Electronically switched optically coupled line interface

The method of providing the switch hook function in a communication line circuit having an input connected to a communication line and an output connected to a utilization circuit and whereas said line circuit includes a semi-conductor operable for blocking communication signals received from said input when the line circuit is in the on-hook condition and operable for passing said communication signals when the line circuit is in the off-hook condition, said method comprising the steps of:

• receiving from said input an alternating alerting voltage;
• passing said received voltage across the open condition of said semi-conductor;
• providing a drive current from said utilization circuit for driving said semi-conductor into the off-hook condition, whereby said drive current remains separate from signals passed through said semi-conductor when said semi-conductor is in the off-hook condition;
• and
• terminating said communication line across an electronic inductor which presents a low impedance to DC voltages and a high impedance to AC signals, said electronic inductor providing a regulated biasing voltage to both transmit and receive optocoupler circuits.

The method of interfacing between a telephone line and a device, said method comprising the steps of:

• receiving a communication signal over said telephone line and presenting said received signal to a hook switch circuit said circuit having a parallel connection of an optocoupler and a zener diode with the output of said hook switch circuit passing its output to one side of an electronic inductor, the other side of the electronic inductor connected to said telephone line;
• passing said presented communication signals to an interface circuit having first and second connections connected across said electronic inductor, said interface circuit including first and second optocouplers each connected across said interface circuit;
• said first optocoupler having one side of its input connected to said electronic inductor to control the impedance presented by said interface circuit to said telephone line via said connected input and the other side of its input connected to said second connection of said interface circuit, said second optocoupler having one side of its output connected to said first connection of said interface circuit and the other side of its output connected to said second connection of said interface circuit;
• wherein an output of said first optocoupler is connectable to a receive portion of said device and wherein the input of said second optocoupler is connectable to a transmit portion of said device;
• bidirectionally presenting telephone line signals to and from said device through said switch hook when said switch hook is in the off-hook state;
• and
• passing only alerting signals from said telephone line to said device when said device is in the on-hook state.

An interface circuit for connection between a telephone line and a communication device, said circuit comprising:

• a diode bridge for controlling the polarity of voltage signals on said telephone line;
• an electronic switch hook connected to the output of said diode bridge, said bridge having means for passing in one direction alerting signals while said switch hook is in the on-hook state and for passing bidirectional communication signals when said switch hook is in the off-hook state;
• means for receiving a drive current for controlling said on-hook and said off-hook states of said switch hook;
• an electronic inductor connected to the output of said hook switch for terminating said telephone line, said electronic inductor operable for presenting a low impedance to DC voltage and a high impedance to AC signals, said electronic inductor providing a regulated biasing voltage to both transmit and receive optocoupler circuits;
• means for presenting signals received from said telephone line to the LED portion of said receive optocoupler;
• and
• means for presenting signals received from said communication device to the LED portion of said transmit optocoupler.