WO1996005686A1

WO1996005686A1 - Bilateral switch for modem applications

Info

Publication number: WO1996005686A1
Application number: PCT/US1995/010246
Authority: WO
Inventors: Michael Bugnacki; Ronald P. Clark; Michael J. Gambuzza; Azzouz Nezar
Original assignee: Cp Clare Corporation
Priority date: 1994-08-10
Filing date: 1995-08-10
Publication date: 1996-02-22

Abstract

A full wave rectifying diode bridge (D1-D4) and photosensitive, solid state switch (Q2) that provides the 'off-hook' switch for telephone signals, where the 'off-hook' switch comes after the full wave bridge (D1-D4) such that the 'off-hook' switch need pass current in only one direction. Also, such an arrangement allows the bridge (D1-D4) and the photosensitive, solid state switch (Q2) to be constructed on the same wafer or die or alternatively in the same package to reduce cost and improve reliability.

Description

PC17US95/10246

1 -

BILATERAL SWITCH FOR MODEM APPLICATIONS

FIELD OF THE INVENTION The present invention relates generally to interfaces between a telephone line and a computer or other such system where digital data is received or sent, and more particularly to the data access arrangements (DAA) that are usually an integral part of the modem that accomplishes this interfacing.

BACKGROUND OF THE INVENTION

For discussion purposes herein, the DAA circuitry is shown and described separately from the modem. Fig. 1 is a schematic/block diagram of a DAA 1 circuit. An important function of the DAA is the transferring of data between the telephone office and the user. Herein, AC signal and data are used interchangeably and are defined as currents or voltages that alternate in polarity, e.g. tones. When data is to be transferred from the user to the telephone lines the user, which is often a computer system 2 including a modem 4, accepts digital signals (RS232 digital signals) from the computing system and converts these signals to AC signals. These AC signals are input to the DAA and via the DAA to the telephone lines TIP and RING. When AC signals are input to the DAA from the telephone lines, the DAA conditions and directs these signals to the modem. The modem converts the received AC signal to a digital signal (an RS232 signal) that is input to the computing system. The DAA accomplishes these functions while providing high voltage isolation between the telephone system and the modem system. In Fig. 1, the computing system receives the RING DETECT SIGNAL 6 or the "ring" signal and answers the call by activating an "off-hook" relay 20 that completes the connection of the telephone lines, TIP 14 and RING 16, to the DAA circuitry. AC data signals may then be transferred between the modem and the telephone lines. Both the "off-hook" and RING DETECT circuitry provide isolation between the signal return of the computing system, GND2, and the signal return (or ground) of the telephone system, GND1. The "off-hook" relay provides a switch contact closure 22 (shown as a photodiode 21 driven FET 22, although a mechanical relay is often used). A protection fuse 24 and another component 26 are provided to protect the DAA and computer circuitry from surges or other destructive signals that may come in on the telephone lines. The component 26 may be combinations of Zener or breakdown diodes or MOVs or other such components as are known in the art. Still referring to Fig. 1, when the "off-hook" FET 22 is on, a DC (defined herein as a substantially constant value signal) current of 20 to over 100 mA is established from the TIP line, through diode Dl , Ql (a Darlington connection), Rl , R2 and R3, and returning through GND1 and D2 to the RING line. This current signals the telephone central office that the receiver is "off -hook". At this time, the central office provides a dial tone to the DAA, then to the modem and on to the user. The user then dials the number desired and establishes a connection for transferring data to and from the user. When data is being sent to the user, that data is embodied in an AC signal riding on the DC current. This AC signal travels through diode Dl and then through C2 to Tl to the modem. The circuitry component selection of Rl, R2, R3, Ql and Cl provides a high impedance to ensure that substantially the entire AC signal is directed through C2. This circuitry acts as an electronic inductor or gyrator. Rl and R2 provide a DC voltage at the base of the Darlington Ql . The combination Rl , R2 and Cl form a low pass filter designed such that virtually none of the AC signal frequency appears at the base of Ql - and so Ql remains as a high impedance to this AC signal. The Zener diode ZD1 provides a protective upper voltage limit of about 60 volts with respect to GND1. When data is being sent from the modem to the telephone lines the signal travels through Tl, C2, the diodes Dl and D2. A requirement of telephone installations is that the DAA-modem combination be insensitive to polarity reversals between TIP and RING. Such a reversal might occur due to an inadvertent wiring error. Diodes D3 and D4 are provided to accommodate this occurrence by steering the current such that the collector of Ql is always positive with respect to the emitter. In this case, the DC current travels from the RING through D3 and returns through D4 to the TIP. Without D3 and D4 the DAA circuitry will not work, and the reversed nominal TIP to RING 48 volts could damage the DAA circuitry, especially Ql. A limitation of the circuit shown in Fig. 1 is the requirement that the DC current be bidirectional between the TIP and RING, as described above, this results in as much as 120 mA flowing through the FET 22 in both directions. To practically implement circuitry using such a FET increases costs. To overcome this limitations, others have devised a bridge-type circuit, as shown in Fig. 2, including photodiode 29 driven FET 22. In the Fig. 2 circuit, the current through the photosensitive FET 28 is undirectional. When TIP is higher in voltage than RING, D5 and D6 conduct the signal through the FET in the direction of the source arrow 30. When the TIP is lower in voltage than RING, D7 and D8 direct the current in the same direction through the FET. This circuit has the disadvantage of adding four additional diodes and at least two diode voltage drops in the DC current path. This results in more power dissipation, the use of more printed circuit board space. A secondary, less important limitation is reduced (by the two diode drops) voltage compliance ability of the DAA circuitry. This voltage compliance reduction increases the possibility of distortion of the signals passing through these diodes if the peak AC signal nears the cross-over region where the diodes start to switch off. It is an object of this invention to provide a low cost, off-hook" solid state relay and bridge insensitive to TIP and RING wiring reversals. It is another object of this invention to provide better matched devices in a solid state switch with low power dissipation, and a circuit that is suitable for construction in a single package, thereby making more efficient use of space, costing less, requiring less handling, and having improved reliability.

SUMMARY OF THE INVENTION The foregoing objects are met with a bridge circuit and an adjacent solid state, optically coupled switch, the bridge having two input connections and two output connections, including: diodes Dl, D2, D3, and D4, arranged in a full wave rectifying bridge, with the anode of Dl connected to the cathode of D4 and to the TIP line, the anode of D3 is connected to the cathode of D4 and to the RING line, the anodes of D2 and D4 are joined together forming a first output, and the cathodes of diodes Dl and D3 are joined together forming a second output, a photosensitive, solid state switching device including a first and a second terminal, said first terminal joined to said second output, a light emitting diode constructed and arranged to transfer light energy to the photosensitive solid state switching device when the light emitting diode is activated by driving current through the light emitting diode, said the RING lines, and where, when the light emitting diode is not activated, the diode bridge, in conjunction with the photosensitive solid state switching device, block any signals traveling between the TIP and the RING line. In a preferred embodiment, a modem is included and constructed and arranged to transmit and to receive AC signals, and where the circuit means includes electronic circuitry arranged and constructed to transfer the DC current appearing at the second terminal to the first diode bridge output and includes means to transfer an AC signal between the modem and the output of the diode bridge via the photosensitive solid state switching device connects said second connection of the photosensitive switch to a modem, where the electronic circuitry receives a signal appearing at the second connection, where the signal also includes an AC part superimposed onto the DC signal, and where the electronic circuitry is arranged and constructed to transfer the alternating part to the modem and return the AC and DC signals back to the anode of bridge diode Dl. The electronic circuitry incudes means to isolate the bridge and photosensitive switch circuitry from the modem. The photosensitive , solid state switch may comprise bipolar transistor devices , e.g. , a Darlington pair, or field effect transistor devices. An advantage of the bipolar transistor is that the bridge diodes and such a transistor may be made by the same process on the same wafer. Such construction will result in fewer components and, therefore, less handling, less cost, less printed circuit board space used, and increased reliability. Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic/block diagram of a DAA circuit; Fig. 2 is a detailed schematic of an alternative "off-hook" solid state relay; and Fig. 3 is a schematic of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows, in block diagram form, an example of typical DAA circuitry as described earlier, and Fig. 2 details alternative circuitry used to implement Fig. 1. Fig. 3 shows a detailed circuit embodying the present invention that replaces the "off-hook" and diode bridge circuitry of the previous Figs. 1 and 2. Referring to Fig. 3, when the system is in the "on-hook" state, LED 40 is inactive and the photosensitive Darlington transistor Q2 is off. In other preferred embodiments, Q2 may be a FET or MOSFET or other photosensitive devices or circuit combinations of devices, e.g. use of photodiodes and transistors, etc., known in the art. In the "on-hook" state, the full central telephone office voltage appears across the bridge and the collector/emitter of the Darlington transistor Ql . The bridge and the Darlington transistors are specified to 350 volts. No current passes through the diodes bridge between points 44 and 46. Of course, the RING detector will still monitor the telephone lines and be able to respond when called. When a RING is detected, and, in response thereto, the LED 40 is activated by the user thereby turning on Q2, the "off-hook" switch is on, and the system is ready for dialing and/or sending or receiving data. Even when no information data signal is being transmitted to or from the modem via the telephone lines, the telephone office generates the DC current described above from the TIP line to the RING line. This current travels through the bridge rectifier diode Dl, through the "off-hook" switch Q2, through Ql and R3 and returns to the RING terminal through D2 completing the signal path. When data is being sent to or received from the modem, an AC signal is impressed on the DC current signal as described above. Operation of the diode bridge is identical with the above description of Fig. 1. When the TIP is higher than RING, diodes Dl and D2 are on; and when the RING signal is more positive than the TIP signal, the signal path is through D3 returning through D4. A difference compared to the "off-hook" switches of Fig. 1 is that current in Fig. 3 always travels through the "off-hook" switch Q2 in the same direction, whereas current through FET 22 is in both directions, and no steering diodes are needed as in Fig. 2. When the modem transmits data signals out to the telephone line, the data signals travel through the "off-hook" switch again, even though the information signal is traveling from the modem to the TIP/RING lines, the current through Ql is in the same direction. The modem via the transformer drives the emitter of Q2 and the bridge diode Dl and D2 remain on (assuming that the TIP is high compared to RING). The signals being sent appear between the TIP and RING telephone lines. Preferably, the diode bridge, and the photosensitive transistor Q2 are fabricated on the same wafer to form a monolithically integrated structure. It will now be apparent to those skilled in the art that other embodiments, improvements, details and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

Claims

What is claimed is:

1. An interface circuit for isolating a first telephone connection and a second telephone connection from a user terminal, said circuit comprising: diode bridge means having a first branch and a second branch coupled together at each of a first and a second control junction, wherein each branch includes first and second diodes serially connected at a common circuit point coupled to a respective telephone connection, and wherein said first and said second diodes of a branch are arranged in a conductive direction from said first control junction to said second control junction; radiation generating means for generating radiation; circuit means for providing a coupling network between a circuit junction and the first control junction of said diode bridge means and between said circuit junction and said user terminal; and detection means, coupled to said radiation means and coupled at a first terminal to said second control junction of said diode bridge means and coupled at a second terminal to said circuit junction and including a photo- sensitive transistor device, for providing with said diode bridge means a conduction path for the transmission of a signal through said detection means and said circuit means from either of said telephone connections to the other of said telephone connections in response to radiation received from said radiation means by said photo-sensitive transistor device.

2. The circuit as recited in claim 1, wherein said radiation generating means comprises: a light emitting diode.

3. The circuit as recited in claim 1, wherein said photo-sensitive transistor device comprises: a photo-sensitive bipolar junction transistor having a base, a collector coupled to said second control junction of said diode bridge means, and an emitter coupled to said circuit junction.

4. The circuit as recited in claim 1, wherein said photo-sensitive transistor device comprises: a photo-sensitive bipolar junction transistor configuration including said photo-sensitive transistor device having a base electromagnetically coupled to said radiation generating means, a collector coupled to said second control junction of said bridge circuit means, and an emitter, and further including a second transistor having a base coupled to said emitter of said photo-sensitive transistor device, a collector coupled to said second control junction of said bridge circuit means, and an emitter coupled to said circuit junction.

5. The circuit as recited in claim 1, further comprises: control circuit means, responsive to a control signal indicative of a call request, for placing said circuit into a switching state by activating said radiation means to generate radiation.

6. The circuit as recited in claim 1 , wherein said diode bridge means and said detection means are monolithically integrated on a common platform.

7. The circuit as recited in claim 1, where in said circuit means further comprises: coupling means for providing a bidirectional path for AC signal transmission between said circuit junction and said user terminal.

8. The circuit as recited in claim 7, wherein said coupling means includes a transformer.

9. The circuit as recited in claim 1, further comprises: a modem circuit coupled to said user terminal.

10. An interface circuit for isolating a first telephone connection and a second telephone connection from a user terminal, said circuit comprising: bridge circuit means having four junctions including first and second control junctions, and first and second terminal junctions, each of said terminal junctions being coupled to a respective telephone connection, wherein said bridge circuit means includes a circuit path of uni-directional conductivity from said first control junction to each of said first and second terminal junctions, and a circuit path of uni-directional conductivity from each of said first and said second terminal junctions to said second control junction; radiation generating means for generating radiation; circuit means for providing a coupling network between a circuit junction and the second control junction of said diode bridge means and between said circuit junction and said user terminal; and detection means, including a bipolar junction transistor coupled at a first terminal to said first control junction of said bridge circuit means and coupled at a second terminal to said circuit junction, for establishing a conduction path through said bipolar junction transistor in response to radiation received from said radiation means by said detection means, so that signals can be bidirectionally transmitted between said telephone connections.

11. The switch as recited in claim 10, wherein said bridge circuit means comprises: a diode bridge circuit including first, second, third, and fourth diodes, each having an anode and cathode and arranged in a bridge configuration; wherein the anode of said first diode is coupled to the anode of said third diode at said first control junction, the cathode of said first diode is coupled to the anode of said second diode at said first terminal junction, the cathode of said third diode is coupled to the anode of said fourth diode at said second terminal junction, and the cathode of said second diode is coupled to the cathode of said fourth diode at said second control junction.

12. The switch as recited in claim 10, wherein said radiation generating means comprises: a light emitting diode.

13. The switch as recited in claim 10, wherein said detection means includes a photo-sensitive transistor device.

14. The switch as recited in claim 13, wherein said photo-sensitive transistor device comprises: a photo-sensitive bipolar junction transistor having a base, a collector coupled to said second control junction of said bridge circuit means, and an emitter coupled to said circuit means via said circuit junction.

15. The switch as recited in claim 13, wherein said photo-sensitive transistor device comprises: a photo-sensitive transistor configuration including a first transistor having a base optically coupled to said radiation means, a collector coupled to said second control junction of said bridge circuit means, and an emitter, and further including said bipolar junction transistor having a base coupled to said emitter of said first transistor, a collector coupled to said second control junction of said bridge circuit means, and an emitter coupled to said circuit junction.

16. The switch as recited in claim 10, further comprises: control circuit means, responsive to a control signal indicative of a call request, for placing said switch into a switching state by activating said radiation means to generate radiation.

17. The circuit as recited in claim 10, wherein said diode bridge means and said detection means are monolithically integrated on a common platform.

18. The circuit as recited in claim 10, wherein said circuit means further comprises: coupling means for providing a bidirectional path for AC signal transmission between said circuit junction and said user terminal.

19. The circuit as recited in claim 18, wherein said coupling means includes a transformer.

20. The circuit as recited in claim 10, further comprises: a modem circuit coupled to said user terminal.