PLC currently has 5 standard programming languages, do you know?
PLC, or Programmable Logic Controller, is an electronic device that was previously known as a "sequence controller." In 1978, the National Electrical Association officially named it PLC and defined it as an electronic system used primarily to read input signals from devices such as buttons, sensors, switches, and pulses. Based on these input signals and the pre-programmed logic stored in its memory, the microprocessor performs operations like logic, sequence, timing, counting, and calculation. It then generates output signals to control external devices like relays, solenoid valves, and motor drivers, enabling automated mechanical control or machining programs.
Today, PLCs support five standard programming languages, including both graphical and textual formats. Graphical languages include Ladder Diagram (LD), Function Block Diagram (FBD), and Sequential Function Chart (SFC). Textual languages consist of Instruction List (IL) and Structured Text (ST). These languages are standardized by IEC 1131-3, a global standard developed by the IEC working group after analyzing PLC programming languages worldwide. This standard not only applies to PLC systems but also extends to broader industrial control applications, significantly contributing to the international standardization of PLC programming.
The Ladder Diagram (LD) is the first and most widely used programming language for PLCs. It evolved from relay control schematics and retains the visual style of traditional relay diagrams. However, instead of physical relays, PLCs use "soft relays," which are memory units representing input/output states. When a soft relay is "energized" (state "1"), its normally open contact closes, and the normally closed contact opens. The concept of "power flow" is also essential in ladder logic, where current flows from left to right, simulating the execution order of the program. The busbar, or vertical line, represents the power supply in the diagram, and the logic solution proceeds from top to bottom and left to right.
Ladder logic has several advantages: it is intuitive, similar to electrical schematics, and easy for electricians to learn. However, it differs from traditional relay logic in that it uses abstract concepts like "power flow" rather than actual current. It also has a one-to-one relationship with instruction lists, making it easier to convert between the two.
Function Block Diagram (FBD) is another graphical language that uses symbols similar to digital logic gates. It is intuitive and effective for solving complex logic problems. FBD allows users to describe functions graphically, making it easier to design and understand control schemes, especially for large-scale systems.
Sequential Function Chart (SFC) is used for describing sequential control programs. It is ideal for complex systems involving concurrency and selection. SFC helps break down large programs into manageable steps, improving clarity and reducing debugging time.
Instruction List (IL) is a low-level, mnemonic-based language similar to assembly code. It is simple and efficient, allowing direct control over the PLC’s operations. While it lacks the visual appeal of graphical languages, it is straightforward for experienced programmers.
Structured Text (ST) is a high-level language resembling Pascal. It supports advanced programming features like variables, functions, and loops, making it suitable for complex control tasks. However, it requires more programming knowledge and is less intuitive for beginners.
Not all PLCs support all languages—many low-end models lack support for FBD or SFC. High-end systems typically offer all five standard languages. Some specialized systems use non-standard languages, like the Continuous Function Chart (CFC) in D7-SYS, designed for continuous process control. These tools simplify complex tasks without requiring deep programming expertise.
Learning PLC programming requires patience, confidence, logical thinking, and good habits. Beginners should practice regularly, think logically, and develop structured approaches. A solid understanding of programming basics and theory is also essential. With persistence and proper guidance, anyone can master PLC programming and achieve great results in automation projects.
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