The whole process of the PLC work - — Part four work is introduced

In order to meet the requirements of industrial logic control, at the same time, combining with the characteristics of computer control, the PLC works using the cycle of sequential scan works. Each scan time referred to as the scan cycle or work cycle. CPU from the first instruction execution, detailedly in order to perform a user program until the end of the user program, and then return to the first instruction to start a new round of scan. PLC controller is such scan cycle to repeat the cycle. The whole process of PLC work available running diagram in figure 1. The whole process can be divided into the following several parts: the first part is on processing. After PLC electricity to an initialization of system, including hardware initialization and software initialization, power range setting and other initialization processing, etc. The second part was the diagnosis treatment. PLC to every scan, - Since the diagnostic work-up, determine whether the action of the PLC itself is normal. Such as CPU, battery voltage, program memory, I/O and communications whether exception or error, such as detect anomalies, the CPU on the front panel leds and abnormal relay will get through, will be deposited in the error code in the special register. When a fatal error, the CPU was forced to STOP, all the scanning will STOP. The third part of the figure 1 running diagram of PLC is communication services. PLC controller self-diagnosis after processing is completed in the process of communication services. First check whether the communication task, if there is a call the corresponding process, accomplishes the communication with other devices, and accordingly the communication data processing; Then the clock, special register update processing, etc. The fourth part is the scanning process program. PLC in the electrical treatment, self-diagnosis and communication services is complete, if the job selection switch in the RUN position, the scanning phase into one program. To complete input processing, namely the input terminal of status register the reading input image, and then execute user programs, and finally the output processing result refresh to the output latch. In the above sections, communication services and programs the scanning process is the main part of the PLC work, the work cycle is called scan cycle. Can see scan cycle directly affect the control signal of the real-time and accuracy, in order to ensure correctly in real time to control, at the end of each scan cycle, communication task assignment time must be controlled within a certain range. When running normally, PLC scan cycle length and speed of the CPU, and the situation of the I/O points, and the length of the user application and programming and so on. Usually l KB is executed with PLC to illustrate its scanning speed, generally for a fraction of ms to hundreds of ms. It is important to note that different instruction execution time is different, ranging from zero a few hundreds of mu mu s to s, therefore, choose different instructions used in the scanning time will be different. For some need high speed processing of signal, it need special software and hardware measures to deal with.
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