line and the t line respectively in the digital service unit thereafter a communication link is established between the digital service unit and the isdn basic interface equipment similarly to the transmission procedure and communication is executed next the stop procedure that is the procedure by which the information channels 1 and 2 are changed to the state where they are not used as the isdn basic interface passive bus but used as the original twist pair line homebus will be explained it is supposed as the initial state that the communication link is established between the digital service unit and the isdn basic interface equipment and that communication is effected when the communication is terminated and the system enters the stopped state there are no signals on the r line in the digital service unit this is detected by the info 2 signal detector and the control section is informed thereof the control section disconnects the cross point and also disconnects the information channel 1 and the information channel 2 from the r line and the t line respectively in the digital service unit in this way since the supplied electric power detecting device in the information plug socket cannot detect the phantom electric power supplied to the information channels 1 and 2 the relay disconnects the information channel 1 and the information channel 2 from the terminal resistor now several cases where the present invention is applied to the homebus system will be explained referring to fig1 a and 3b fig1 is a block diagram showing the construction of an embodiment of the.