Apparatus and method for determining the state of a telephone line

Method for determining the state of a telephone line, comprising the steps of:

• charging a capacitor by the voltage across the telephone line via a resistance sufficiently large to satisfy loading requirements on a telephone line in On-Hook state;
• providing a low impedance voltage sensing device having an on-state when a voltage above a predetermined threshold value is sensed;
• connecting said low impedance voltage sensing device to said capacitor when the state of the telephone line is to be determined;
• and
• using an on-state of said voltage sensing device while said capacitor is connected to the voltage sensing device as indication that the telephone line is in the On-Hook state.

Apparatus for determining the state of a telephone line, comprising:

• a modem device for automatic dialing and control of the apparatus;
• an Off-Hook switch for connecting said modem device to the telephone line in response to a control signal from said modem device;
• means for determining the On-Hook/Off-Hook state of the telephone line, including
• a resistor and a capacitor connected in series across said telephone line, said resistor having a resistance sufficiently large to be an acceptable load on a telephone line in an On-Hook state;
• a voltage sensing device with a low impedance sensing circuit and an output circuit indicating an on-state when the sensing circuit is connected to a voltage above a predetermined threshold level;
• a normally open switching device connected across said capacitor in series with said low impedance sensing circuit in said voltage sensing device;
• means for closing said switching device in response to a polling signal from said modem device;
• and
• means for detecting an on-state in said voltage sensing device coincident with a polling signal.

Modem device connected to a telephone line via an Off-Hook switch, comprising:

• means for protecting the modem device from an overload condition, including
• a voltage sensing device comprising an opto-coupler having a sensing circuit and an output circuit with an on-state when current is flowing in the sensing circuit, and a zener diode connected in series with said sensing circuit;
• means for effectively connecting said sensing circuit in series with said zener diode across input terminals of the modem device;
• means responsive to an on-state in said output circuit coincident with an Off-Hook state of the Off-Hook switch for switching the Off-Hook switch to an On-Hook state;
• and
• a ring detector circuit connected to the telephone line, and including said sensing circuit in said opto-coupler.