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Old Li Talks Technology: The Essentials of PLC Programming

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Starting from an Emergency Repair

Last night, the packaging line in Workshop 2 suddenly stopped, and the manager was extremely anxious. Xiao Wang called me: “Old Li, come quickly, the machine won’t move!” When I arrived on site, I saw that the conveyor belt and robotic arm were unresponsive, but the PLC indicator lights were normal. My over ten years of repair experience told me that this was likely a program issue.

Sure enough, upon opening the program, I found that a sensor signal was not connected properly, causing the safety interlock to remain engaged. I told Xiao Wang: “See, this is why you need to learn to read ladder diagrams first; a ten-minute fault turned into two hours of hassle for you guys.”

plc1

What Exactly is a PLC?

Many apprentices are intimidated when they first encounter PLCs, but there is really no need to be. A PLC is the brain of the factory, responsible for controlling equipment actions. In the past, we used relays for control, which involved complex wiring and was prone to faults. Now, with PLC programming, we can change production plans just by modifying the program—how convenient!

I explained to Xiao Wang: “Think of a PLC as a person with eyes and limbs. The input points are like eyes and ears, responsible for receiving signals from buttons and sensors; the output points are like hands and feet, controlling motors, cylinders, indicator lights, etc.; the program in between is like the brain, deciding what actions to take based on the signals received.”

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Ladder Diagram Programming: A Language Understandable by Electricians

I remember a university student who interned last year, holding a thick programming book, and after a long time, he still couldn’t figure out how to make the motor run. I told him: “Don’t overthink it; PLC ladder diagrams are essentially just drawings of relay control circuits.”

The left side of the ladder diagram represents conditions, while the right side represents results; if the conditions are met, the results are executed. It’s that simple!

For example, I can draw the simplest start-stop circuit:

|——[ Start Button ]——[ Stop Button ]——[ Self-Holding Contact ]——( Motor Running )——|
|——[ Motor Running ]—————————————————————————————————( Self-Holding Contact )——|

This indicates that pressing the start button will run the motor while maintaining itself; pressing the stop button will break the circuit, stopping the motor. It’s the same principle as the circuits we connect with relays, easy to understand at a glance.

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Common Instructions are Actually Just a Few

Xiao Li asked me yesterday: “Old Li, there are so many PLC instructions, how long will it take to get started?” I laughed: “Actually, 80% of applications in factories use just these few instructions:

Normally Open Contact (X), Normally Closed Contact (B), and Coil (Y) are the basic elements of ladder diagrams. Normally open contacts are open by default and only close when there is a signal; normally closed contacts are closed by default and only open when there is a signal. The coil is the executing element, which can be understood as a relay coil or motor.

Additionally, with timers (T) and counters (C), you have enough to work with. A “timer” is like a kitchen timer; when the set time is up, it triggers an action; a “counter” is a counting device that triggers an action when it reaches a set value, such as pushing once after packaging 20 products.

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Practical Tips

I remember once repairing an injection molding machine, and I found that the machine kept getting stuck at a certain step. When I opened the program, I discovered that the designer had, for convenience, written all safety interlocks in one network, making it impossible to identify which sensor was causing the problem.

A good PLC program should be clearly divided into blocks, with each function independently forming a network. This way, when a fault occurs, you can easily identify the problem by looking at the indicator lights without having to check each one individually. This is the experience of us old hands, which is not taught in school.

Also, variable naming is very important. I’ve seen too many programs using default names like X0, Y1, which become incomprehensible after a few days. We should directly name them “Start Button”, “Conveyor Motor”—much clearer!

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Start Learning from Simple Sequential Control

Xiao Wang asked me how to get started the fastest. I suggested he start with simple sequential control:

“For example, make a simple conveyor control: press the start button, the conveyor runs; when a product is in place, the cylinder extends; after the cylinder is in place, wait for 1 second, then retract the cylinder; then continue to detect the next product. This small program can be written in 30 minutes and debugged in an hour.”

Doing three to five of these small projects will build a solid foundation. Remember, the key to learning PLCs is hands-on practice; just reading books is not enough. Each brand of PLC has a different operating interface, but the principles are the same.

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Old Li’s Advice

In the automation industry, the biggest fear is having high aspirations but low skills. Some people read a lot but want to jump into complex tasks without understanding the basics. My advice is: first master the basic instructions, start with small programs, and frequently review actual programs on-site; gradually, you will become proficient.

Additionally, always pay attention to safety on-site. Always back up before modifying programs, and keep a safe distance from equipment during testing. I’ve seen too many cases where a single line of code was written incorrectly, causing equipment to suddenly operate and injure people.

Remember Old Li’s words: PLCs are not hard to learn; what is difficult is having a diligent and hardworking mindset. Start simple, progress step by step, and you too can become a technical expert in the workshop!