[Core Knowledge] Understanding PLC Logic Operations from 0 to 1: A Single Trick to Enhance Your Program Logic Tenfold!

Do you remember the first time I faced that “mysterious” control cabinet on the factory production line? That was 15 years ago, right after I graduated from school, staring at the complex wiring and that little box with an LCD screen (back then it was still the S7-200) with a blank expression. The experienced worker patted my shoulder and said, “Young man, this thing is the ‘brain’ of the factory. Once you learn it, you will grasp the soul of the entire production line.”

PLC Logic Operations: The Basic Language of Industrial Automation

A PLC (Programmable Logic Controller) is essentially a computer designed specifically for industrial environments. Its “thinking method” is based on logic operations. Imagine if the PLC is likened to a diligent factory manager, then logic operations are the way he makes decisions.

Basic Logic Operations: The Building Blocks of Industrial Automation

There are three basic logic operations: AND, OR, and NOT. These seemingly simple operations can be combined to achieve astonishing control functions.

• • AND Operation: Like a “double insurance” in the factory. The next step will only be executed when both conditions are met. For example, the machine can only start when “the safety door is closed AND the operation button is pressed.” In a ladder diagram, this is represented by two normally open contacts in series.
• • OR Operation: Equivalent to providing the system with “multiple paths.” Any one condition being met can trigger the result. For instance, either “the local start button OR the remote start signal” can start the device. In the program, this is represented by parallel contacts.
• • NOT Operation: The “reverse thinking” in industrial control. It inverts the signal and is an important element of safety control. For example, “the safety fence NOT activated” means the safety fence has not been triggered, and the device can operate.

From Simple to Complex: The Power of Logic Combinations

I remember once I was responsible for the renovation of a packaging line. The client required that the packaging process must only start when “the product is in place, the packaging paper is ready, and the heat sealing machine temperature is normal,” but they also wanted it to stop immediately when “the emergency stop button is pressed OR the safety door is opened OR there is a system fault.”

This seemingly complex requirement became clear when broken down using logic operations:

Start Condition = Product in place AND Packaging paper ready AND Heat sealing machine temperature normal
Stop Condition = Emergency stop button pressed OR Safety door opened OR System fault
Running Status = Start Condition AND (NOT Stop Condition)

Understanding this logical construction method means you have grasped the core thinking of PLC programming.

The Secret to Enhancing Your Program Logic Tenfold

Years of on-site experience have taught me that high-quality PLC programs are not about the amount of code, but about the clarity of the logical structure. Here I share my summarized “Pyramid Logic Method”:

1. 1. Bottom Layer Processing Signals: Preprocess all input signals (filtering, inversion, buffering, etc.)
2. 2. Middle Layer Constructing Conditions: Use basic logic to combine various working conditions and state judgments
3. 3. Top Layer Executing Actions: Execute corresponding control actions based on conditions

This structure makes the program easy to debug and maintain. Once, late at night, I was called to a factory to handle an emergency fault. The equipment had intermittent shutdowns, and after two hours of investigation, I found that a sensor signal jitter caused a logical judgment error. Because the program structure was chaotic, with signal processing and control logic mixed together, it was difficult to troubleshoot anomalies. If the “Pyramid Logic Method” had been applied, this problem could have been avoided during the design phase.

Advanced: Deep Applications of Logic Operations

Once you master the basic logic, you might want to try some advanced applications:

• • Edge Detection: Capturing the rising or falling edge of a signal to achieve precise timing control
• • Latch Circuit: Maintaining a state until specific conditions are met, suitable for process control
• • State Machine Design: Using logic operations to construct complex state transition systems

These techniques combined can make your PLC programs both stable and flexible. I once used the state machine method to restructure the control system of an old production line, simplifying the originally chaotic program of over 300 networks to less than 100, reducing the failure rate by 85%.

Final Thoughts

I remember my experienced worker often said: “Those who can write programs are programmers, but those who can design logic are engineers.” I deeply agree. In the world of industrial automation, logical thinking is our most fundamental and powerful tool. I hope you can build your own industrial control technology system starting from this foundational knowledge.

If you are learning PLC programming, remember: clarify the logic first, then write the code. When faced with a new control requirement, first sketch out the logical relationships on paper before starting programming. This simple habit will make your automation journey further and steadier.

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