Abstract: A pulse density signaling input/output (PDSIO) scheme is implemented to provide a two-way, two-wire hybrid analog/digital communications system. PDSIO signals are sent as logic pulses from a "master" port to one or more "slave" ports over a twisted pair. From one to a predetermined maximum number of logic pulses are transmitted over two wires at baseband by a master port during a defined period. The modulation used by the master port is pulse density modulation at base band so that the transmitted signal appears to be a d.c. logic signal. The pulses transmitted to the slave port are also used to power the slave port. The slave port communicates with the master port over the same two wires using a pulsed current modulation that multiplexes data on the pulse signals from the master port.