GB2264422A

GB2264422A - Tranformerless modem

Info

Publication number: GB2264422A
Application number: GB9300256A
Authority: GB
Inventors: Steven Jay Young
Original assignee: Apple Computer Inc
Current assignee: Apple Inc
Priority date: 1992-02-18
Filing date: 1993-01-08
Publication date: 1993-08-25
Also published as: GB9300256D0; JPH0670058A

Abstract

The invention improves electrical isolation in a modulator/demodulator 21, 22 coupled to a telephone communications cable 30 and a digital computer 20. The circuit comprises a matched pair of light emitting diodes 12, 16 and phototransistors 14, 18. This matched pair may be formed as part of an integrated circuit which incorporates most, if not all, of the components of a modulator/demodulator. The present invention thus reduces the size of such devices, as well as reducing their power consumption, improving upon the known signal-to-noise performance and reducing the overall cost. <IMAGE>

Description

TRANSFORMERLESS MODEM Background of the Invention This invention is in the field of digital communications. In particular, the present invention relates to modulators/demodulators ("modems") used to couple digital computers to telecommunications networks.

The use of modems to enable communications between computer systems using ordinary telephone lines is known. The modem accepts the computer's digital information, converts it to a form which may be transmitted over a telephone line, and transmits the converted signal. At the destination computer the signal is reconverted by a second modem and forwarded to the destination computer.

Known modems contain a standard Data Access Arrangement ("DAA") interface between the telephone line and the other circuitry which comprises the modem. This DAA interface is used to isolate the computer and its operator from any high voltage which may be on the the telephone line including, for example, a voltage surge caused by a lightning strike. At the same time, the interface prevents possible current drain from the computer system to the telephone network. In known interfaces, a transformer is used to accomplish this electrical isolation.

Although transformers perform the task of electrical isolation well, they have several drawbacks. All modems must, by law, be able to isolate either the computer or telephone system from an over-voltage of at least 2700 volts. Given the physical principles that dictate a transformer's performance, a transformer capable of providing this amount of protection will be of a certain irreducible size and weight. This minimum size, which is about 1 cubic inch, presents a major design problem in portable computers, where space is at a premium and where weight is also a major consideration.

An apparatus that can replace the transformer within the modem's DAA interface and which would be smaller and lighter than the transformer would be a significant improvement upon known modems.

Summary of the Invention In a first preferred embodiment, the present invention comprises an opto-electronic coupler which replaces the transformer used in known modems.

Incoming and outgoing electronic signals are converted into light pulses by a light emitting diode ("LED") and an associated phototransistor reconverts the optical pulses into an electronic signal. The opto-electronic coupler is significantly smaller than a transformer weighs less, and provides a higher level of voltage protection.

In other embodiments, the opto-electronic coupler is part of a single integrated circuit ("IC") which also incorporates much of the remaining modem circuitry, including the 2-to-4 wire hybrid circuit, the impedance matching resistors, the ring detection circuitry and, possibly, an electronic on-hook relay.

These embodiments are discussed below with reference to the figure described below.

Brief Description of the Figure Fig. 1 is a schematic diagram of a first embodiment of the present invention.

Detailed Description of the Preferred Embodiment A first embodiment of the present invention is shown in Fig. 1.

System 10 comprises first LED 12, first phototransistor 14, second LED 16, second phototransistor 18 and first and second modulator/demodulator circuits 21 and 22, respectively. Coupled to LED 12 and first phototransistor 14 through modulator/demodulator 22 is computer system 20 which incorporates a portion of the modem circuitry not shown in Fig. 1, including a 2-to-4 wire hybrid circuit. LED 16 and phototransistor 18 are in turn coupled through modulator/demodulator 21 and a second 2-to-4 wire hybrid circuit to telephone system 30.

As system 10 functions in a symmetrical fashion with regards to incoming and outgoing transmissions, only the case of an outgoing signal will be discussed herein. LED 12 flashes each time a pulse is received from computer 20.

The duration and frequency of these outgoing pulses causes phototransistor 18 to conduct at exactly the same frequency and for the same duration as the incoming pulses. These electronic pulses are then further processed before they are transmitted over telephone system 30.

Although not a part of the present invention, it should be understood that any one of several known encoding techniques may be used to generate the appropriate light pulses. Voltage-to-frequency modulation and pulse width modulation are just two of the many possible encoding techniques.

Numerous advantages additional to reducing component size and weight are realized by replacing the transformer with an opto-electronic coupler. As the use of fiber optic telephone systems becomes more and more common, the coupler described herein could accomplish both the needed electrical isolation and the transmission of light signals into the fiber optic cable in one step and one component. As the opto-electronic coupler requires only a small amount of the area available on a standard IC, it is possible to fabricate a majority of the modem's circuitry on the same IC as the coupler further reducing the space and weight of the modem.

Power requirements are also reduced, an important factor in portable computers. In general, this type of coupler also has a better signal-to-noise response than a transformer isolation circuit. Finally, and very important in the present computer marketplace, the coupler described herein can be fabricated and installed much more cheaply than can a transformer.

Although the invention has been explained with reference to the fcregoing embodiment, it should be apparent to one skilled in the art that numerous changes and modifications may be made thereto without departing from the scope or spirit of the invention. These changes, modifications and applications of the present invention will become apparent to one of skill in the art in view of this disclosure. Thus the invention should be limited only in accordance with the appended claims.

Claims

1. In a modulator/demodulator for enabling digital communication between digital computers over telephone networks, an electrical isolation circuit comprising: first light emitting diode means coupled to a first digital computer for converting first digital electrical pulse information to first light information pulses; first phototransistor means optically coupled to the first light emitting diode and electrically coupled to a telephone transmission line for converting the first light information pulses into second electrical pulse information for transmission on the telephone transmission line; second light emitting diode means coupled to the telephone transmission line for converting third electrical pulse information received from the telephone transmission line into second light information pulses; and second phototransistor means optically coupled to the second light emitting diode means and electrically coupled to the first digital computer for converting the second light information pulses into fourth electrical information pulses for transmission to the first digital computer.

2. A method for providing electrical isolation in a modulator/demodulator, the modulator/demodulator being coupled to a telephone communications cable and to a digital computer the method comprising the steps of: coupling an output of the digital computer to a first light emitting diode; coupling the telephone communications cable to a second light emitting diode and to a first phototransistor; coupling an input to the digital computer to a second phototransistor the first light emitting diode and the first phototransistor being optically coupled and the second light emitting diode and the second phototransistor being optically coupled; transmitting digital signals from the digital computer through the optically coupled first light emitting diode and the first phototransistor to the telephone communications cable; and transmitting digital signals from the telephone communications cable through the optically coupled second light emitting diode and second phototransistor to the digital computer.

3. An apparatus for providing electrical isolation in a modulatorldemodulator, the modulator/demodulator being coupled to a telephone communications cable and to a digital computer the apparatus comprising: means for coupling an output of the digital computer to a first light emitting diode; means for coupling the telephone communications cable to a second light emitting diode and to a first phototransistor; means for coupling an input to the digital computer to a second phototransistor the first light emitting diode and the first phototransistor being optically coupled and the second light emitting diode and the second phototransistor being optically coupled; means for transmitting digital signals from the digital computer through the optically coupled first light emitting diode and the first phototransistor to the telephone communications cable; and means for transmittirig digital signals from the telephone communications cable through the optically coupled second light emitting diode and the second phototransistor to the digital computer.

4. In a modulator/demodulator for enabling digital communication between digital computers over telephone networks, an electrical isolation circuit substantially as hereinbefore described with reference to the accompanying drawing.

5. A method for providing electrical isolation in a modulator/demodulator, the modulator/demodulator being coupled to a telephone communications cable and to a digital computer, substantially as hereinbefore described with reference to the accompanying drawing.

6. An apparatus for providing electrical isolation in a modulator/demodulator, the modulator/demodulator being coupled to a telephone communications cable and to a digital computer, substantially as hereinbefore described with reference to the accompanying drawing.