Kenneth S Brooks

US Patent:

46792325, Jul 7, 1987

Filed:

Aug 2, 1985

Appl. No.:

6/761888

Inventors:

Kenneth L. Brooks - Naperville IL
Paul R. Bufkin - Wheaton IL
David G. Mihal - Oswego IL
Jerry G. Miller - Wheaton IL

Assignee:

American Telephone and Telegraph Company, At&T Bell Laboratories - Murray Hill NJ

International Classification:

H04M 900

US Classification:

379413

Abstract:

When a floating battery feed circuit is used to power telephone lines, it is desirable to provide an anti-corrosion biasing circuit to keep the voltage on the tip and ring conductors from becoming positive. This can be done using an anit-corrosion circuit that sinks a dynamic amount of current from the tip conductor. A line circuit including such biasing is, however, incompatible with special lines such as PBX lines that rely on a grounded tip conductor in the talk state. In accordance with the present invention, a constant current is supplied from a ground reference circuit to the ring conductor of such special lines only when loop current is flowing. The magnitude of this supplied current is such that the anti-corrosion circuit holds the tip conductor sufficiently close to ground potential to allow the proper operation of circuits connected to the special lines.