Loop-start/ground-start line interface circuit

DETAILED DESCRIPTION In FIG.
1, a private branch exchange (PBX) 10 is coupled through a two-wire telephone line circuit, including tip and ring conductors 11 and 12, to an interface circuit 13.
That circuit provides battery line feed to the PBX, as well as performing a two-wire-to-four-wire translation with respect to a four-wire communication path.
Included in interface circuit 13 is a two-wire transmit circuit 16 and a two-wire receive circuit 17.
The latter circuits extend a communication path for voice signals toward a telephone central office (not shown).
Interface circuit 13 is a part of a line interface unit at the edge of such an office or, alternatively, it is one interface in a channel unit which includes channel path treatment circuits (not shown), such as gain insertion, or equalization, which are provided on a four-wire basis.
Details of the PBX 10 comprise no part of the present invention and are, therefore, illustrated only partially in order to provide a schematic representation of subscriber loop closure and ring-ground functions often provided to a special service circuit extending to such a PBX.
That special service circuit is, in FIG.
1, the conductors 11, 12 to which the interface circuit of the present invention must provide accommodation.
A controller 18 in the PBX operates a first relay RG having make contacts (illustrated as an "X" mark across a lead) in a lead connected between ring conductor 12 and ground, and a second relay LC having a set of transfer contacts.
The latter contacts include make contacts, which can be operated to connect a resistor 19 between the tip and ring conductors, and a set of break contacts (illustrated as a short line segment perpendicularly intersecting a lead) in a connection between the tip conductor 11 and a ground detector 20.
Resistor 19 schematically represents a PBX user telephone set impedance as seen through the PBX.
Detector 20 is otherwise connected at its opposite terminal to a negative voltage supply 21.
That supply, and others to be indicated herein, are schematically represented by a circled polarity sign to indicate a lead connection to a supply (not shown) of the polarity indicated and having a terminal of opposite polarity connected to ground