Plc's software consists of system programs and user programs. System program written by the PLC manufacturer design, and stored in the PLC's system memory, the user can not directly read and write and change. System programs generally include system diagnostic procedures, input processing procedures, compilers, information transfer procedures and monitoring procedures. PLC's user program is the user's programming language using PLC, prepared according to the control requirements of the program. In the PLC application, the most important thing is to use PLC programming language to write user programs, in order to achieve the purpose of control. Because PLC is specially developed for industrial control devices, the main user is the majority of electrical and technical personnel, in order to meet their traditional habits and ability to master, PLC's main programming language than the computer language is relatively simple, easy to understand, the image of the special Language. Compared with the general computer language, PLC programming language has obvious characteristics, it is different from the high-level language, but also different from the general assembly language, it is necessary to meet the ease of preparation, but also to meet the requirements of easy debugging. At present, there is no programming language that is compatible with each manufacturer's product. But no matter what type of PLC, the programming language has the following characteristics.
1. Graphical instruction structure
The program is represented graphically and the instructions consist of different graphical symbols that are easy to understand and remember. The software developers of the system have compiled the independent arithmetic functions needed in industrial control into symbolic graphics. Users can combine these graphics according to their own needs and fill in the appropriate parameters. In the logic part, almost all manufacturers are similar to the relay control circuit ladder, it is easy to accept. Such as the Siemens company also uses the control system flow chart to indicate that it follows the binary logical components graphical symbols to express the control relationship, intuitive and easy to understand. For more complex arithmetic, timing counting, etc., are generally given reference to the ladder diagram or logic components, although not as symbolic part of the logical operation, but also by the user.
2 clear variable constant
Graphic symbols correspond to the operation code, provides the computing function, the user-entered operands, such as K400, T120 and so on. For PLC variables and constants and their value range is clearly defined by the product model, you can consult the product manual.
3. Simplified program structure PLC program structure is usually very simple, for the block structure, different blocks to complete different functions, you can make the program debugger control of the entire program functions and control the order has a clear concept.
4. Simplify the application software generation process
The use of assembly language and high-level language programming, to complete the editing, compilation and connection of the three processes, the use of programming language, only need to edit a process, the rest is done automatically by the system software, the entire editing process are man-machine dialogue state, Does not require users to have profound software design capabilities.
5. Strengthen the debugging means
Either assembler or high-level language program debugging, are editorial staff a headache, and PLC program debugging provides a complete set of conditions, the use of programmers, the use of PLC and programmer buttons, display and internal editing, debugging , Monitoring, etc., and the software support, diagnosis and debugging operations are very simple. In short, PLC programming language is user-oriented, does not require users with advanced knowledge, do not need long time specialized training.