US 4,536,618 A · Telephone line interface

Abstract

A telephone line interface for direct connect telephone line connections characterized by low cost and minimal package size requirements. The telephone line interface utilizes a small signal transformer with a primary winding couplable to the line by actuation of a relay. A secondary winding of the transformer is coupled to an integrated circuit which performs the functions of ring detect and dial tone detect, and separation of the received signal and the transmit signal when transmitting and receiving simultaneously. Ring signal detection on the secondary side of the coupling transformer substantially reduces the size and cost of the ring detect circuitry, as the secondary of the transformer may be provided with over-voltage protection, whereas the line side of the transformer must survive particularly high surge voltages, thereby requiring high voltage isolation not obtainable by way of conventional integrated circuits. Other aspects and features of the invention are disclosed.

Drawings

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Drawings

Claims8

Dependent claims

Apparatus for detecting the ring signal on first and second telephone lines comprising;

a coupling transformer having a primary winding and a secondary winding, each having at least two leads thereon,
a switch means for controllably changing between open and closed conditions to provide electrical conduction between first and second switch contact means when in said closed condition, said first switch contact means being a means for connection to the first telephone line, said second switch contact means being coupled to said first primary lead for said coupling transformer, said second primary lead of said coupling transformer including means for coupling to the second telephone line,
open line load means coupled between said first and second switch contact means for providing a predetermined linear load on the phone lines when said switch means is open, and
ring signal sensing means coupled to said first and second secondary leads of said coupling transformer for sensing the presence of a ring signal on the telephone lines when said switch means is open, said ring signal sensing means including filter means for attenuating signals not within the predetermined frequency range of the ring signal, and sensing means coupled to said filter means for sensing the presence of an output of said filter means within the predetermined frequency range of the ring signal and providing a ring detect output signal in response thereto.

1 more element

Apparatus for detecting the ring signal on first and second telephone lines comprising;

a coupling transformer having a primary winding and a secondary winding, each having at least two leads thereon,
a switch means for controllably changing between open and closed conditions to provide electrical conduction between first and second switch contact means when in said closed condition, said first switch contact means being a means for connection to the first telephone line, said second switch contact means being coupled to said first primary lead for said coupling transformer, said second primary lead of said coupling transformer including means for coupling to the second telephone line,
open line load means coupled between said first and second switch contact means for providing a predetermined linear load on the phone lines when said switch means is open,
filter means coupled to said first and second secondary leads of said coupling transformer, said filter means being a means for providing a filter means output primarily responsive to signals within the predetermined frequency range of ring signals received from said coupling transformer,
threshold means coupled to said filter means for imposing a predetermined threshold level on the output of said filter means,
zero crossing detector means coupled to said threshold means for detecting zero crossings in the signal received therefrom and for providing a square wave output in response thereto, and
output means coupled to said zero crossing detector means for detecting the square wave output and providing a ring signal detection signal in response thereto.

4 more elements

Dependencies are inferred from the claim text.

Description3,390 words

Summary880 words

#1. Field of the Invention.

#The present invention relates to the field of telephone line interfaces.

#2. Prior Art.

Brief description of the drawings1 figure

#FIG. 1 is a circuit diagram for the preferred embodiment circuit of the present invention.

Detailed description2,495 words

#The circuit of the preferred embodiment of the present invention may be seen in FIG. 1. The circuit shown is coupled to the phone line through a modular jack 20. The tip line of the modular jack 20 is coupled to one side of the primary of the load and coupling transformer 22, with the other side of the primary of the transformer being couplable through relay switch 24 to the ring line. The series connection of resistor 26 and capacitor 28 comprising an open line load means is coupled to the normally open relay switch connections to provide an open line load consistent with FCC requirements and to protect the relay contacts.

#Typically resistor 26 is on the order of 6.8 K ohms and capacitor 28 is on the order of 0.47 uf, whereas the primary impedance of the transformer is on the order of 600 ohms and normally considered too low to be driven by a ring signal through resistor 26 and capacitor 28 to provide meaningful coupling to the secondary. It should be noted that the relay and resistor 26 and capacitor 28 are the only components on the phone line side of transformer 22. The secondary of the transformer, on the other hand, may be protected by a pair of back-to-back zeners 30 which limit the secondary voltage to the value consistent with integrated circuit voltage capabilities. In this embodiment, resistor 42 provides a predetermined resistive AC load on the line as seen by the line itself, as well as the rest of the circuitry shown in FIG. 1, with coupling capacitor 34 coupling the secondary voltage (AC) to the circuit shown within the dashed enclosure 36, in the preferred embodiment part of a custom integrated circuit.

#Capacitor 34 is coupled to the positive input of amplifier 38 through resistor 40 and also to the negative input of that amplifier through resistors 42 and 44. The output of amplifier 38 is also coupled to the negative input of the amplifier through feedback resistor 46. Finally, the negative input of amplifier 38 is also coupled to ground through resistor 48 and capacitor 50, the combination having a corner frequency of approximately 50 Hz in the preferred embodiment.

Facts

Application

06/541,986Filed 14 Oct 1983

Granted

20 Aug 1985

Term ended

14 Oct 2003

Estimated · how

Pre-1995 term; greater of 17 years from grant or 20 years from filing when eligible. Maintenance-fee lapses, terminal disclaimers and statutory extensions have not been checked. The continuity chain may be incomplete; this is not an enforceability determination.

Classification2 codes

H04M 3/005
Automatic or semi-automatic exchanges: interface circuits for subscriber lines
H04M
Telephonic communication

Telephone line interface

Life of this patent

Telephone line interface

Life of this patent

Life of this patent

Abstract

Drawings

Drawings

Claims8

Description3,390 words

Drawings

Classification2 codes

Life of this patent

Description3,390 words