The basic working principle of PLC are briefly introduced

PLC microprocessor as the core, has many characteristics of microcomputer, but the way it works with microcomputer are quite different. Microcomputer work generally use for order way. PLC controller is a focused input, output, cycle scanning way. Each time the loop scanning time is called a scan cycle. For each program, the CPU from the first instruction execution, in order to perform detailed instructions do periodic program loop scanning, if without the jump instruction, is from the first instruction to order a detailed user program, until the end and return to the first instruction, so the cycle cycle. PLC in the process of each scanning in addition to execute user programs, and complete internal processing, input sampling, communication services, program execution, self-diagnosis, output refresh, etc. The whole process of the PLC work includes three parts, namely the electrical treatment, the scanning process, and error handling. The whole process of PLC work available running diagram shown in figure 4. PLC after electrify, under the control of CPU in the system program for internal processing, including hardware initialization, the I/O module configuration scope of inspection, the power to keep setting and other initialization processing work. PLC has a strong self-diagnosis function, PLC to scan every a self-diagnosis inspection, determine whether the action of the PLC itself is normal, such as power test, the internal hardware is normal, program if there is a wrong grammar, etc. If detect anomalies, the CPU panel leds and abnormal relay will get through, will be deposited in the error code in the special register; CPU can type and degree according to the error signal, even for the corresponding error handling, the PLC controller STOP scans or forced into the STOP state. PLC controller operating normally, the scan cycle length and the length of the user application, CPU speed, such as I/O point position. Usually 1 KB is executed with PLC to illustrate its scanning speed, General 1 - 10 ms / KB) 。 It is important to note that different instruction execution time is different, so choose different instructions used in the scanning time will be different. If used in high speed system to shorten the scan cycle, can consider from the hardware and software. PLC cycle way of scanning the salient features are: high reliability, strong anti-interference ability, but the response lag, slow speed.
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