公开(公告)号：US3840710A

公开(公告)日：1974-10-08

申请号：US36219073

申请日：1973-05-21

Applicant: BELL TELEPHONE LABOR INC

Inventor： LIMIERO A ,  SMITH J

IPC: H04M9/00 ,  H04M1/00

CPC classification number: H04M9/006

Abstract: A key telephone line circuit is disclosed having improved hold control features. The improved circuit is balanced against longitudinal-to-metallic noise conversion by an arrangement which divides the hold impedance equally between both conductors of the central office or PBX line, while still maintaining the line relay operable for detecting loop current. The new arrangement also contains a delay timer for maintaining the hold bridge operated during inadvertent central office loop open conditions. Circuitry is provided for immediately releasing the hold bridge when the subscriber returns from a held condition.