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In order to solve the problem of data transmission slow and poor real-time control in the distributed system composed of PC extended card CP 5611 and multiple plc s7-200, their communication patterns and RS-485 protocol standards are analyzed. PC and multiple PLC build distributed application system through serial port to convert RS232 to RS485. Slave PLC based on free-port mode, custom command word and data packet format receive commands and data from PC to realize fast data communication. The practical application has proved that this distributed application system is feasible, real-time control is excellent and data transmission is also reliable.