable logic controller , how it works, programmed, and look at some advanced technologies and types of PLCs

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Summary: This Article by PolyCASE describes the Advantages and Disadvantages of PLCs and how important protecting PLCs is. Safety procedures are put into very specific detail to show their importance. Pros and Cons of PLCs are a fantastic showcase for this article, because if someone was to be learning how to install PLCs, they would need to know which one to choose,

Reflection: I Really enjoyed this article purely based off the amount of detail that it goes into regarding the Pros and Cons of different PLCs. It also explains how to safely enclose and protect the PLC and prevent damage to the controller itself as well as the computer that is programmed inside of the controller.

Citation: PolyCASE What Is a Programmable Logic Controller (PLC)?  September 10th, 2021 https://www.polycase.com/techtalk/electronics-tips/what-is-a-programmable-logic-controller.html Accessed on October 26, 2023.

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Summary: This Company, AMCI, describes some of the basics of PLCs, what language the controllers communicate in, as well as some of the history behind how they came to be. They also explain the different types of Input devices and how to choose the right one for the job.

Reflection: One of the few articles that explain how the programming language works and how it is used to communicate between the controller and receiver. The Company also displayed pictures of the newest product of the PLC world. Knowing the basics is the best way to get your foot into the doorway of mastering PLCs.

Citation: AMC Inc. What is a PLC? February. 2020 https://www.amci.com/industrial-automation-resources/plc-automation-tutorials/what-plc/ Accessed On October 26, 2023.

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AMCI : Advanced Micro Controls Inc :: What is a PLC?

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A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

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Summary: This Article’s main object is to explain the logic behind PLCs, and what they are used for. And how to work the ladder logic to correctly diagram the system you are looking to build. As there are many different scenarios, even ladder diagram will be different for most things, so there are many examples given in the article.

Reflection: I enjoyed this article purely over the fact that the logic is easy for me to understand, but as a beginning electrician it was quite difficult to understand. This article would be great for beginners.

Citation: All the circuits Programmable Logic Controllers (PLC) July 25th, 2021,

https://www.allaboutcircuits.com/textbook/digital/chpt-6/programmable-logic-controllers-plc/ Accessed on November 3, 2023

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A GUIDE TO PROGRAMMABLE LOGIC CONTROLLERS 

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A GUIDE TO PROGRAMMABLE LOGIC CONTROLLERS 

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Summary: c3controls explains PLCs in easy to understand language, they talk about how introduce yourself to PLCs. There is also a diagram showing the cycle of how PLCs operate showing you what come after what.

Reflection: I think they did a really good job describing PLCs in this article. as well as making sure the reader understand what a PLC is and how there are different sizes and shapes for different uses.

c3controls- A GUIDE TO PROGRAMMABLE LOGIC CONTROLLERS (PLCS): BACK TO BASICS, August 2022

https://www.c3controls.com/white-paper/back-to-plc-basics-guide-programmable-logic-controller/ Accessed on December 13th, 2023

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Introduction to PLCs

Today's industrial automation systems are more advanced and tech-savvy than ever before. Yet, they didn't get this way on their own!

Programmable Logic Controllers (PLCs) form their backbone, allowing internal components to function together as a seamless unit. Versatile and modifiable, these digital computers are essential to many of the systems and devices we rely on today.

Join us as we take an in-depth look at PLC basics, allowing even the non-initiated to understand how the components work and why they're so important.

What is a PLC?

Before we get too far into technical specifics, let's review how a PLC system operates .

It's a specialized, hardened, computer device designed for use within industrial control systems. It controls devices and equipment by connecting different units and enabling them to work in a coordinated manner.

What gives it this power?

At the core of every PLC is a basic computer processor that gathers various inputs and evaluates them to achieve the desired output. The inputs can be digital or analog. As users can program the system in multiple ways to fit a certain scenario, PLCs within many applications across various industries, including conveyor systems, oil refineries , manufacturing lines and more.

While these might be more sophisticated scenarios, consider the interaction between a simple light switch and a light bulb. By flipping the switch, a user can only turn the light on or off. There is little versatility or flexibility in this application.

Now, say a PLC joins the mix. Suddenly, the user can create more complex controls. For instance, he can make the light blink on or off or make the light turn off one minute after it turns on. Now think of thousands or even tens of thousands of circuits all being controlled in coordination with each other. While that is a simple explanation, that is essentially what a PLC does.

How Does It Work?

We've covered that a PLC relies on a computer processor to turn myriad inputs into logic to control a myriad of outputs. Yet, how does this occur?

As the PLC scans inputs from multiple sources, it scans them and internalizes them. Then, it executes the user programming to enact the desired outputs. Next, it communicates any necessary information to a control network such as Modbus or Ethernet IP. Because PLC's are in charge of mission critical systems and there are usually people near machines, a series of diagnostics are run to make sure everything is in order, before it scans the inputs again. This entire process is the "scan cycle." The larger the number of inputs, the larger the PLC program, the longer the scan cycle. The scan cycle is measured in milliseconds, more commonly known as "fast." However, there are some applications where fast isn't fast enough. A Programmable Automation Controller, or PAC, may be needed. A PAC uses multiple CPUs in a single system or chassis to provide parallel processing, or specialized processing of different facets of the application.

Therein lies the importance of user programming, as the program is what causes the PLC to produce the desired results. Without it, the PLC is just an expensive doorstop.

In this way, a PLC is often described as a small, specialized computer. It shares similar terminology with traditional computing systems, including memory, software, CPU, I/O system and more. Yet, a PLC is made to function in an industrial environment, controlling concrete inputs and outputs from devices, machines and workers, while a personal computer is built to exist in your home or office.

Basic PLC Components

PLCs come in many sizes and feature different levels of capabilities. Some main classifications include smart programmable relays, compact PLCs, modular PLCs, and small-medium-large PLCs.

Compact PLCs

Also known as integrated PLCs, or smart programmable relays, these systems feature a complete system all packed into one small case. Due to this design, the manufacturer, rather than the user, will decide the number and types of inputs and outputs.

Modular PLCs

These PLCs comprise multiple pieces all plugged into a single rack. Modular PLCs can come in many sizes, with varying levels of power and capability.

Small, Medium and Large PLCs

These PLCs differ in size, according to the specific applications for which they are suited.

Though these PLCs are all far from one-size-fits-all, regardless of category, each will include some form of these four components:

Central Processing Unity

Rack or Mounting

Power Supply

Input/Output (I/O) Section

Let's review each of these in detail so you understand how the components work together.

1. Power Supply

As its name implies, the power supply is the component that keeps the PLC running, delivering 24VDC or 120VAC line voltage in most cases. Most power supplies contain a battery back-up that prevents data loss during a power outage.

2. CPU is the Central Processing Unit

The CPU executes the PLC program. In addition to running the PLC program, the CPU interfaces with the unit's other components. The CPU is where you will find the microprocessor, responsible for coding, decoding and computing data.

ROM is Read Only Memory. ROM can be read by not written to. It is used to store programs and parameters that should not be altered. It is where the PLC operating program is stored.

RAM is Random Access Memory. RAM can be written and read. Information in RAM can be altered. RAM is where the user program is stored. Information in RAM can be lost during a power outage. A battery back is used to save the RAM information should a power outage occur.

EAPROM is Electronically Alterable Programmable Read Only Memory. Information in an EAPROM is not subject to loss due to a power outage. EAPROM is used to store user data without the need for external power. Think of your Roku device or something similar. You can unplug it, store it away, come back months later, plug it back in and hook it up. All your favorites and applications are still there and ready to go.

3. Input/Output Systems

The PLC's I/O system is the system that accepts new information from outside sources and creates a new function in the form of an output.

Operating System vs. User Program

In the world of PLC programming, you'll often hear of the CPU executing two different types of programs: the operating system and the user program. Let's take a look at key differentiators between the two.

Operating System

The operating system (OS) of a PLC includes tasks and programs designed to run automatically, meaning they don't require direct user commands to function.

It organizes all the functions, sequences, and operations of the CPU not associated with a specific control task. Key OS tasks include:

Initiating a warm restart and hot restart

Updating and outputting I/O process image tables

Executing the user program

Managing memory areas

Detecting and calling interrupts

Creating a connection with programmable devices

User Program

The user program is the combination of the various functions a PLC requires to process a given automated task. In other words, it's the part of the PLC that allows operators to dictate the outputs they desire, stored in the PLC's internal memory.

Though it's in charge of the more visible functions, the user program works with the PLC operating system.

Programming a PLC

One common way to program, modify or troubleshoot a PLC is through a PC-based device in conjunction with the manufacturer's software. Also used are proprietary handheld devices hooked up to the PLC via a cable.

While a handheld device is often preferred for its portability and convenience, it may lack a traditional keyboard and have limited capability. On the other hand, though a PC device is a little bulkier, it will typically possess more robust processing power. For instance, it allows users to run a program in either online or offline mode in addition to editing, monitoring, diagnosing and troubleshooting the program.

Regardless of the system, you can use your laptop or handheld device to enter the PLC program. From there, you can edit the code and transfer it to the CPU.

Then, disconnect your programming device, as the code is now stored inside the CPU, where it can instruct and govern the operations of the rest of the unit.

An Intro to Ladder Programming

When discussing PLC programming basics, you may hear the term "ladder programming." What does this mean?

There are two main categories of PLC programming languages. Specific languages used will vary depending on the manufacturer. While some have their own, specialized languages, standard ones fit into one of two categories: textual language or graphical language.

Textual language includes:

Instructions Lists (IL)

Structured Text (ST)

Graphical language includes:

In most cases, users prefer graphical languages over text-based ones, as they are simpler and more convenient. Within this category, ladder diagrams tend to prevail for their ease of use.

The Structure of Ladder Logic

This graphical language has been around since the dawn of modern PLCs in the early 1970s , first used because it borrowed from the relay diagrams with which plant electricians were already familiar.

Now, the number of available symbols has grown over time, leading to more advanced and diverse functionalities. The PLC relies on these symbols to simulate real-world relay logic controls, connecting them through lines of circuitry and power to direct the flow of the electrical current.

Often called ladder logic , ladder programming gets its name because the code that results assembles a ladder, comprised of the following symbols:

A power rail to the left

A power rail to the right

Individual circuits ("rungs") connecting the left and right power rails

Common Logic Systems

While ladder logic is a full language of symbols, there are some that you will see more than others, especially in diagrams. Let's review a few.

These come in two types: "Normally Open" (NO) and "Normally Closed" (NC). A light switch is an example of a NO circuit, as it stays off unless someone turns it on. Other uses for the NO symbol include power buttons and other internal programmers.

On the other hand, an NC contact represents a circuit that remains active until an input triggers a shutdown. Uses for the NC symbol include fail-safe features, heat monitoring, and "Stop" buttons.

The output symbol is a common representation for warning signs, indicator lights, and motor contactors. The output for each turns on when the corresponding input becomes energized.

One-Shot Positive Edge Connections

These outputs are turned on if a given condition changes from "false" to "true" over the time it takes for the PLC to complete one scan. It's often used for counters and math commands.

Timer instructions allow for on-delayed or off-delayed events. A Timer Delay-On system begins a timer to turn the system on when the PLC input energizes, giving it time to warm up before operation. This works well for delaying siren sounds and facilitating sequence start delays.

A Timer Delay-Off system puts this delay at the end of operation, allowing for time to pass between the shut-off operation and the actual system shutdown. It's ideal for automatic displays and any machinery that's susceptible to accidental shutoff.

This system will determine if a given value is greater than, less than, or equal to another value. One application is in batching systems, which will use comparisons to ensure that all package components are the same weight.

Math instructions or calculators enable simple addition and subtraction functions, allowing PLCs to calculate data such as motor speed. In addition to these typical numerical functions, they can also power more complex transcendental functions such as square roots, as well as trigonometric sine, cosine, and tangents along with their inverse forms (arc sine, arc cosine, and arc tangent).

Advanced instructions may include communication directions, PID loops, drum sequences, shift registers, ramp generators, and more.

When building a PLC ladder, users can incorporate function blocks that feature more complex instructions. In addition to improving scan time, these blocks can replace entire physical components of your system, such as hard-wired mechanical timers. One example includes:

And/Or Logic

Using function blocks, you can condense the rungs of your ladder by incorporating "AND" logic that requires the ignition of two or more inputs to trigger an output. For example, a technician will have to press two buttons for a given machine to turn on.

You can also apply the same concept to "OR" logic. If "AND" logic is akin to a series circuit, "OR" logic is its counterpart or a parallel circuit. It's used within function blocks to require one input or another.

This makes it ideal for machines that have more than one control panel with an ON/OFF switch at either end. With "OR" logic, users can press either switch to achieve the same effect.

Creating a PLC Program

If you want to use ladder logic to create a PLC program, let's review the steps to take.
1. Determine Program Functions
First, determine what you want your program to do.

Do you want to power on a light switch? What about turn off a conveyor belt or enable a machine to pause every three minutes? There is no right or wrong answer, but you'll need a clear view of the output function before you begin.
2. List Program Conditions
Next, list all the conditions that will play into your program. Using the light switch example, one condition would be that when you flip the switch upward, the light comes on. Then, when you flip it downward, the light turns off.

Create a flowchart referencing these conditions for visual representation.
3. Configure Your Software
Now, you're ready to open your handheld or PC-based device and load your programming software. Configure it with the required settings and set your language to "ladder logic language." Finally, select the appropriate hardware processor and give your new program a name.
4. Add Your Rungs
With your power rails in place, you can begin programming your ladder "rungs." Add your necessary number of rungs into the program, paying close attention to each input and output.
5. Scan for Errors
It's essential to scan your program for errors at this juncture, catching them now rather than going back later.
6. Download the Program
Your last step is to download the program to your PLC. Download it to the system's memory. When the transfer is done, you can disconnect your programming device.

A Comprehensive Look at PLC Basics

As industrial control systems continue to advance in terms of sophistication and function, PLCs will become even more multi-faceted and diverse. While this guide to PLC basics is a helpful start, it's important to stay on top of these trends to maintain your competitive advantage.

Want more information on industrial controls? Looking for top-of-the-line components that can help you build a future-focused system that delivers?

That's where we come in.

We offer a wide range of control products in our online shop . From motor controls to terminal blocks, our inventory is diverse and designed to meet any need.

to learn more about what we provide and how we can deliver robust solutions to power your next project.

Disclaimer:
The content provided is intended solely for general information purposes and is provided with the understanding that the authors and publishers are not herein engaged in rendering engineering or other professional advice or services. The practice of engineering is driven by site-specific circumstances unique to each project. Consequently, any use of this information should be done only in consultation with a qualified and licensed professional who can take into account all relevant factors and desired outcomes. The information was posted with reasonable care and attention. However, it is possible that some information is incomplete, incorrect, or inapplicable to particular circumstances or conditions. We do not accept liability for direct or indirect losses resulting from using, relying or acting upon information in this blog post.

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A Guide to Programmable Logic Controllers (PLCs): Back to Basics - c3controls

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Understand the fundamentals of programmable logic controllers (PLCs) and learn how these versatile digital computers are crucial to modern industrial automation systems. Get a comprehensive overview and access to free PLC templates.

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Summary: This Article by Inductive Automation is the most well rounded of all the articles I have found. Inductive Automation brought up many points about what a PLC is and how to use it, as well as the Pros and Cons. They also added different ways to protect the controller, and the effects PLCs will have in the future.

Reflection: As PLCs are such a vast topic, it is nice to see an article that talks about every subject in some detail. Although nothing is covered in depth, I think the article works well for beginners.

Citation: Inductive Automation PLC: Programmable Logic Controller. February 24th, 2020, https://inductiveautomation.com/resources/article/what-is-a-PLC Accessed on November 3, 2023

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If you're familiar with industrial automation, you may have heard of PLCs. So, what is a PLC and why is it so important in the world of automation?

What Is a Programmable Logic Controller?

Programmable Logic Controllers (PLCs) are industrial computers, with various inputs and outputs, used to control and monitor industrial equipment based on custom programming.

PLCs come in many different sizes and form factors. Some are small enough to fit in your pocket, while others are large enough to require their own heavy-duty racks to mount. Some PLCs are more modular, with only basic I/O (Inputs and Outputs), but can be customized with additional back planes and functional modules (such as analog I/O, communications modules, or display modules) to fit different types of industrial applications.

What Are the 5 Standard PLC Programming Languages?

PLCs are widely used in a variety of industries because they are fast, easy to operate, and considered easy to program. There are five standard PLC programming languages. The most commonly used language is Ladder Logic, but it is also possible to use Function Block Diagrams, Sequential Function Charts, Structured Text, or Instruction Lists to achieve the same functionality.

A PLC's Role in SCADA & HMI Systems

SCADA and HMI systems enable users to view data from the manufacturing floor and provide user interfaces for control and monitoring - and PLCs are an essential hardware component element in these systems.

PLCs act as the physical interfaces between devices on the plant or manufacturing floor and a SCADA or HMI system. PLCs can communicate, monitor, and control complex automated processes such as conveyors, temperature control, robot cells, and many other industrial machines.

How Does a PLC Work?

A PLC's operation is broken down into three stages: inputs, program execution, and outputs. PLCs capture data from the plant floor by monitoring inputs from any connected machines or devices. These inputs are checked against the program logic, which changes the outputs to any connected output devices. It is possible to have the same machine connected to both inputs and outputs on the same PLC, such as a valve position sensor connected to the inputs with the control of that valve position connected to the outputs. A program could read the current position of that valve, check to see if it needs to move, then move the valve position with the output.

PLCs often make a distinction between Digital (or Discrete) and Analog I/O. Digital I/O acts like a standard light switch where the state is either on or off, with no states between. Analog I/O acts like a dimmer switch, where the state can be anywhere between on and off.

It is easy to think of there being two sources of input data for PLCs: Device input data, automatically generated by a machine or sensor, or User input data, generated by a human operator using an HMI or SCADA system.

The Device input data comes from sensors and machines that send information to the PLC. This can include:

On/Off states for things like mechanical switches and buttons

Analog readings for things like speed, pressure, and temperature

Opened/Closed states for things like pumps and valves

Human-facilitated inputs can include button pushes, switches, sensors from devices like keyboards, touch screens, remotes, or card readers.

PLC outputs are very similar to inputs, but can also include audible or visual indicators for the user, such as turning on a warning light, or sounding an alarm beacon. Other outputs can include:

Opening or closing a valve

Adjusting the speed on a motor

Turning a heater On or Off

PLC programs operate in cycles. First, the PLC detects the state of all input devices that are connected to it. The PLC executes the user-created program, using the state of the inputs to determine the state that the outputs should be changed to. The PLC then changes the output signals to each corresponding device. After completing all these steps, the PLC then does a housekeeping step, which includes an internal diagnostic safety check to ensure that everything is within normal operating conditions. The PLC restarts the cycle each time the process is completed, starting again by checking inputs.

Using PLCs With Ignition

With a wide range of available Ignition device drivers, you can connect Ignition with just about any modern or legacy PLC. Once the device driver is installed, data can be viewed or sent to the PLC. With PLC data now available to Ignition's tag system, you can do so much more with Ignition's robust core modules.

With Ignition, it is possible to create a comprehensive SCADA and MES system, HMI system, Alarming and Reporting solution, or an enterprise-wide solution to view and control data on a PLC at any level of an organization.

How to Use PLCs With the IIoT

Traditionally, PLCs communicate using the poll-response method. In local plant and manufacturing environments, this type of communication method is usually fine, since the communication distances are short and predominantly hardwired. With poll-response, PLCs are constantly communicated with to check for any data changes.

As the Industrial Internet of Things (IIoT) becomes more popular, there is an increased need for data from remote locations. This translates to more PLCs and computing devices at the edge of the network. Cellular networks are frequently used in communications with edge devices that require data transmission across long distances. However, due to the high frequency of poll-response communication, cellular networks can incur an incredibly high cost when used this way.

To address this issue, solutions such as MQTT, in conjunction with the Sparkplug B Specification, employ a publish-subscribe protocol to streamline communications from the edge of the network. While modern PLCs employ modern communication protocols, legacy PLCs that are still at the edge of the network require additional hardware to deliver the same functionality. Edge gateways, such as Ignition Edge IIoT along with an MQTT broker, pull data from legacy PLCs using poll-response and then transmit the data using a publish-subscribe protocol.

This IIoT architecture allows industrial organizations to build IIoT solutions on top of brownfield systems, improving bandwidth usage and making data from edge-of-network PLCs widely available throughout the organization.

What Is the Future of PLCs?

The industry continues to see new products entering the market ranging from devices like Programmable Automation Controllers (PACs), which combine the functionality of PLCs with higher-level PC functionality, all the way to industrial embedded hardware.

Even with these new products, PLCs remain popular because of their simplicity, affordability, and usefulness. And software like Ignition will enable organizations to maximize their usefulness for many years to come.

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Learn Programmable Logic Controller (PLC) basics: what it is, what its functions are, how it operates, and its importance in the world of automation.

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PLCs Explained: Programmable Logic Controller Basics

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PLCs Explained: Programmable Logic Controller Basics

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Summary-Explains the history, language, and process of working with PLCs, including their advantages, the computing units needs, and how to wire them. another big thing that is mentioned is the ladder logic that a person who understands PLCs would have to make in order to start working with PLCs

Reflection- I think that this would be the article i most recommend to someone starting in the PLC field because of the sheer amount of information they provide their readers.

POSTED BY TECHNICAL COMMUNICATIONS TEAM · MAY 22, 2020
PLCs Explained: Programmable Logic Controller Basic

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What is a PLC?

A PLC, or programmable language controller, is a compact industrial computer designed to control system processes from one location. You may have heard about PLCs but haven't fully grasped the concept. PLCs are like the brains of industrial automation equipment. Instead of having several machines or workers managing different systems and equipment, PLC controllers can complete various tasks at once with a built-in I/O module (inputs and outputs). Programmable logic controllers are everywhere and heavily used across many industries. This post will dive into the main components of a PLC, programming languages, types of PLCs, and PLC systems in the future.

Traffic light signals are a great example of a PLC application. Instead of having someone reading data and pushing buttons, there is now a PLC system in place to control and monitor this function within one housing unit. If programmable logic controllers are relatively a new concept for you, then it might be hard to imagine the many types of possible PLC applications around you. Other recognizable PLC applications include automatic car washes, elevators, automatic doors, and amusement rides.
PLC in automation equipment
History of PLCs

To understand the purpose of a PLC, we'll need to go back in time to the 1960s. Picture transistor radios, Barbie dolls, G.I. Joe action figures, and Chevrolet Impalas. Machine processes during this era were controlled through the use of electromechanical relays. We're not talking about a few relays either, these systems took up entire walls. However, the size of the relay-powered machines was not the only disadvantage.
Downsides of relay-powered machines:

Hard-wiring had to be done in a very specific order.

Time-consuming troubleshooting.

Constant troubleshooting due to worn relay contacts.

Strict maintenance schedule.

Large and cumbersome.

Changes involved entire system rewiring.

It wasn't long before engineers began brainstorming solutions to these pain points. With the introduction of computer control in the industrial sector, the first version of the PLC was developed. The invention of the programmable logic controller is attributed to Richard Morley and his company, Bedford and Associates.
Example of a relay room before PLCs
PLC Advantages

Richard and his team focused on ensuring maintenance electricians and plant engineers could easily understand and use this new programmable controller. Naturally, the process took much trial and error to get to a controller that solved all the previous pain points while still being user-friendly. Eventually, they were able to achieve this with the MODICON 184.

Nowadays, PLCs offer several advantages over the hardwired relays including reliability, flexibility, cost effectiveness, speed, better communication, and advanced data processing functions. However, the most unique advantage to PLC systems is the ability to reprogram them. Therefore, they're highly adaptable to changes in processes or monitoring application requirements.

In addition, programmable logic controllers feature several diagnostic tools that facilitate easy troubleshooting such as:

Report generation

Fault Logs

Step-by-step program simulation

Manually run inputs/outputs

PLC Components

When thinking of PLCs, you might be thinking of a large device with many buttons and plugins, but in fact, PLC controllers are small compact devices integrated with other hardware and software components. To understand a PLC system, it is best to breakdown the system into two primary parts: the central processing unit (CPU) and the input/output (I/O) interface system.

PLC CPU

The central processing unit contains memory and a communication system needed to instruct the PLC on how to perform. Hence, the reason why CPUs are the "brain" of programmable logic controllers. This is also where data processing and diagnostics take place. The memory unit provides a permanent storage location to store programs without losing information when turning off machines. You can choose memory sizes based on the tasks assigned by the PLC. The communication system allows the CPU to communicate with other devices such as I/O devices, programming devices, and other PLC systems.

You will also need a communication port, a chassis or rack, and a programming device such as an external computer. Depending on the type of PLC, many of these components are combined into a single device. For example, a compact module may have a CPU with a set number of I/O and a specific communication port. We will discuss each component and the options available for each.

PLC I/O

Input/output (I/O) modules relay the necessary information to the CPU and communicate the required task in a continuous loop. Input and output devices can either be digital or analog form: digital devices are finite values represented in values of 1 or 0, analog devices are infinite values, and measure ranges of currents or voltages. Inputs or providers are switches, sensors, and smart devices in analog or digital forms. Outputs can be motor starters, lights, valves, and smart devices. Many types of I/O modules are available to get the right configuration to run your machine.

Additional Notes: The power supply either comes integrated with the base module or sold as a separate component. Two of the most common voltage ranges available for PLCs include 12-24VDC and 110/220 VAC. Make sure the voltage range you choose is enough to power the CPU and I/O modules.
PLC control panel
PLC Programming

PLCs require programming before they can function and perform their given tasks. These programs are usually written in a programming device or software. The program is then downloaded into the memory or RAM of the programmable logic controller. Based on the written program the CPU can read the information provided by the inputs and instruct the PLC to execute the desired outputs. There are many programming languages available to configure your PLC system.

According to IEC 61131, there are 5 standard programming languages available:

Ladder Diagram or Ladder Logic (LD)

Function Block Diagram (FBD)

Structured Text (ST)

Sequential Function Chart (SFC)

Instruction List (IL)

Ladder Logic

Ladder logic is the most popular programming language for PLCs. It is a highly used and recognized programming language in many manufacturing industries. Because of its visual layout it is easy to read and program. Ladder logic is a program written in symbols that translate opening and closing relays, timers, shift registers, counters, and mathematical operations. The written symbols deliver the user's program to the PLC system.

PLC Operation
Ladder logic example
If you are trying to understand a PLC system better, it will be beneficial to learn how a PLC controller operates once the program is written and the appropriate I/O modules are selected. The programmable logic controller will process the given information (inputs), execute the instruction from the program, and deliver the results (outputs) based on the provided information and written logic. PLCs operate in a repeating loop once the user determines the inputs and outputs.

1. Input Scan - Detects the state of input devices connected to the PLC

2. Program Scan - Executes program created by user

3. Output Scan - Operate all output devices connected to the PLC

4. Housekeeping - Communicates with other devices and runs diagnostics

The four basics steps in PLC operation are:
Unitary PLC

Unitary PLCs, also known as compact or integrated PLCs, are the simplest type of PLC controller and are suitable for simple processes. A unitary PLC will have a set number of I/O points and an integrated CPU with connection ports. Usually, you can directly connect these controllers to the device or application in question.

Modular PLC

Modular PLCs, also known as rack-mounted, give you more flexibility. You'll still need a base module which will include your CPU, inputs, and power supply. However, you can customize the PLC by adding additional I/O modules. You can basically go wild here adding as many modules as the PLC allows. You could even insert an analog to digital signal converter.

How Do I Choose Which PLC Type I Need?

Now that you understand the basic components of a PLC and the types available, you may wonder how to narrow down your choices. The first thing to consider is whether you'll be implementing a new system, or adding to an existing one. If it's a new system your options remain wide open. However, if there's a system already in place, you'll need to comply with that system's requirements.

Below are 6 questions to ask yourself when purchasing a programmable logic controller:

1.Are you implementing this PLC into a new or existing system?

2. What type of environment will the PLC reside in?

3. What type and how many I/O points does your system require?

4. How complex are the processes you're looking to control?

5. How will your PLC communicate with other networks, systems, or field devices?

6. What programming language will you be using to dictate the PLC's processes?

PLCs Moving Forward

PLCs are everywhere and they exist in places where we might have never thought, like traffic signals. PLC controllers have been around for more than 50 years and will continue to stay, but, with the rise of "big data", robotic integration and cloud computing PLCs will have to adapt to these new changes as manufacturing continues to become faster and more efficient.

In the future, PLCs will have to become more robust to withstand extreme climate changes, operate remotely without needing someone to be on-site, and communicate with sensors through the Industrial Internet of Things (IIoT) to collect and analyze data needed to make better choices. PLCs will have to be equipped with higher security as cybersecurity is needed across all industries.

Hopefully, you have a newfound appreciation for the PLC. It's definitely better than trying to manage hundreds of electromechanical relays. Convinced yet about the value PLCs can bring to your processes? We're proud to offer you micro PLCs by Lovato Electric in our web store. If you still have questions regarding PLCs, please don't hesitate to contact us.

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PLCs Explained: Programmable Logic Controller Basics

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PLCs are like the brains of industrial automation equipment. They control inputs and outputs behind the scenes making sure all is running as it should. PLC is short for programmable logic controller and it's basically a computer designed to work in an industrial environment.

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Summary- Explains how PLCs have helped the current operation of many industries. they also go into some detail regarding the PLCs such as how the computer inside the PLC operates. PLCs also have replaced the need for lines and lines of relays and timers and compiled them into one small unit for a faster and more controllable unit

Reflection- Although this is relatively short article I think it goes into a very good amount of detail withe the basics of PLCs

A3, Understanding the Basics of PLCs, POSTED 09/12/2018 | By: Daron Underwood, CTO, Vice President R&D