Signal multiplexing circuit

A signal multiplexing circuit selectively operable as a line card interface for a digital access circuit of a central data and voice conmunication facility and as a station set interface for a data and voice station, and which is controllably oprable for bidirectional communication between the line card interface and addressable ones of a plurality of station interface interconnected via a two wire telephone line, comprising:

• line sampling means responsive to a carrier signal for detecting an active line state in the presence of the carrier signal on the telephone line and an inactive line state in the absence thereof;
• transmit register means responsive to a data message bit field input thereto for storing the bits until the occurrence of a predetermined inactive line state;
• controller means operably responsive to data message bit fields input thereto and to the detected inactive line state for enabling a serial data bit output of the transmit register means;
• modulator means having an input coupled to the serial data bit output for modulating a high band carrier signal and outputting the modulated high band signal on the telephone line;
• and
• equalizer means having analog signal receiving and transmitting paths including respective automatic gain control circuits connected between the telephone line and corresponding signal inputs and outputs for controlling signal gains, and gate means for selectively enabling and disabling the receiving and transmitting paths in response to predetermined ones of the data message bit fields input to the controller means.