PURPOSE: To omit a specific signal line for transmitting a switching signal by switching a data transmission line in accordance with a selection signal sent from a host device and connecting an interface of the host device to an interface of an objective subordinate device.

CONSTITUTION: To select a terminal equipment for transmission, the host device 1 encodes the machine number of the terminal equipment, converts the code into a serial signal and sends the serial signal from a driver 6 to a receiver 19 in a branching circuit 18. The receiver 19 sends the machine number code to a switching circuit 21, which extracts the code and sends the extracted code to a serial/parallel conversion circuit 22. The circuit 22 converts the machine number code into a parallel signal and send the signal to a decoder 23, which decodes the singal and sends the decoded signal to a multiplexer 24. The multiplexer 24 selects a driver 25 and a receiver 28 which correspond to the machine numbers of the specified terminal equipment 3, e.g.