TELEPHONE LINE INTERFACE CIRCUIT WITHOUT HOOKSWITCH

摘要

A novel electronic inductor circuit is used to establish a DC current drawn from the telephone line needed for indicating to a central office (or PBX) that a telephone device (i.e. a modem) has seized the line. The electronic inductor's transistor is used to perform the functions of a hookswitch, thereby avoiding the introduction of unwanted impedance in the AC signal path of the device. In order to go on-hook, the electronic inductor's transistor (upon receiving a control signal) stops drawing current from the central office loop indicating that it is now in the on-hook state. Unlike a switch, the present invention operates by starting and stopping the DC current while the entire circuit remains connected to the line at all times. The line is "seized" by providing a control voltage to turn on the electronic inductor and thereby initialize line current to flow through its transistor - thus achieving the "off-hook" state.