Abstract: Upward/downward data arranged in a time-division multiplexed frame, including a common subchannel for priority transmission and private subchannels for transmission request/enable signals, are exchanged via an upward/downward data signal line between a plurality of slave stations and a master station having a priority circuit and an internal communication device. A slave station establishes the transmission request signal and transmits it to the priority circuit through the upward data signal line. The priority circuit selects one transmission request signal to assign priority for transmission to a corresponding slave station and then establishes the transmission enable signal and transmits it through the downward data signal line based on the selected signal. When the signal is acknowledged, the corresponding slave station establishes a message signal and transmits to the master station through the upward data signal line.