Abstract: A four-wire line unit interface circuit used on four-wire line units in a line shelf of a digital loop carrier provides all of the logic necessary to access a subscriber line to a line unit interface bus connected to common equipment within the line shelf. Information received from the common equipment includes signaling data, configuration data and provisioning data which is reconfigured and processed by the line unit interface circuit for controlling the subscriber line. Configuration data from the common equipment is decoded to assign time slots on a line unit interface bus to the various channels serviced by the line shelf and to further provide for a timing offset between the transmit and receive strobes provided to each subscriber line circuit. An asynchronously operating microprocessor is provided on the four-wire line unit, and a microprocessor interface is provided to interface the asynchronously operating microprocessor with the data provided on the line unit interface bus.