The programming language of PLC has obvious characteristics compared with general computer language. It is different from general high-level language and general assembly language. It is easy to write and easy to debug. At present, there is no programming language that is compatible with all manufacturers' products. At present, PLC provides users with a variety of programming languages ​​to adapt to the needs of programming user programs. The programming languages ​​provided by PLC usually have the following types: ladder diagram, instruction list, sequential function diagram and function block diagram. Next, we will introduce these commonly used plc programming languages ​​in detail. 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 line. Because electrical designers are familiar with relay control, the ladder programming language has been widely welcomed and applied. The characteristics of the ladder programming language are: corresponding to the electrical operation schematic diagram, with intuitiveness and correspondence; consistent with the original relay control, the electrical designer is easy to grasp. The difference between the ladder programming language and the original relay control is that the energy flow in the ladder diagram is not the actual meaning of the current, and the internal relay is not the actual relay. When applied, it needs to be treated differently from the original relay control concept. . 2, instruction list language (IL) The instruction list programming language is a mnemonic programming language similar to assembly language, and consists of opcodes and operands just like assembly language. In the absence of a computer, it is suitable to program the user program using a PLC handheld programmer. At the same time, the instruction list programming language corresponds to the ladder programming language diagram one by one, and can be converted to each other under the PLC programming software. The instruction table programming language features: mnemonic to indicate the operation function, easy to remember, easy to master; mnemonic representation on the keyboard of the handheld programmer, easy to operate, can be programmed without computer ; has a one-to-one correspondence with the ladder diagram. Its characteristics are basically the same as the ladder language. 3, functional module diagram language (FBD) The functional 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. Different function modules have different functions. Function module diagram programming language features: function module diagram programming language features: function module as a unit, analysis and understanding of the control scheme is simple and easy; function module is expressed in the form of graphics, intuitive, for digital logic circuits The basic designer is easy to master the programming; for the control system with large scale and complex control logic, the function module diagram can clearly express the functional relationship, which greatly reduces the programming debugging time. 4, sequential function flow chart language (SFC) The sequential function flow chart language is a programming language designed to satisfy sequential logic control. During programming, the process of the sequential flow action is divided into steps and transition conditions, and the functional flow sequence of the control system is assigned according to the transition condition, and the actions are performed step by step in sequence. Each step represents a control function task, represented by a box. The ladder contains logic for completing the corresponding control function tasks. This programming language makes the program structure clear, easy to read and maintain, greatly reducing the amount of programming work and shortening the programming and debugging time. It is used in the case where the scale of the system is large and the program relationship is complicated. Sequential function flow chart The characteristics of programming language: function as the main line, according to the order of function flow, clear and easy to understand the user program; avoid the defects of ladder diagram or other language can not be sequenced, but also avoid the use of ladder language When programming sequential actions, the user program structure is complicated and difficult to understand due to mechanical interlocking; the scanning time of the user program is also greatly shortened. 5, structured text language (ST) A structured text language is a programming language that uses structured descriptive text to describe a program. It is a programming language similar to a high-level language. In large and medium-sized PLC systems, structured text is often used to describe the relationship of various variables in the control system. Mainly used for user programming that is difficult to implement in other programming languages. Structured text programming languages ​​use computer descriptions to describe the various operational relationships between various variables in the system to perform the required functions or operations. The structured text programming language used by most PLC manufacturers is similar to high-level languages ​​such as BASIC, PASCAL, or C. However, for the convenience of application, the expression of statements and the types of statements are simplified. The characteristics of structured text programming language: programming in high-level language can complete more complicated control operations; need to have certain knowledge and programming skills of computer high-level language, and have higher requirements for engineering designers. Intuitive and operability is poor. Different types of PLC programming software support different types of the above five programming languages. Early PLCs only supported the ladder programming language and the instruction list programming language. 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