Expandable link telephone intercom system

Abstract

A multi-link intercom system having a large number of stations selectively connected to one of eight links by respective station controllers. When a calling station goes off-hook, its station controller connects the station to the first available link during the one of four link access periods corresponding to that link. The calling station then applies appropriate signals to the link identifying the called station. These signals are decoded by a link controller for the link which generates called station address signals during its corresponding link access period. All of the link controllers generate called station address signals on the same bus, but the signals are time multiplexed to occur during a portion of each link access period corresponding to each link. The station controller for the called station recognizes its address and connects its station to the link corresponding to the portion of the link access period in which the station address was received. Two-way communication between the calling and called station is then effected. Each of the station controllers utilizes a pair of self-contained controller circuits which are inherently adapted to operate with four link access periods and selectively connect its station to one of four audio links. The system includes circuitry for allowing two of the four-cycle, four-link controller circuits to operate in parallel utilizing relatively few additional signal lines as compared to a four-link system.