The working principle of programmable controller and input/output processing principle

Any kind of relay control system is composed of three parts, namely the input part, logical part and output part, which input part refers to all kinds of buttons, switches, etc. ; Logical part is composed of all kinds of relay and contact to realize the control circuit of a certain logic functions; Output part refers to all kinds of electromagnetic valve coil, turn on the motor of contactor and the signal light electrical appliances. As shown in figure 1, it is a kind of simple relay control system. Figure 1 X1, X2 in light control chart is two button switch, Y1, Y2 are two relay, T1 & # 127; Is time relay. Its job is to process is: when the X1, X2 any press a button, coil Y1 connected, normally open contact closure of Y1, light is red light. At this point of time relay T1 connect and start delay at the same time, after the delay to 2 s, coil Y2 through, normally open contact closure, the green light. Can know from the above example, & # 127; Relay control system is based on a variety of input conditions to execute the logic control circuit, the & # 127; The logic control circuit is based on the needs of the control object in a certain fixed line connection good, so can't flexible changes. And relay control system, PLC is composed of input part, logical part and output part. As shown in figure 2: the main function of each part is: input parts: gather and save the actual operation data of the object information, The various switch quantity information of controlled object or operation command, etc. ) 。 Logical parts: process the input part of information, and according to the controlled object correctly reflect the actual action requirements. Output part: to provide the device being controlled, which operates processing equipment are in need. User program through the programmer or other input device input coexist in the PLC user memory. When PLC began to run, the CPU according to the regulation of system monitoring program order, by scanning, complete state of each input point or enter data collection, the execution of user programs, each output point status updates, programmer typed response and display updates, CPU self-checking function, etc. PLC scan can be fixed procedures, can also according to the variable order specified by the user program. Centralized PLC controller sampling, concentrate output work, reduce the interference from the outside. By the above analysis, can the working process of PLC for three phases, namely the input sampling stage, program execution stage and output refresh stage. ( 1) Input sampling stage PLC in the input sampling stage, first scan all inputs, and each input into memory each image corresponding to the input registers. At this point, the input image register be refreshed. Then enter the procedure of phase, the program or output phase, the input image register isolation with the outside world, no matter how changes in the signal, the content remains the same until the next scan cycle of the input sampling stage, only to write new content at the input. ( 2) Program execution stage according to the principle of PLC program scans, PLC left right after the first, after the first step under the order statement every scan. When instruction involves input, output, state, 'read' from the input image register of PLC corresponding to the current state of the input image register, and then, for the corresponding operation, operation results in the component image register, again for component image register, each element will change as the program execution process. ( 3) Output refresh stage after all the instruction execution is completed, the output image register all the state of the relay output in the output refresh stage in the archive to the output latch register, output through a certain way, drive the external load. Concentrated sampling, the characteristics of the output work way is: in the sampling period, all the input signal ( No matter the signal at that time whether to adopt) , read together, then in the process of the whole process of PLC system cut off from the outside world until the output control signal to the next working cycle'll deal with the outside world, fundamentally improve the anti-interference of the system improves the reliability of the work. PLC in the process of input and output must obey the following principles: (1) the input image data registers, depends on the input terminal board on each input terminals in a state of the on and off during the week. (2) how will the program depends on the user program and input and output image the contents of the registers. (3) the output image register data depends on the output instruction execution results. (4) the data in the output latch, & # 127; By the last output during the refresh output image register data in the decision. (5) the output terminal of the on off state, determined by the output latch.
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