What is a Programmable Logic Controller? The Complete LED Controls Guide to PLCs

What is a Programmable Logic Controller (PLC)?

A Programmable Logic Controller (PLC) is an industrial-grade computer designed to control machinery and processes. It’s the intelligent “brain” behind factory automation, processing inputs, making pre-programmed logic decisions and driving outputs in a consistent, reliable way. You’ll find them at the centre of almost every factory or production facility, quietly ensuring everything runs smoothly.

At LED Controls, we supply a full range of PLCs from leading brands such as IMO, ABB and Unitronics, along with all the expansion modules, HMIs and accessories you need to build or upgrade your system.

Machine Computing with Programmable Logic Controllers:

So a PLC is like a Personal Computer that - instead of handling spreadsheets and emails - is wired into sensors, actuators, motors and switches. The PLC constantly monitors data from these inputs and sends out responding control signals to equipment across a system or plant.

A PLC might control conveyor belts, robotic arms, pumps, lights, fans or packaging machines. In every case, it takes the place of traditional, hard-wired control panels, allowing you to programme logic rather than physically rewire circuits. This makes PLCs flexible, cost-effective and far easier to adapt as your production needs evolve.

How Does a PLC Work?

Although it may look like a simple grey box from the outside, a Programmable Logic Controller is always busy on a continuous loop of operations. This process happens in milliseconds and is repeated constantly while the system is powered on. Engineers call this the scan cycle.

The scan cycle is made up of three main steps: Input, Logic (or Processing), and Output:

Step 1: Input – Reading the Real World

The PLC gathers information from the devices connected to its input terminals. These devices may include:

• Push-buttons and switches – used by operators to start, stop or reset equipment
• Proximity sensors – to detect when an object is present on a conveyor
• Limit switches – to confirm that a machine component has reached its position
• Temperature sensors – to monitor heating or cooling processes
• Pressure and flow transmitters – to measure process conditions in systems like water treatment

Each input sends an electrical signal which the PLC reads and stores in its memory. These signals can be digital (on or off) or analogue (variable values, such as temperature or pressure).

Once all inputs are collected, the PLC builds an internal snapshot of the system’s current state. This snapshot is then used in the next stage of the cycle.

Step 2: Logic / Processing – Making Decisions

Based on the data collected in step 1, the PLC’s central processing unit (CPU) runs through its pre-programmed instructions to determine what needs to happen next. This is where the “logic” part comes in.

The logic is created by engineers using a programming language such as Ladder Logic, Function Block Diagram (FBD) or Structured Text. Each instruction tells the PLC how to respond under specific conditions. For example, a simple ladder logic rule might read:

If sensor A is triggered, and temperature is below 60°C, then start motor B.

More complex programmes may include timers, counters, mathematical functions and safety interlocks. Timers might delay an action for a few seconds, while counters track how many times an event occurs. Safety interlocks ensure that machines only operate when all protective conditions are met, for instance, stopping a motor if a safety door is open.

This processing stage happens extremely quickly - often hundreds or thousands of times per second - ensuring that the PLC reacts to changes in the system almost instantly.

Step 3: Output – Acting on Decisions

So the PLC applies its pre-programmed logic to the data in order to decide on an action to take. Action is taking by sending electrical signals through output terminals. These outputs connect to components that physically make things happen - motors start or stop, valves open or close, alarms sound and indicator lights turn on and off.

Typical output devices include:

• Electric motors – used to drive conveyor belts, pumps or fans
• Solenoid valves – that control the flow of air, water or other fluids
• Relays and contactors – electrically operated switches that allow the PLC to control high-power equipment indirectly
• Indicator lamps and buzzers – to show system status or warn operators of problems

Imagine a packaging machine that uses a motor to move a conveyor belt. The PLC continuously monitors a safety guard switch and an overload sensor on the motor.

• Under normal conditions, when the guard is closed and the motor is not overloaded, the PLC energises the motor output, allowing the conveyor to run.
• If the safety guard is opened, or if the overload sensor reports excessive current draw (a sign of a jam or fault), the PLC immediately de-energises the output controlling the motor’s contactor. This cuts power to the motor almost instantly, stopping the conveyor and helping prevent damage or injury.

When outputs change state, the PLC updates its internal output table to match. The system then begins the scan cycle again, reading new inputs, reprocessing the logic, and updating outputs accordingly.

Putting It All Together – The PLC Scan Cycle

To summarise, each PLC scan cycle consists of:

1. Reading all inputs and storing their current states
2. Executing the user programme based on those inputs
3. Updating outputs according to the programme’s decisions

This loop is repeated continuously, often every few milliseconds. That’s why PLCs are ideal for real-time control: they react faster than a human could and maintain precise timing even in complex systems.

How to Program a PLC

Programming a PLC involves creating the logical instructions that define how it should respond to inputs and control outputs. The process varies slightly between manufacturers, but the main principles remain consistent.

Planning the Control Process

Before writing any code, engineers define the sequence of operations. This includes identifying all inputs, outputs, sensors, actuators, and safety conditions. A flowchart or process diagram is often created to map how the system should behave under different circumstances.

Choosing a Programming Language

PLCs can be programmed using several internationally recognised languages defined by the IEC 61131-3 standard. The most common are:

Ladder Logic (LD)

Ladder Logic is by far the most widely used PLC programming language. It’s called “ladder” logic because the program looks like a ladder on screen: two vertical lines on the sides (representing power rails) with horizontal “rungs” running between them. Each rung represents a rule or instruction for how the system should behave.

For example, a basic rung might say:
If the Start button is pressed, energise the Motor output coil.

Ladder Logic can handle more than just simple on/off controls. It can include timers (to delay actions), counters (to track how many times something happens), and comparators (to compare numerical values such as temperatures or pressures). These building blocks make it extremely versatile while remaining easy to read visually.

Originally, Ladder Logic was created to help electricians move from traditional relay-based control panels to digital systems. The symbols and layout look like electrical schematics, which makes it easier for anyone familiar with relays, switches and contacts to understand.

Function Block Diagram (FBD)

The Function Block Diagram approach is a more graphical, modular way to build control logic. Instead of wiring up rungs, you create boxes - called function blocks - that each perform a specific job. These blocks are then connected by lines to show the flow of signals or data.

So they're kind of like Lego pieces: each block serves an individual purpose, but put them all together and you've only gone and built the Millennium Falcon.

Common examples of function blocks include:

• AND / OR / NOT logic gates – to combine or modify signals
• Timers and counters – to delay actions or count events
• Mathematical blocks – to add, subtract or average sensor readings
• PID controllers – specialised blocks that automatically regulate variables such as temperature or flow

Because you can see how each function connects visually, FBD is especially popular for process control applications such as managing temperature, pressure or flow. It allows engineers to visualise entire processes at a glance, which makes troubleshooting and optimisation much easier.

Many PLC software platforms, including those used for Unitronics systems, offer both Ladder and Function Block programming options so users can choose whichever best suits their project or skill level.

Structured Text (ST)

Structured Text is a text-based programming language, more like what you’d find in computer coding. It looks similar to older programming languages such as Pascal or BASIC, and it uses written instructions to control the PLC rather than graphical symbols.

Where Ladder Logic and Function Blocks are visual, Structured Text is purely written. For example, a rule might look something like this:

IF Temperature > 60 THEN Start_Fan := TRUE; ELSE Start_Fan := FALSE; END_IF;

In this example, if the temperature reading goes above 60 °C, the PLC will turn on a fan. If it drops below that, the fan turns off.

Structured Text is especially useful for complex calculations, data manipulation, and loops (repetitive operations). It’s also ideal when you need to process information from multiple sensors or perform mathematical control such as averaging or scaling values.

Because it’s written rather than drawn, Structured Text can seem intimidating to beginners, but it gives programmers precise control and is very powerful for advanced automation systems. Many engineers combine it with Ladder Logic or Function Blocks, using each language where it makes the most sense.

Sequential Function Chart (SFC)

A Sequential Function Chart is a visual method used to break a process into steps and transitions, making it perfect for multi-stage or time-based operations.

Imagine you're filling a tank with liquid. You might want to:

1. Open the inlet valve until the level sensor detects the tank is full
2. Close the inlet valve
3. Start the mixer for 60 seconds
4. Open the outlet valve to empty the tank
5. Wait until the tank is empty before repeating

In Sequential Function Chart programming, each of these actions would be a step, with a transition between them that defines when to move to the next stage (for example, “level sensor detects full” or “timer expires”).

The PLC moves through these steps one at a time, ensuring each stage completes before the next begins. This makes SFC ideal for batch processes, robotic sequences, and production lines where timing and order are critical.

Visually, it's basically a flowchart. Which makes it easy to follow even for non-engineers. Each block represents an action or condition, and arrows show how the process transitions between each stage.

How These Languages Work Together

In many modern PLC systems, you’re not limited to just one language. You can mix and match them within a single project. For example, you might use:

• Ladder Logic to handle simple on/off control and safety interlocks
• Function Blocks for process control or PID regulation
• Structured Text for calculations or advanced logic
• Sequential Charts to manage the order of operations

Below are some popular PLC systems available from LED Controls that support multiple programming languages. This flexibility allows engineers to choose the method that best fits their process, team skills and application complexity.

Product	Description	Supported Programming Languages
Unitronics UniStream	A compact all-in-one controller combining a powerful PLC, colour touchscreen HMI and built-in I/O options. Ideal for OEM machinery and process automation.	Ladder Logic (LD), Function Block Diagram (FBD), Structured Text (ST), Sequential Function Chart (SFC)
ABB AC500	Highly versatile modular PLC range for complex automation projects, supporting distributed control and high-level integration.	Ladder Logic (LD), Function Block Diagram (FBD), Structured Text (ST), Sequential Function Chart (SFC)

Writing the Programme

Once the control process has been planned and you've got your programming language down, it’s time to translate your process plan into a logic that your PLC can understand. This is the "programming" bit.

Most PLC programming today is done on a PC or laptop using manufacturer-specific software, in whichever language you have chosen. Different PLC manufacturers provide their own programming environments, each tailored to their hardware but following similar principles. For example:

• Unitronics UniLogic® – an intuitive environment for UniStream PLCs, combining Ladder Logic, Function Blocks and Structured Text with built-in HMI design and simulation tools.
• ABB Automation Builder – ABB’s professional engineering suite for AC500 modular PLCs, supporting Ladder, Function Block, Structured Text and Sequential Function Chart languages along with advanced diagnostics and simulation features.

You write and test the logic on the computer first, then download it to the PLC via USB, Ethernet connection or Wifi. Ethernet is generally preferred because it allows faster and safer communication and can be monitored live while the PLC is running.

Downloading to the PLC

Once the programme has been tested in simulation mode, it is sent to the PLC via Ethernet or USB. The controller stores the logic in its internal memory and begins executing it continuously in a scan cycle - reading inputs, processing decisions and updating outputs.

Testing and Commissioning, Monitoring and Maintenance

After downloading, the system is tested to ensure each input and output behaves as expected. Engineers can stay connected through the ethernet network in order to check response times, error handling and safety features, making adjustments where necessary. This phase is known as commissioning and is essential before full production use.

Monitoring can continue into operation through connected HMIs (Human-Machine Interfaces) or supervisory systems. Data from sensors can be logged for diagnostics, energy management or predictive maintenance, helping reduce downtime and optimise performance.

Why Use a PLC?

Flexibility – When your process changes, you can reprogramme the logic instead of rewiring the panel. That means less downtime and lower costs.

Durability – Industrial PLCs are engineered to withstand vibration, electrical noise and extreme environments, so they can keep running where ordinary computers would fail.

Scalability – Many PLC systems can grow with your operation. You can add extra I/O modules or communication units as your system expands, rather than replacing the entire controller.

Efficiency – By automating repetitive tasks, PLCs improve productivity, reduce human error and increase reliability.

Integration – Modern PLCs communicate easily with networks, sensors, HMIs and supervisory systems, using standard industrial communication protocols. This allows your factory to share data and operate as a connected system.

Where Are PLCs Used?

PLCs are found wherever machines need to follow precise, repeatable instructions. They’re used in manufacturing plants, food and beverage processing, bottling and packaging, logistics, warehousing, water treatment, utilities and building automation. From assembly lines to HVAC systems, PLCs provide the control backbone that keeps everything coordinated.

If your process involves multiple steps, sensors or actuators, a PLC is almost always the most efficient and adaptable way to control it.

Choosing the Right PLC for Your Application

Selecting a PLC may seem daunting, but it becomes straightforward once you consider a few core factors.

Factor to Consider	What It Means / Why It Matters	Recommended Example or Range
1. Input and Output (I/O) Requirements	The PLC must have enough input and output points to connect to all your sensors, switches, motors and indicators. Consider both digital and analogue signals, and whether your system might grow in future.	IMO XGB Series – a compact modular PLC with optional expansion modules to increase I/O capacity as your system develops.
2. Processing Power	More complex or high-speed operations need faster CPUs and more memory to handle calculations and logic efficiently. Simpler systems can use smaller, integrated units.	Unitronics UniStream All-in-One PLC + HMI – combines strong processing performance with an intuitive touchscreen interface, ideal for mid-level control and motion applications.
3. Communications	The PLC must support the communication protocols used in your plant — such as Modbus, Profibus, Profinet or Ethernet/IP — so it can exchange data with other devices, drives or SCADA systems.	ABB AC500 Modular PLC – supports a wide range of industrial communication standards and integrates easily with networked automation systems.
4. Environment	PLCs installed in harsh or demanding environments need protection against dust, moisture, vibration and temperature extremes. Always check IP ratings and ensure a suitable enclosure is used.	Always check environmental requirments against your PLC's IP Rating. Find out more about IP Ratings here.
5. Software and Programming	PLCs use dedicated software for programming, simulation and maintenance. Some are graphical and intuitive (Ladder Logic or Function Blocks), while others use more advanced code. Choose one suited to your skill level and needs.	Unitronics UniLogic® Software (for UniStream PLCs) – user-friendly, with integrated HMI design tools and free download from the manufacturer.
6. Expansion and Lifecycle	Select a modular design that allows you to add new I/O, communication modules or features later. Also check that spare parts and manufacturer support are available long-term.	IMO iSmart PLC Range – modular architecture with add-on I/O and communication modules, backed by long-term manufacturer support.
7. Application Size / Type	Match the PLC to the scale of your application. Smaller, all-in-one units suit compact machines, while modular systems work best for larger installations with many I/O points.	All-in-one Controllers: Ideal for OEM or small machine control. Modular PLC System: Suited to production lines or multi-stage processes. Expansion Modules: For growing systems.

Each of these PLC types and ranges is available through LED Controls, with full technical advice and support from our expert team to help you select, configure and commission the best option for your application.

Frequently Asked Questions

Can I replace a relay-based control panel with a PLC?
Yes. A PLC can take over all the logic that used to be handled by hard-wired relays, timers and contactors, but with far greater flexibility and less wiring.

What’s the difference between a PLC and an HMI?
A PLC is the controller that performs logic; an HMI is the touchscreen interface used by operators to monitor and interact with the system. Many units combine both functions in one device, such as the Unitronics all-in-one range, but programming is generally still carried out on a separate computer.

Is a PLC suitable for small systems?
Absolutely. Even small applications can benefit from automation, especially if you plan to expand later or want consistent, reliable operation.

How do I keep my PLC system future-proof?
Choose a modular design with open communication options, document your programming, and work with a trusted supplier like LED Controls for long-term parts and support.

Are PLCs safe?
Many PLCs include safety-rated features and modules designed to meet international safety standards. Always ensure your control system complies with the relevant regulations for your industry.

PLCs @ LED Controls

A PLC is the intelligent core of modern automation, allowing machines and processes to work seamlessly together. Whether you’re managing a small pump or an entire production line, the right PLC can make your operation faster, safer and more efficient.

From selecting the correct controller to providing the modules, HMIs and accessories you need, the team at LED Controls are here to help you find the perfect solution. If you’re planning a control upgrade or building a new automated process, get in touch today to make your automation smarter.

01706 242050
[email protected]
ledcontrols.co.uk

Related Guides from LED Controls

If you’d like to learn more about the wider world of automation and electrical control, take a look at some of our other expert guides:

• What Are Industrial Communication Protocols? The Complete LED Controls Guide to ICPs
• Ingress Protection: What Your Electrical Enclosure’s IP Rating Means
• What Is a Residual Current Device? The Complete LED Controls Guide to RCDs