PLC ladder diagram and instructions in the programming statements table used in combination with examples are introduced

So-called programming, it is the user according to the requirements of control object, the use of PLC manufacturers provide a programming language, described a control process. PLC ladder diagram language is the most commonly used programming languages and directive statements table language, and often a combination of both. 1) Ladder diagram ( Language) Ladder diagram is a kind of evolved from the relay contact control circuit diagram of graphic language. It is with the aid of move, similar to the relay contact, coil, as well as terms and symbols in series, in parallel, according to the requirements of control connection and the graph of PLC logic relationship between input and output, intuitive and easy to understand. Ladder diagram graphic symbol respectively is commonly used in PLC programming components move and move off contact; With ( ) Their coil. The kinds of ladder diagram programming elements with a graphic symbol and indicate the number of letters or distinction. Contact and coil referred to as a network composed of independent circuit, ladder diagram and the statement list is generated by the programming software applications have a network id, allowed to take the network as the unit to the ladder diagram to add comments. Ladder diagram design should pay attention to the following three points: (1) ladder diagram according to the order of from top to bottom, from left to right. Each logical line ( Or cascade) Began in the left bus bar, and then is contact list, and connection, the last is the coil. (2) each rung in the ladder diagram flow is not a physical current, but the concept of 'current', from left to right, its both ends there is no power supply. The concept of 'current' is used to visually describe the user program execution of coil connected to meet the conditions. (3) input registers used to receive the external input signal, but not by PLC internal other relay contacts to drive. Therefore, ladder diagram only appear in the input register contact, but not in the coil. Output register output program execution results to the external output device, when the output register in the ladder diagram, the coil to electricity, signal output, but not direct drive output devices, and to go through the relay output interface, transistors or thyristor can be achieved. Output register contact is for internal use. 2) Instructions statement table statement table is a kind of instruction mnemonic to PLC controller program language, it is similar to the computer assembly language, but better than assembly language easy to learn, several instruction program is composed of directive statements table. An instruction statement by step sequence, instruction and the function device Numbers of three parts. Under the cases of PLC to realize three-phase squirrel cage motor/stop control of the representation method of two kinds of programming languages: figure 1
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