Distributed intercom system with audio bus

A distributed intercom system that interconnects a hub and various keypad and door zone units having microphones and/or speakers via CAT5 or similar cable having multiple twisted pairs. Audio is carried between all devices over a first audio wire pair, in analog signal format, forming an audio bus. Audio twisted pair acts as a data communication bus for control messages. One or more of receiver units may be active, while only one keypad device can be individually instructed to connect its corresponding microphone circuit to the audio bus. Determination of which unit is selected to be the transmitter occurs under the control of the hub. For example, in a case where a first user wishes to speak to another user in a different room, the first user presses a "talk" button at his keypad, causing that keypad to send out a message on a data communication bus to the hub. The hub unit then sends a reply on the data communication bus that grants permission to the first user's keypad to couple its microphone to the audio bus. The message is interpreted by the other stations as an instruction to keep their own microphones disconnected from the bus. A message is also sent to the desired receiving station(s) to couple their speakers to the bus to receive the audio signal.