Abstract: The communication performance of advanced field bus protocols used in NC systems was analysed, and a new data communication protocol was defined based on dual CAN(controller area network) for position control system in NC—CANsmc(CAN for synchronous motion control). CANsmc, defined as a master/slave topology, consisted of one master and at most 61 slaves with two communication channels. Its communication control was polling with synchronous telegram. The protocol was based on CAN chip, and CANsmc telegram head was defined with CAN telegram descriptor. Three types of telegram were transmited in CANsmc: MST(master synchronous telegram), MDT(master data telegram) and DT(drive telegram). Master transmited MST and MDT to slaves through the master channel, and slaves fed DT back to the master through the slave channel when they received their own MDT. Finally, a CANsmc prototype was built, and protocol was implemented by using software. The master was based on PC ISA interface and the slave was a position control unit with position control, speed control and zero return functions. The prototype system proves that CANsmc could meet the satisfiction of industrial robot and CNC machine control.