Abstract

A telephone interface circuit for use in a digital loop carrier system. A transformer for coupling analog information and a switch responsive to a hook condition signal is provided. The hook condition signal provides an indication of the "on/off hook" condition of a corresponding telephone subscriber. The switch provides a closed circuit condition to "sink" a DC current when the corresponding telephone subscriber is "off hook" and an open circuit condition when the corresponding subscriber is "on hook". The telephone interface circuit further includes a capacitor serially coupled with the primary winding of the transformer, to prevent DC current from passing through the primary winding of the transformer. A combination telephone interface circuit that can be adapted for use as a remote terminal interface circuit or as a central office terminal interface circuit is also included. The combination interface circuit includes first and second circuit paths disposed in parallel with each other. The first circuit path includes a transformer, switch, and capacitor as described above. The second circuit path includes a subscriber line interface circuit. The combination interface circuit includes jumper wires for selectively coupling either the first circuit path or the second circuit path between a pair of input teriminals and a pair of output terminals of the combination interface circuit.