Digital loop transceiver having a power up/down circuit

TLDR: In this paper, a digital private automatic branch exchange (PABX) has a plurality of digital line cards for coupling the PABX to respective digital subscriber sets via subscriber lines.

Abstract: In a digital telephone system, a digital private automatic branch exchange (PABX) has a plurality of digital line cards for coupling the PABX to respective digital subscriber sets via subscriber lines. In each digital line card, a digital loop transceiver operates in a master mode to couple the digital data bus of the PABX to the subscriber line via a subscriber line interface circuit in response to control signals provided by the PABX on the control bus thereof. In each digital subscriber set, a digital loop transceiver operates in a slave mode to couple the subscriber set to the subscriber line via a subscriber line interface circuit and to provide the several control signals required by the other components thereof. The digital loop transceivers may provide communication on each of two communication channels, with the digital data words of the first channel being treated the same as the digital data words of the second channel. In each digital loop transceiver, a power up/ power down circuit powers down all circuitry associated with each digital loop transceiver except for enough circuitry to detect an incoming data burst. Power up of each digital loop transceiver is accomplished via a power control portion which controls a data transmission controller in response to a detect signal and a power enable signal. A data control portion controls the transmission of bursted data to guarantee the avoidance of data collisions in a two-wire full duplex communication system.