Essential Knowledge for Learning PLC Programming

Learning PLC programming does not require a deep professional background, but mastering some basic knowledge can help you understand the core logic more quickly and avoid detours. Below is a framework of essential knowledge for beginners, categorized as “Basic Understanding → Electrical Basics → Programming Thinking”:

1. Basic Understanding of Industrial Control

  1. The Core Function of PLC Understand that a PLC is an “industrial computer” whose main function is to receive input signals (buttons, sensors) → execute preset logic → control output devices (motors, valves), replacing traditional relay control circuits. In simple terms: PLC = Input Interface + Processing Core + Output Interface, essentially “using programs to replace hard-wired logic”.

  2. Understanding Typical Industrial Scenarios Familiarize yourself with the application scenarios of PLCs (such as production lines, machine tools, elevators, packaging machines), and know that “sequential control” (executing actions step by step) is the most common control requirement, for example:

  • Washing Machine: Water Inlet → Washing → Draining → Spinning (executed in order);
  • Conveyor Belt: Start → Run → Detect Object → Stop (condition-triggered switching).

2. Basic Electrical Circuit Knowledge (Core Requirement)

PLC programming directly interfaces with electrical devices, and without understanding circuit logic, you will find it “difficult to make progress”. Focus on mastering:

  1. Basic Circuit Components and Symbols

  • Input components: Buttons (Normally Open SB, Normally Closed SB), Limit Switches (SQ), Proximity Switches, Photoelectric Sensors (NPN/PNP types);
  • Output components: Contactors (KM), Relays (KA), Solenoid Valves (YV), Indicator Lights (HL);
  • Protection components: Fuses (FU), Thermal Relays (FR, motor overload protection). Be able to read simple electrical schematics (such as motor start-stop control circuits) and understand the roles of “normally open contacts”, “normally closed contacts”, and “coils”.

  • Basic Logic of Low Voltage Circuits

    • Series: “AND” logic (all contacts must be closed for the coil to be energized);
    • Parallel: “OR” logic (any contact being closed energizes the coil);
    • Self-locking: Using the normally open contact of the coil in parallel with the start button to achieve “continuous operation after pressing start”;
    • Interlocking: Using the normally closed contact of coil A in series with the circuit of coil B to prevent both A and B from operating simultaneously (e.g., motor forward and reverse). These are the “prototypes” of PLC ladder diagrams, which are essentially “electronic versions of electrical circuits”.

  • Common Voltage and Wiring Knowledge

    • Common industrial DC voltage: 24V (for sensors, PLC input/output);
    • AC voltage: 220V (for contactor coils, indicator lights), 380V (for motors);
    • Wiring principle: PLC input/output circuits must form a “closed loop”, for example, one end of the button connects to the PLC input point (I0.0), the other end connects to 24V+, and the PLC common terminal connects to 24V-.

    3. Basic Computer and Programming Knowledge (Entry Level is Sufficient)

    1. Binary and High/Low Levels PLCs use binary operations (0 and 1):

    • 1 (ON): Indicates the signal is on (e.g., button pressed, output activated);
    • 0 (OFF): Indicates the signal is off (e.g., button released, output stopped). Understand that “high level (e.g., 24V) corresponds to 1, low level (0V) corresponds to 0”.

  • Concept of Variables and Addresses

    In PLCs, “addresses” are used to identify inputs, outputs, and internal variables, for example:

    • Input (I): I0.0, I0.1 (corresponding to external buttons/sensors);
    • Output (Q): Q0.0, Q0.1 (corresponding to external indicator lights/contactors);
    • Internal Relays (M): M0.0, M0.1 (temporary variables within the program, with no external wiring). Understand that “addresses are the locations in PLC memory that store the 0/1 states”, similar to “room numbers in a house”.

  • Simple Logical Thinking Be able to understand conditional judgments like “if… then…”, for example:

    • “If button I0.0 is pressed (I0.0=1), then turn on indicator light Q0.0 (Q0.0=1)”;
    • “If temperature sensor I0.2 detects high temperature (I0.2=1), then stop motor Q0.1 (Q0.1=0) and trigger alarm Q0.2 (Q0.2=1)”.

    4. Basics of Mechanics and Sensors (Auxiliary Understanding)

    1. Understanding Simple Mechanical Actions Know the basic action modes of devices such as motors (rotation), cylinders (extension/retraction), and conveyor belts (transportation), and understand that “PLC controls the sequence and conditions of these devices’ actions”.

    2. Common Sensor Principles

    • Proximity Switch: Detects whether a metal object is nearby (e.g., robotic arm positioning);
    • Photoelectric Sensor: Detects whether an object obstructs light (e.g., conveyor belt counting);
    • Limit Switch: Detects whether mechanical components have moved to a specified position (e.g., elevator door limit). There is no need to delve into the principles, just know that “sensors can convert physical quantities (presence of objects, position) into electrical signals (1/0) for the PLC”.

    Conclusion

    Core foundational knowledge: Electrical circuit logic (self-locking, interlocking, series and parallel) + PLC input/output concepts + simple conditional judgment thinking. This knowledge does not require systematic study of electrical textbooks; you can quickly master it by “looking at simple circuit diagrams + hands-on connecting small circuits (like button-controlled lights)”. Beginners can learn PLC programming while supplementing these basics, as the combination of both is the most efficient— for example, when learning “self-locking control”, simultaneously understanding the self-locking wiring in electrical circuits will quickly bridge the logical connection between “hardware → software”.

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