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In order to improve the reliability of the acquisition/output link in the PLC control system, the design principle of the redundant system is analyzed, combined with the field application environment and actual needs, a distributed I/O redundancy method based on domestic PLC is developed. The article introduces the digital input/output and analog input/output redundancy schemes respectively for the type of data collected. This method takes the self-diagnosis results of the self-diagnosis periodically during operation as the basis for redundant diagnosis, combined with the user-configured redundancy scheme to improve accuracy of collecting/outputting data. It can be known from experiments and on-site base application that this method improves the reliability of the PLC control system and can ensure that the PLC system operates stably, efficiently and safely for a long time.