1. Ladder language (LD)
　　 Ladder language is the most commonly used programming language in PLC programming. It is a programming language similar to the relay circuit. Because electrical designers are more familiar with relay control, the ladder diagram programming language has been widely welcomed and applied.
　　 Trapezoidal programming language is characterized by: corresponding to the electrical operation principle diagram, with intuitiveness and correspondence; consistent with the original relay control, electrical designers are easy to grasp.
The difference between the ladder diagram programming language and the original relay control is that the energy flow in the ladder diagram is not a current of actual significance, and the internal relay is not actually a relay. In application, it needs to be treated differently from the original concept of relay control. .
　　 2. Instruction List Language (IL)
　　Instruction list programming language is a mnemonic programming language similar to assembly language. It is composed of opcodes and operands like assembly language. In the case of no computer, it is suitable to use PLC handheld programmer to compile the user program. At the same time, the instruction list programming language corresponds to the ladder diagram programming language diagram one by one, which can be converted under the PLC programming software.
The characteristics of the instruction table programming language are: the use of mnemonics to indicate operating functions, easy to remember, and easy to master; the use of mnemonics on the keyboard of the handheld programmer, which is easy to operate, and can be programmed without a computer. ; There is a one-to-one correspondence with the ladder diagram. Its characteristics are basically consistent with the ladder diagram language.
　　 3. Function block diagram language (FBD)
　　Function block diagram language is a PLC programming language similar to digital logic circuits. The function module diagram is used to represent the functions of the module, and different function modules have different functions.
The features of the functional module diagram programming language: The features of the functional module diagram programming language are: it is simple and easy to analyze and understand the control scheme with the functional module as the unit; the functional module is to express the function in the form of a graph, which has strong intuitiveness and is suitable for digital logic circuits. The basic designer can easily master the programming; for the control system with large scale and complicated control logic relationship, the function module diagram can clearly express the function relationship, so the programming debugging time is greatly reduced.
　　 4. Sequential Function Flowchart Language (SFC)
　　Sequential functional flowchart language is a programming language designed to satisfy sequential logic control. During programming, the process of sequential flow actions is divided into steps and transition conditions, and the functional flow sequence of the control system is assigned according to the transition conditions, and the steps are followed in sequence. Each step represents a control function task, represented by a box. The box contains the ladder logic for completing the corresponding control function tasks. This programming language makes the program structure clear, easy to read and maintain, greatly reduces the workload of programming, and shortens the programming and debugging time. Used for occasions where the scale of the system is large and the program relationship is complicated.
Sequential function flow chart programming language features: function as the main line, assigned in accordance with the order of the function flow, clear and organized, easy to understand the user program; avoid the defects of ladder diagram or other languages ​​that can not be sequentially operated, and also avoid the use of ladder diagram language When programming sequential actions, due to mechanical interlocks, the structure of the user program is complex and difficult to understand; the scan time of the user program is also greatly reduced.
　　 5. Structured Text Language (ST)
　　 Structured text language is a programming language that uses structured description text to describe programs. It is a programming language similar to high-level languages. In large and medium-sized PLC systems, structured text is often used to describe the relationship between variables in the control system. It is mainly used for user programming which is difficult to realize in other programming languages.
　　Structured text programming language uses computer description to describe various operation relationships between various variables in the system to complete the required functions or operations. Structured text programming language and BASIC language, PASCAL language or C
adopted by most PLC manufacturers 　　Languages ​​and other high-level languages ​​are similar, but in order to facilitate the application, the expression methods and types of sentences have been simplified. The characteristics of structured text programming language: programming in a high-level language can complete more complex control operations; it requires a certain level of knowledge and programming skills in a high-level language of the computer, which requires high engineering designers. Intuitiveness and operability are poor.
　　 Different types of PLC programming software support different types of the above five programming languages. Early PLCs only supported ladder diagram programming language and instruction list programming language. The current PLC supports the programming languages ​​of ladder diagram (LD), instruction list (STL) and function block diagram (FBD).