In the design of the control system for the double spindle CNC cylindrical grinder,the speed of the two wheel spindle motors was controlled by analog module,but the NC system can only support the analog control of one spindle.In order to solve this problem,the Profinet bus communication interface of 808D AD numerical control system was designed to connect S7-1200 PLC to extend the second analog spindle.When NC programming for the second spindle,the R parameter was used to instead of the S speed command,the GET command and S7 communication protocol were used to transmit the speed command to the remote PLC terminal and then be output from the PLC analog module;the encoder of the second analog spindle was connected to the entrance of the high-speed counter of the PLC,and the actual speed was transmitted to the CNC system side through PUT command;the user interface of 808D AD was designed for double spindle grinding process,which could display the command speed and actual speed of two spindles.The results show that the system has strong anti-interference capability by using the bus communication,and the speed error of the second analog spindle can be controlled within 10%,which has a certain reference value for spindle speed control or non linkage displacement shaft control.