Radiant energy – Photocells; circuits and apparatus – Signal isolator
Reexamination Certificate
1999-03-17
2002-03-12
Evans, F. L. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Signal isolator
C324S514000
Reexamination Certificate
active
06355936
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic circuits, and more specifically to a circuit for electrical insulation between a two-way bus and a peripheral circuit.
2. Description of Related Art
There are conventional circuits based on optocouplers that provide electrical insulation between two systems having different electrical natures. An optocoupler is generally in the form of a package having a light-emitting diode and a phototransistor. When a current flows through the diode, it emits photons that make the phototransistor conductive. Thus, the optocoupler enables the transmission of electrical signals while providing excellent electrical insulation between its input and output. For example, an optocoupler makes it possible to couple low voltage circuits with circuits connected to the main without any risk of recovering the main's voltage in the low voltage circuits.
This type of electrical insulation circuit is especially suited for setting up a connection between peripheral circuits and the communications bus of a computer system. In particular, there are conventional insulation circuits for linking an RS-232 bus with peripheral circuits. The bus includes a first data wire for transmitting data in the peripheral circuit-to-bus direction and a second data wire for transmitting data in the opposite direction. The peripheral circuit can be an external supply of the inverted rectifier type that can sometimes produce dangerously high voltages. Thus, an optocoupler is typically positioned on each of the data wires between the bus and the inverted rectifier in order to insulate the inverted rectifier with respect to the bus.
The use of an optocoupler to provide such insulation does not raise any particular difficulties with a bus that has one wire for transmission in one direction and another wire for transmission in the opposite direction. However, a problem involving parasitic feedback of the signals arises when one wire is used to transmit data in both directions. For example, such a situation arises with a USB (universal serial bus), which has a supply wire, a ground wire, and two data wires that convey differential electrical signals in both directions.
SUMMARY OF THE INVENTION
In view of these drawbacks, it is an object of the present invention to overcome the above-mentioned drawbacks and to provide an electrical insulation circuit for connection between a two-way bus and a peripheral circuit that averts parasitic feedback. The transmission of differential signals in both directions is provided by four optocouplers that are connected to one another so as to selectively turn off the operation of two of the optocouplers in the direction of transmission of the signals. In preferred embodiments, additional circuitry prevents the optocouplers that are off from disturbing the transmission of the signals from the optocouplers that are on towards the destination circuit (bus or peripheral circuit).
One embodiment of the present invention provides an electrical insulation circuit of the type connected between a peripheral circuit and a two-way bus. The electrical insulation circuit includes first and second optocouplers for transmitting differential signals in the peripheral circuit-to-bus direction, and third and fourth optocouplers for transmitting differential signals in the bus-to-peripheral circuit direction. The first and third optocouplers are associated with a first signal terminal on the peripheral circuit side and with a first data wire on the bus side, and the second and fourth optocouplers are associated with a second signal terminal on the peripheral circuit side and with a second data wire on the bus side. Further, the first and third optocouplers and the second and fourth optocouplers are connected such that the first and second optocouplers are off during transmission in the bus-to-peripheral circuit direction and the third and fourth optocouplers are off during transmission in the peripheral circuit-to-bus direction. In a preferred embodiment, the first and second optocouplers each include a light-emitting diode on the peripheral circuit side and a phototransistor on the bus side, and the third and fourth optocouplers each include a light-emitting diode on the bus side and a phototransistor on the peripheral circuit side. The first and third optocouplers and the second and fourth optocouplers are connected so that the light-emitting diodes of the first and second optocouplers are off during transmission in the bus-to-peripheral circuit direction and the light-emitting diodes of the third and fourth optocouplers are off during transmission in the peripheral circuit-to-bus direction.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.
REFERENCES:
patent: 4178504 (1979-12-01), Farmer
patent: 5406091 (1995-04-01), Burba et al.
patent: 5514875 (1996-05-01), Krause
patent: 0 066 180 (1982-12-01), None
European Search Report dated Jan. 19, 1999 with annex on French Application No. 9803500.
Bongini Stephen
Evans F. L.
Fleit Kain Gibbons Gutman & Bongini P.L.
Jorgenson Lisa K.
STMicroelectronics S.A.
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