Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1998-07-01
2001-08-07
Chan, Wing F. (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
C379S379000, C379S093050
Reexamination Certificate
active
06272220
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This inventor relates to a line interface circuit, and more particularly, to such a circuit in which the mechanical switch hook has been replaced by an electronic circuit.
BACKGROUND OF THE INVENTION
In prior art systems, the hook switch function is typically implemented between the telephone line and the diode bridge using a mechanical connection, such as a relay or the switch hook itself, as shown in FIG.
2
. The hook switch relay can be electro-mechanical or optical, but in prior art configuration it must be able to switch current flowing in either direction. This approach is used so that the hook switch relay contact can be bridged by a resistor in series with a capacitor in order for telephone ringing signals to be capacitively coupled into the line circuit while the switch hook contact remains open (BC on-hook condition).
It is desired to replace the mechanical switch hook to alleviate cost and/or size reductions. Any such replacement must handle the power variations and polarity reversals which occur on telephone lines.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a circuit and method which interfaces between a telephone transmission line and a communication device without using a mechanical contact at the input of the line on circuit. Related patent application Ser. No. 09/108,576 discloses an optically coupled circuit having pairs of optical couplers which provide high voltage isolation in a telephone circuit. The optical coupler pairs are biased such that AC signals are optically coupled across the isolation barrier. The optically coupled data access arrangement is connected to the telephone line through a hookswitch relay which conducts current in either direction when the contacts are closed and conducts no current when the contacts are open.
A diode bridge, in prior art systems, is connected between the transmission line and the DC loop holding circuit of the data access arrangement. The diode bridge functions to ensure that the DC voltage received from the transmission line always has the same polarity when it enters the line matching circuit. Typically the tip connection on a telephone line is considered to be positive for polarity. However, in some cases the ring connection will have a positive polarity. To compensate for ring-positive polarity connections, the diode bridge routes the incoming telephone voltage to the desired side of the line matching circuit.
The present invention uses an optical coupler in between the diode bridge and the remaining portion of the optically coupled data access arrangement circuitry. The purpose of this additional optical coupler is to act as a hookswitch for the circuit. The selected optical coupler need only conduct current in one direction, since the diode bridge ensures that the required current flows through the photo transistor element in the proper direction.
In the present invention, the AC ringing signal cannot be capacitively coupled around the optical coupler performing the hookswitch function due to the diode bridge which causes the current to flow in only one direction through the switch hook optical coupler. The present invention uses a zener diode in series with a resistor in order to couple the telephone ring signal around the optical coupler. The zener diode is selected such that its zener breakdown voltage is greater than the maximum steady state DC voltage of the telephone line when on hook. This method provides a clipped version of the ringing signal at the optically coupled receive output of the circuit which is sufficient to accurately detect ringing as required by a modem connected to the device output of the circuit.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
REFERENCES:
patent: 5020100 (1991-05-01), Gardiner
patent: 6192125 (2001-02-01), Norsworthy et al.
Brady III W. James
Chan Wing F.
Hernandez Pedro P.
Sentient Communications, Inc.
Telecky , Jr. Frederick J.
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