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PLC
Old Li Talks Technology: What’s So Difficult About PLCs? Discussing Those Pitfalls from a Practical Perspective
Last week, Xiao Zhang rushed over to me, saying: “Old Li, the production line 3 has stopped again, the system is alarming, and I’ve been looking at it for a long time without any clue!” I put down my teacup and walked with him to the site. After glancing at the alarm information on the touchscreen, I knew it was probably that old problem—abnormal sensor signals.
Sure enough, after checking around, I found that the proximity switch was loose, causing intermittent signals. Xiao Zhang looked at me in awe after I solved the problem in five minutes: “Old Li, how did you know where the problem was just by looking?”
To be honest, it’s not a technical job; it’s purely a matter of experience. After so many years, I’ve seen this type of fault many times. Today, I’ll talk to you about PLCs. Many beginners find them complex, but once you grasp the basic ideas, it’s not that mysterious.
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What is PLC? Don’t overthink it
PLC stands for Programmable Logic Controller, which is essentially the brain of the computer that controls equipment in factories. You see those automated devices in our workshop? Behind them is a control cabinet, and inside that dark little box is the PLC.
Many beginners feel overwhelmed when they see PLC programming software, with screens full of ladder diagrams and function blocks, not knowing where to start. In fact, just think of the PLC as a black box that processes information:
“Input points are like the PLC’s eyes and ears, collecting external information; output points are its hands and feet, controlling various device actions. The program in between is its thought process.”
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Common Faults: Don’t Panic, Check These First
I remember one night when we were urgently repairing the packaging line, and the electrician Xiao Li had been working for two hours without finding the fault. I went over, and without saying a word, I first checked three things: the power indicator light, communication status, and forced status. As a result, I found that an output point had been forced ON, and after canceling the force, the device immediately returned to normal.
When troubleshooting PLC faults, first check the “three-piece set”: power, communication, and force. This is the experience I’ve summarized over the years, solving about 70% of urgent faults. Don’t laugh; such simple checks can save a lot of emergency repair time!
Especially regarding forced status, many beginners overlook this. During debugging, if a certain point is forced and forgotten to be canceled, problems arise when the device is officially running. Guess what? The program logic is correct, but it just doesn’t run according to logic, and you can’t find the reason after searching for a long time.
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Basic Instructions: Understanding Them is Enough
With so many PLC instructions, beginners often ask: “Old Li, do I have to learn all these instructions?” I just laugh; do you need to learn all cooking techniques to make a home-cooked meal?
In actual work, 80% of automation tasks only use these basic instructions: normally open contacts, normally closed contacts, coils, timers, and counters. Mastering these instructions will allow you to handle most industrial control scenarios.
Take timers for example. Last month, Old Wang’s production line needed to be modified to require materials to stay for 10 seconds before being conveyed to the next station. Old Wang racked his brain writing a bunch of programs, and I shook my head when I saw it; it could be done with a single TON timer. Why make it so complicated?
“You see, it’s that simple. When the input conditions are met, the timer starts timing, and when it reaches the set value, it outputs an action. This is no different from the timer in your kitchen.”
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Tips for Program Debugging
Apprentices often ask me: “Teacher, the program is finished, but the device just won’t move, what should I do?” I ask him: “Do you know how to use online monitoring and single-step execution?” He looks confused.
Alas, this is rarely taught in textbooks, but it’s one of the most commonly used techniques in actual work. Knowing how to use monitoring mode to track program execution flow can increase your debugging efficiency by more than ten times.
For example, if you wrote a startup sequence and the motor just won’t turn, opening the monitoring mode allows you to see directly which condition hasn’t been met, whether it’s the limit switch not being in place or the safety door not being closed properly; it’s clear at a glance. This is much better than guessing with the drawings and equipment.
Last year, an apprentice modified a segment of the program, and the device kept alarming. I had him use the single-step execution function to go through it line by line, and quickly discovered he had written a normally open contact as normally closed, causing the logic to be reversed. Such problems are hard to spot just by looking at the program, but using monitoring mode makes them apparent.
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Let’s Talk About Communication
As factory automation levels increase, devices need to communicate with each other. I remember when I first encountered the Modbus protocol, I was as nervous as on a first date, and ended up setting the communication address incorrectly, wasting an entire afternoon.
Communication isn’t that complicated; the key is to understand the relationship between the master and slave. Just think of it as making a phone call; you must know the other party’s number to connect, and the communication address is the “phone number”. Parameters like baud rate and parity are like speaking the same language; both sides must set them consistently.
Recently, Xiao Wang asked me: “Old Li, how do two PLCs exchange data?” I told him a lazy trick: instead of studying complex communication blocks, just use the simplest read/write register function, mapping one PLC’s output register to the other’s input register, and they can “talk” to each other.
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Conclusion
After saying all this, what I want to express is: PLCs aren’t that difficult; don’t be intimidated by those technical terms. Start with the basics, practice more, and when you encounter problems, observe and think more, and you’ll gradually discover the way.
Remember Old Li’s words: first master the basic instructions and learn the basic ideas of fault diagnosis; the rest can be learned when needed. The biggest taboo in this field is talking without practice; just looking without doing. If you have the opportunity, go to the site and learn from the experienced workers; that’s the real skill!
