A multiple access digital communications system comprises a plurality of remotely located terminals coupled to a communications bus including a pair of oppositely directed, unidirectional signal paths (10, 12) and a unidirectional path coupler (14) for transferring signals from the inbound path (10) to the outbound path (12). To send a message from one terminal to another desired terminal, a corresponding bus interface unit (20A) coupled to both the inbound and outbound paths (10, 12) is adapted to perform an rf. carrier sensing operation to determine as to whether or not any other subscriber is transmitting on the inbound path (10). If no carrier is detected, the bus interface unit (20A) confirms this determination by first beginning to transmit a message packet and then monitoring the outbound path (12) during a collision window. If the transmitted message is received intact within this collision window, the bus interface unit (20A) may transmit any remaining portion of its message packet on the inbound path (10).