conversion circuit 132 the rotary encoder 6 is of an incremental type and supplies a phase and b phase signals to the second parallel serial signal conversion circuit 133 and a z phase signal to the first parallel serial signal conversion circuit 132 the abnormal condition sensor 31 detects an abnormal condition of the servo motor 4 for instance an abnormal temperature increase thereof and supplies a sensor output signal to the first parallel serial signal conversion circuit 132 the first parallel serial signal conversion circuit 132 receives the pu phase pv phase and pw phase signal the z phase signals and the sensor output signal and converts them into a corresponding serial signal which is given to the second parallel serial signal conversion circuit 133 as shown in fig6 and 7 the second parallel serial signal conversion circuit 133 converts the serial signal from the first parallel serial signal conversion circuit 132 and the a phase and b phase signals into another serial signal which is transmitted to a line driver 135 of a signal transmission circuit unit 134 via the signal transmission lines 3 referring to fig7 a clock frequency divider circuit 136 divides a transmission clock signal and supplies its output to the first parallel serial signal conversion circuit 132 the signal transmission lines 3 include a pair of signal lines 137 and 138 for transmitting the serial signal from the servo motor unit 1 to the servo driver unit 2 another pair of signal lines 139 and 140 for sending 5 volt and 0 volt lines from the servo driver unit 2 to the servo motor unit 1 thus according to this embodiment a significant reduction in the number of lines in the signal transmission lines 3 can be accomplished as compared with the.