PURPOSE: To enable one serial port arranged in an NC control unit to execute the input/output of plural terminal devices, by providing the body of the NC control unit with a device to send format data at prescribed time intervals.

CONSTITUTION: A signal outputted from the serial port 12 in the NC control unit 10 is sent to respective interface cards 22. Each interface card 22 receives a signal from the NC control unit 10 successively only when the sent signal coincides with its card number. Data sent from the unit 10 are stored in a data memory device of the 1st I/O device 24, and after being edited so as to be received by the fine terminal device, sent to the terminal device. Consequently, the terminal device executes its own operation and simultaneously sends data corresponding to the input data to a signal line 100 through the 2nd I/O device 26. The format data are sent from the unit 10 to each card 22 at fixed time intervals.