PURPOSE: To eliminate adverse effect of a reflected wave produced at a branching path on transmission of a signal by coupling directly a connection adaptor to a transmission line of bus form and applying branch connection to a terminal device from the adaptor via a connection cable.

CONSTITUTION: The adaptor APi is coupled directly to a socket STi(i=1∼n) of a time division multiplex transmission line BUS of 4 wires extended from a master terminal device ME, and a slave terminal device TEi is subjected to branch connection from the APi via a cable TCi. The BUS is terminated with a terminator TP having characteristic impedance R0. The TEi receives a transmission code on the BUS via a transformer T1 of the APi by using a high-impedance reception circuit RCV, converts the code into a logical level and gives it to a reception signal processing circuit RDP. The RDP gives received information to an input/output circuit IOC and transmits a transmission timing signal ST to a transmission signal processing circuit SDP. The SDP brings a control signal HZ to logical 1 at the signal ST, transmits the signal SD by using a transmission circuit DRV of low impedance and brings the DRV to high impedance in other cases.