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If you're an engineer who wants to give your resume an additional useful and worthwhile skill, then PLC programming is a great option. The skill is required in almost every industrial field, so whether you're involved in manufacturing and robotics or energy and water treatment, there's a good chance you'll be able to apply your new skill. For engineers who already possess a system and logic mindset, learning to program PLCs is a natural next step.
PLC Fundamentals: The "What" and "Why" of Industrial Control
First, you must have a clear understanding of what a PLC is and how it came to be the standard in automation for the industry. Building this foundation first will make all of the programming concepts that follow much easier to learn.
What is a PLC (Programmable Logic Controller)?
A Programmable Logic Controller (PLC) is a specialized, rugged computer designed to operate reliably in harsh industrial environments. While a desktop PC would fail quickly in conditions with dust, vibration, and extreme temperatures, a PLC is built to last. It reads inputs from sensors and switches, executes a program you write, and then controls outputs like motors, valves, and lights.
It's different from a microcontroller such as an Arduino since it's a finished, packaged system with protection for its inputs and outputs (I/O). It's built for high reliability and ease of mounting in industrial panels and thus is the workhorse of today's automation.
Why Use a PLC? The Key Benefits
Engineers choose PLCs for several key reasons:
- Reliability: They can endure harsh conditions and operate 24/7 for years without failure.
- Real-Time Operation: PLCs run their programs in a very rapid, repetitive cycle known as a scan cycle. This predictability is important for machine control with safety and accuracy.
- Modularity: It is easy to add additional inputs, outputs, or communications modules as your system expands. This scalability feature makes them flexible for any size of the project.
- Simple Programming: The programming languages, such as Ladder Logic, were designed to be understood by technicians and electricians, not just computer scientists, so that they could troubleshoot easily on the plant floor.
The Anatomy of a Typical PLC System
A whole PLC system is not a single box. It consists of several essential components functioning together:
- Central Processing Unit (CPU): The brains of the operation. It executes the program, does calculations, and controls all other parts.
- Input/Output (I/O) Modules: These are the PLC's connection to the real world. Input modules take in signals from devices such as pushbuttons or sensors. Output modules provide signals to control devices such as motors or indicator lights.
- Power Supply: This module supplies stable, regulated DC power to the CPU and all modules attached.
- Programming Device: A desktop or laptop computer running the PLC manufacturer's software, used to write, download, and monitor the program.
Core PLC Concepts Every Programmer Must Know
To program a PLC effectively, you first need to grasp a few key operational principles. These concepts are universal and apply to any brand of controller you might encounter, from Siemens to Allen-Bradley.
The PLC Scan Cycle: The Controller's Heartbeat
A PLC does not run its program like a standard computer. Instead, it operates on a continuous, high-speed loop called the scan cycle. This cycle has four main steps:
- Read Inputs: The PLC checks the status of every input terminal and saves the values (on/off) in its memory.
- Execute Program Logic: It then runs your program from top to bottom, using the stored input values to solve the logic and determine what the outputs should be.
- Diagnostics and Communication: The PLC performs quick internal health checks and manages any communication tasks.
- Write Outputs: Finally, the PLC updates the physical output terminals based on the results calculated during the program execution. The cycle then repeats.
Understanding PLC I/O
I/O, or Input/Output, refers to how the PLC talks to the machine.
- Digital vs. Analog Signals: Digital signals are merely on/off states, such as a pushbutton. Analog signals are a series of values, such as a temperature sensor that measures 0 to 100 degrees.
- Sinking vs. Sourcing: These are the electrical wiring methods for I/O modules. You should know whether your module is sinking or sourcing for the physical wiring of devices to the PLC.
Memory and Data Organization
PLCs have special data structures to organize information.
Tags vs. Direct Addressing: Newer PLCs utilize descriptive tags, i.e., names ("Motor_Start_Button") to identify data. It is far more readable than the direct address method of the past (%I0.1).
Common Data Types: PLCs represent various types of data in a number of forms:
- BOOL: A simple true/false or 1/0 value.
- INT: An integer (whole number).
- DINT: A "double integer" for bigger whole numbers.
- REAL: A figure with a decimal point.
The Languages of PLC Programming (IEC 61131-3)
An international standard, IEC 61131-3, defines the official programming languages for PLCs. While you have several options, there is a clear choice for where a beginner should start.
Ladder Logic (LD): The Most Common Language
Ladder Logic is the most widely used PLC language. Its design comes from traditional electrical relay logic diagrams, which is why it looks like a ladder with rungs of logic. Because it's so visual and easy for technicians to read, it is the best starting point for any new PLC programmer.
Function Block Diagram (FBD)
This is a visual language also in which you link pre-built code blocks. Every block has some specific function, such as a counter or timer. FBD is a good choice for programming systems that involve continuous process control.
Structured Text (ST)
Structured Text is a text-based language that reads much like Pascal or C. It is powerful for handling complex math, special algorithms, or processing large amounts of data where a graphical language would be too cumbersome.
Sequential Function Chart (SFC)
SFC is used to program tasks that happen in a distinct order. It works like a flowchart, allowing you to organize the program into a series of steps and the transitions that move from one step to the next.
Begin with Ladder Logic. It gives you a solid foundation in the fundamentals of PLC operation in the most understandable manner. Learn it first, and you'll find that you can learn the other languages much more easily afterward if you need them.
Setting Up Your PLC Learning Lab: Hardware vs. Software
You don't require a factory to start PLC programming. You can get started right from your desk with a computer, and there are two broad methods you can take to get your practice established.
The Hardware Path: Getting Hands-On Experience
The ideal way to see how software controls real-world devices is by using a hardware PLC trainer. You can connect the actual buttons and lights and witness your code execute. The biggest drawback is price—any small trainer kit will set you back a few hundred dollars. Well-known hardware brands are Siemens, Allen-Bradley (Rockwell Automation), and Beckhoff.
The Software Route: Accessible and Free
For most beginners, simulation software is the route to take. Major manufacturers provide the programming software with a built-in PLC simulator. You can create, execute, and debug your program on a "virtual" PLC that exists only on your computer. This approach is free or low-cost and completely risk-free—you can't damage anything! Best Simulators to Try:
- Siemens TIA Portal: The modern software for Siemens PLCs. It has a great simulator.
- Rockwell Studio 5000 Logix Emulate: The environment for Allen-Bradley ControlLogix PLCs.
- Codesys: A free, hardware-independent development system that supports many brands.
Recommendation: Begin with a simulator. Learn the programming logic first. Once you are comfortable, you can determine whether investing in hardware is right for you.
Your Step-by-Step PLC Learning Path
Now for the practical part. This learning path breaks down the process into small, manageable steps. Focus on mastering one concept before moving to the next.
Step 1: The "Hello World" of PLCs
Begin with the PLC version of "Hello World": a basic Start/Stop circuit. This program employs a start button to energize an output (such as a motor) and a stop button to shut off. This project introduces the extremely valuable topic of latching logic, also referred to as sealing logic.
Step 2: Mastering the Building Blocks
Next, master the main building blocks of automation: timers and counters.
- Timers (On-Delay, Off-Delay): These control how long an action takes or is delayed.
- Counters (Count-Up, Count-Down): These track how many times an event happens.
These two instructions are found in nearly every PLC program.
Step 3: Expanding Your Logic Toolkit
Make your programs smarter with comparison and math instructions.
Comparison instructions (EQU, GRT, etc.) let your program make decisions by comparing two values. For example, "Is the tank level greater than 90?" Math instructions (ADD, SUB, etc.) can perform basic arithmetic for tasks like totaling production counts.
Step 4: Handling Data
Learn to transfer data inside the PLC. The Move (MOV) instruction is most helpful in copying a value from one location to another. You will use it to load presets into timers, place counts from counters into data registers, or transfer recipe values.
Step 5: Introduction to Analog Signals
Become acquainted with analog inputs. That is, the fundamental principle of scaling—taking a raw electrical signal (such as 4-20mA) from a sensor and translating it into a practical engineering unit, such as a 0-100 degree temperature reading or a 0-300 gallon tank level.
Practice Projects to Build Your PLC Skills
Theory gives you the rules, but practice is how you learn to play the game. The only way to get good at PLC programming is to apply what you've learned by building actual projects. This is where the concepts really start to stick.
Start with beginner projects and then move on to intermediate ones as you get more comfortable. The table below gives you some ideas.
| Project Idea | Skills You'll Use | Level |
| Traffic Light Controller | Basic Logic, Timers (On-Delay) | Beginner |
| Garage Door Opener | Latching Logic, Input/Output Control (Limit Switches) | Beginner |
| Simple Conveyor Belt System | Start/Stop Circuits, Counters | Beginner |
| Tank Filling and Draining System | Comparison Logic (Greater/Less Than), Analog Scaling | Intermediate |
| Automated Sorting System | Multiple Sensors, Data Movement (MOV), Counters | Intermediate |
As you work on these projects, make a point of keeping them recorded. Screen capture your Ladder Logic and a short explanation of what the program is supposed to do and how it does it. This record will become your professional portfolio—it's evidence of your capability to which you can refer during a job interview with potential employers.
Where to Go Next in Your PLC Education
The basics will get you far, but the automation field is enormous. After you're comfortable with the basics, then you can begin to branch out into more advanced areas.
Advanced PLC Topics
- HMI/SCADA: Understand how to design graphical screens(Human-Machine Interfaces) that allow an operator to monitor and control the process.
- Industrial Networking: Become acquainted with protocols such as EtherNet/IP and Profinet that enable PLCs to communicate with other PLCs, as well as with drives, robots, and other intelligent devices.
- Safety PLCs: Learn about specialized controllers and programming methods applied in machine safety and emergency stop systems.
For ongoing education, the manufacturers' official manuals for PLCs are your best friend. Online forums such as PLCTalk and communities on Reddit are also excellent websites where you can ask questions and learn from field-proven experts.
Mastering PLC for Automation Careers
It is a clever decision for any engineer interested in automation to learn PLC programming from the ground up. Begin with the fundamentals, utilize simulation software, and perform hands-on exercises. Be patient, continue to hone your skills, and you will be prepared to handle larger automation projects and grow your career in no time. If you're willing to take it to the next level, look for free online tutorials and join a PLC forum to socialize with fellows on the same path.