According to the international standard IEC 61131-3, there are mainly five programming languages for PLC (Programmable Logic Controller):
1. **Ladder Diagram (LD / LAD):**
* **Description:** The oldest and most widely used PLC language, it is graphically represented and resembles electrical relay control circuit diagrams. It consists of contacts (representing input conditions), coils (representing output results), and function blocks.
* **Characteristics:** Intuitive and easy to learn, it is particularly suitable for electricians and engineers familiar with relay logic and excels in handling sequential control of discrete quantities (switching quantities).
* **Applications:** Control of mechanical equipment, start-stop logic for production lines, safety interlocks, etc.
2. **Function Block Diagram (FBD):**
* **Description:** A graphical language. It uses predefined or custom “function blocks” (representing specific functions such as timers, counters, mathematical operations, complex control algorithms, etc.) and connects them with lines to represent data flow.
* **Characteristics:** Highly modular, with good code reusability, and capable of visually demonstrating signal flow. It is suitable for process control and applications involving complex calculations.
* **Applications:** Process control (such as PID regulation), data processing, recipe management.
3. **Structured Text (ST):**
* **Description:** An advanced text-based language with a syntax similar to Pascal or C. It uses structured statements such as IF-THEN-ELSE, CASE, FOR, and WHILE loops, as well as expressions.
* **Characteristics:** Highly powerful and flexible, excelling in handling complex mathematical operations, algorithms, data structures, and string manipulations. The code is compact and highly readable for those familiar with text programming.
* **Applications:** Implementing complex algorithms, data processing, math-intensive tasks, and interacting with databases.
4. **Instruction List (IL):**
* **Description:** A low-level, register-oriented text language similar to assembly language. It consists of a series of operation instructions (operation code + operand).
* **Characteristics:** The code is very compact and may have the highest execution efficiency (depending on the compiler and platform), but it is difficult to read and complex to write and debug. Its use in modern PLC programming is gradually decreasing.
* **Applications:** Specific situations where there are extreme requirements for execution speed or memory usage, or for maintaining old programs.
5. **Sequential Function Chart (SFC):**
* **Description:** A graphical language used to describe the sequential, selective, and parallel execution steps of control programs. It consists of “steps” (representing specific actions or states), “transitions” (conditions for switching between steps), and “directed connections”.
* **Characteristics:** It is highly effective in clearly depicting complex sequential control processes, with a clear structure that is easy to understand and debug.
* **Applications:** Batch processing, complex robotic arm sequences, and state machine control.
The most common types:
1. **Ladder Diagram (LD/LAD):** It is undoubtedly the most commonly used language, especially in discrete manufacturing and mechanical control fields. Its intuitive electrical representation makes it the cornerstone and the first choice for beginners in PLC programming.
2. **Function Block Diagram (FBD):** It is also very popular, particularly in process industries and where modularization and visualization of data flow are needed. It is often used in combination with LD.
3. **Structured Text (ST):** Its usage is growing rapidly and it is the preferred choice for handling complex logic and algorithms. It is indispensable in applications requiring advanced functions. The use of ST is very common in modern complex projects.
**Summary:**
Ladder Diagrams (LD) dominate in basic logic control due to their intuitive nature. Function Block Diagrams (FBD) are widely used in process control and modular design. Structured Text (ST) is increasingly important in complex algorithms and data processing because of its powerful flexibility. Instruction List (IL) is becoming obsolete, while Sequential Function Chart (SFC) is a powerful tool for describing complex sequential processes. In practical projects, engineers often **mix and match** multiple languages based on task requirements (e.g., using SFC for the main process, LD/FBD for logic, and ST for complex calculations).