Apparatus and method for determining the state of a telephone li

Details Apparatus and method for determining the state of a telephone li Apparatus and method for determining the state of a telephone li

1992-05-13

1994-03-01

Chan, Wing F.

Telephonic communications

Telephone line or system combined with diverse electrical...

Having transmission of a digital message signal over a...

379377, 379379, H04M 1100, H04M 322

Patent

active

052915450

ABSTRACT:
A detector for On-Hook/Off-Hook state of a telephone line includes a capacitor connected to the telephone line in series with a resistance large enough to be allowed as load on an On-Hook telephone line. A low impedance voltage sensing means in series with a normally open switch is connected across the capacitor, and the switch is closed in response to a polling signal to determine if the line is in On-Hook state. The capacitor transforms a very low charging current lasting seconds into a large discharge current lasting only milliseconds. This makes it possible to use an inexpensive low impedance voltage sensing device, such as an opto-coupler in series with a zener diode. Such components provide high-voltage galvanic insulation at low cost, and a very compact apparatus can be made. The invention also provides for inexpensive ring detector circuits and for overload protection for the equipment connected to the telephone line.

REFERENCES:
patent: 4220825 (1980-09-01), Fahey
patent: 4224478 (1980-09-01), Fahey et al.
patent: 4433212 (1984-02-01), Moses et al.
patent: 4647723 (1987-03-01), Voorhies
patent: 4679229 (1987-07-01), Yamaguchi
patent: 4845741 (1989-07-01), Fourdraine
patent: 4958371 (1990-09-01), Damoci et al.
patent: 4987586 (1991-01-01), Gross et al.
patent: 5146384 (1992-09-01), Markovic et al.
General Electric Optoelectronics Manual, 1976 (pp. 70-72).

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