Patent ID: 6480596
Filing Date: 2002-11-12
Classification: H04M

Abstract:
A trunk circuit for interfacing a private branch exchange serving a plurality of extension subscribers to a central office line of a central office exchange, said trunk circuit comprising:tip and ring terminals connected to the central office line of the central office exchange with a power source providing a voltage potential; a transformer having a primary coil respectively connected to the tip and ring terminals, and a secondary coil connected to a switching circuit of the private branch exchange; tone signal detection means connected to the switching circuit of the private branch exchange, for detecting a dial tone generated in response to one of incoming and outgoing telephone calls; direct inward-outward dialing detection means connected to the tip and ring terminals and the primary coil of the first and second transformers, for detecting a polarity of the voltage potential at the tip and ring terminals and the current flowing through the tip and ring terminals to determine reception of one of the incoming and outgoing telephone calls for enabling formation of a speech path between a central office line subscriber and a corresponding extension subscriber of the private branch exchange; a switch connected the primary coil of the first transformer, for selectively enabling the current to flow between the tip and ring terminals at the central office line through said direct inward-outward dialing detection means via one of a low impedance loop and a high impedance loop; and a relay driver for controlling operation of said switch to select between one of the low impedance loop and the high impedance loop for the current to flow through said direct inward-outward dialing detection means between the tip terminal and the ring terminal at the central office line.