Signal multiplexing circuit

Abstract

Bidirectional communication of voice and data signals over a two wire telephone line interconnecting several telephone sets with a digital access circuit of a central data and voice communication facility is achieved by a frequency division multiplexing circuit that functions either as a set interface for a telephone or as a line card interface for the access circuit. Data message signals input to the multiplexing circuit are stored in a shift register for subsequent modulation of a carrier signal, but since only one interface may enter a transmission mode at one time transmission priority is assigned by a controller to the interface that first attempts transmission on an inactive line. All other interface then enter a monitor mode to listen but not act on the transmitted message. Transmission errors caused by line noise, or collision transmissions between two or more interfaces are resolved via message transactions between the interfaces only, and retransmission priority is resolved in favor of an interface having the lowest address. Controllable transmit and receive equalizers define analog signal paths in the interfaces and compensate for line losses.