High reliability is the key performance of electrical control equipment. PLC is highly reliable because it adopts modern large-scale integrated circuit technology and strict production technology, and its internal circuit adopts advanced anti-interference technology. For example, the F-series products produced by Mitsubishi Corporation have an average trouble-free time of 300,000 hours. Some use redundant CPU, and the average failure time is longer. From the external circuit of PLC, compared with the relay system of the same scale, the electrical wiring and switch contacts are reduced to hundreds or even thousands, and the faults are greatly reduced. In addition, it has PLC hardware fault self-checking function, which can send out fault alarm information in time. In the application software, users can also carry out fault self-diagnosis for peripheral devices, system circuits and devices other than PLC. In this way, it is not surprising that the reliability of the whole system is high. This paper mainly studies the control of crane, and adopts the control method of series resistance to control the motor, so as to realize the control of crane. The working mode of PLC is computer control, not relay contactor, so PLC is similar to general computer, such as the same basic structure and the same principle of instruction execution. However, there are important differences between these two working modes, which are reflected in the circular scanning mode of PLCCPU, focusing on input sampling, output refresh and I/o image area respectively. Results The process of entering the state and executing the program in the previous state. The process of scanning mode of PLC work package is generally divided into five stages: internal processing and self-diagnosis, peripheral communication processing, sample input, user program execution and output refresh.

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