What are the plc programming languages?

The IEC (International Electrotechnical Commission) announced the PLC standard (IEC1131) in May 1994. It consists of five parts: general information, equipment and test requirements, programming languages, user guides, and communications. The third part (IEC1131-3) is the programming language standard for PLC. IEC1131-3 details the syntax, grammar and expressions of the five programming languages ​​of PLC.

The five programming languages ​​of PLC are:

1. Sequential function chart: Sequential function chart is a graphical language located on top of other programming languages. It is mainly used to prepare sequential control programs. . The sequential function diagram provides a graphical direction for organizing the program and can be used to describe the functions of the system. According to it, the ladder diagram can be easily drawn.

#2. Ladder diagram: Ladder diagram is the most commonly used graphical programming language for PLC programming. As mentioned earlier, the ladder diagram is very similar to the circuit diagram of the traditional relay control system. It has the advantage of being intuitive and easy to understand. It can be easily mastered by electrical personnel familiar with the relay control system in the factory. It is especially suitable for switching logic control. Ladder diagrams consist of contacts, coils, and function blocks represented by boxes. Contacts represent logical input conditions, such as external switches, buttons, etc. Coils usually represent logical output results and are used to control external indicators, contactors, etc. Function blocks are used to represent timers, counters, or additional instructions for mathematical operations.

#3. Function module diagram: This is a programming language similar to digital logic gate circuits, and it is easy for people with a foundation in digital circuits to master. This programming language uses boxes similar to AND, OR, and NOT gates to represent logical operations. The left side of the box is the input variable of the logical operation, the right side is the output variable, and the signal flows from left to right.

#4. Instruction list: (Siemens calls it statement list) A program composed of several instructions is called an instruction list. The instruction of PLC is a mnemonic expression similar to the instruction in microcomputer assembly language, consisting of operation code and operand. The instruction system of a small PLC is much simpler than that of assembly language. Only about 20 instructions can control the switching value. However, the instruction list program is difficult to read, and the logical relationships are difficult to see at a glance. Therefore, ladder diagrams are generally used when designing more complex switching control programs. The instruction list can handle some problems that cannot be solved with ladder diagram.

5. Structured text: Structured text is a dedicated high-level programming language created by the IEC1131-3 standard, which can enhance PLC's mathematical operations, data processing, graphic display, Report printing and other functions. It can be said to be an advanced application of PLC, so it is mostly used by programmers who have received professional computer programming training.

Among the above five programming languages, the most commonly used ones are ladder diagram and instruction list (statement list). Beginners must master these two programming languages ​​well. There is a certain correspondence between the ladder diagram and the instruction list, and they can be converted into each other. Siemens PLC programming software uses ladder diagram programming and statement list programming as the main interfaces. No matter what language the user writes the program, another program is required. If so, it will be converted automatically.

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