Abstract: An intercommunication system in which a plurality of stations are interconnected by a single common line and each station is set to receive signals within a voltage "window" different from that of all the other stations of the system, and is operable to transmit signals at a voltage level selected in dependence on which of the stations is intended to receive the signals, in which all stations are further provided with a lock-out gate which disables them from transmitting when another station is already transmitting. The signal selection function and signal transfer function on the receiver part of each station are separated to permit equalization of the transmission voltage "window" for all biasing voltage levels.