Optically-coupled differential line receiver

An optically-coupled line receiver that handles multiple-state signals on data communications lines by using a differential circuit at the input.

FIELD OF THE INVENTION 
This invention generally relates to data communications interfaces. 
Specifically, the invention is directed to optically-coupled data 
communications interfaces. 
BACKGROUND OF THE INVENTION 
Data communication lines or channels can connect two data apparatus that 
are adjacent to each other or that are in separate geographic locations. 
As the trend of communicating between geographically-dispersed equipment 
continues, the risk increases of data communication lines accidentally 
contacting electrical power lines, and damaging equipment and injuring 
people. An additional increasing risk is the development of ground loops, 
and thus noisy data transmission, between such equipment. Electrical 
isolation of the data communication lines or channels can reduce the 
aforementioned risks. 
Optocouplers are one class of devices used to transmit signal information 
between respective circuitry while maintaining electrical isolation 
therebetween. This is accomplished since optical signals, and not 
electrical signals, are passed between the light emitters and the 
photodetectors of an optocoupler, which are connected to respective 
portions of the circuitry. The size, cost and performance of optocouplers 
makes these devices preferable to implement electrical isolation for data 
communication lines and interfaces. 
However, optocouplers are generally available only for handling 
single-ended signals, i.e., signals that are referenced to ground. In 
contrast, data communication lines and interfaces most often handle data 
that are in differential-driven and differential-received formats. For 
optically-coupled devices to handle a differential signal, additional 
external circuitry is required. It is desirable therefore to provide an 
optocoupler that will directly accept differential signals. It is also 
desirable to provide an optocoupler that provides an isolated data 
communications interface. 
SUMMARY OF THE INVENTION 
The aforementioned problems are obviated by the present invention which 
provides an optically-coupled differential line receiver. The receiver may 
comprise an input differential amplifier circuit capable of driving a 
light emitter; a light emitter, such as a light emitting diode ("LED"); 
and an output amplifier circuit. The input differential amplifier circuit 
may include a differential amplifier that controls a switched current 
source. The switched current source, in turn, drives the emitter as a 
function of the values of the differential signals received by the 
differential amplifier. 
The output amplifier circuit may include a photodetector that drives an 
output amplifier in response to the emitter operation. Further, the output 
amplifier circuit may have either a single-ended or differential design. 
By incorporating a differential signal detector and an LED drive within an 
electrical isolation device, differentially-coded data signals can be 
received, decoded and transmitted in one integrated operation.

DETAILED DESCRIPTION 
The devices described herein are based in part on the technology used in 
the Siemens.RTM. IL300 family of aluminum gallium arsenide (AlGaAs) linear 
optocouplers, discussed in the Siemens Optoelectronics Data Book 1993, pp. 
5-115 through 5-122, and pp. 11-177 through 11-193. It should be 
understood that other devices can be used as well. Also, the optocouplers 
could be used with other types of electromagnetic radiation, such as 
ultraviolet light. 
An optically-coupled differential line receiver 10 constructed in 
accordance with the present invention is shown in the block schematic 
diagram of FIG. 1. The line receiver 10 is contained in a dual in-line 
package (DIP) 12, having eight pins, but any suitable packaging 
arrangement or device can be used. The receiver DIP 12 has two sides, an 
input side 14 and an output side 16. 
The input side 14 of the receiver DIP 12 has a first voltage supply pin 
V.sub.CC1 20, a first ground pin GND.sub.1 22, and first and second 
differential data inputs 24, 26. The output side 16 of the receiver DIP 12 
has a second voltage supply pin V.sub.CC2 30, a second ground pin 
GND.sub.2 32, a data output 34, and an enable output 36. 
The line receiver 10 contained within the DIP 12 comprises an input 
differential amplifier circuit 40 and an optical differential output 
circuit 42 separated by an optical cavity 44, depicted symbolically by a 
dashed line. As described in detail below, optical radiation 46 generated 
by the input differential amplifier circuit 40 passes through the optical 
cavity 44 to the optical differential output circuit 42. The input 
differential amplifier circuit 40 is connected to the first voltage supply 
pin V.sub.CC1 20, the first ground pin GND.sub.1 22, and the first and 
second differential data inputs 24, 26. The differential output circuit 42 
is similarly connected to the second voltage supply pin V.sub.CC2 30, the 
second ground pin GND.sub.2 32, the data output 34, and the enable output 
36. Note that the optical cavity 44 may be any suitable light conducting 
medium. 
An electrical schematic diagram for the input differential amplifier 
circuit 40 for the line receiver 10 is shown in FIG. 2. The amplifier 
circuit 40 comprises a differential amplifier 100 having a non-inverting 
input 102, an inverting input 104, and an output 106. The amplifier 
circuit 40 also comprises a switched current source 110 having a voltage 
source input 112, an output 114, and a control input 116. The amplifier 
circuit 40 further comprises a light emitter 120, which may be a low or 
high speed light emitting diode ("LED"), that emits optical radiation when 
driven to the "on" state (i.e., is conducting). 
The non-inverting input 102 of the differential amplifier 100 is connected 
to the first differential data input 24 of the receiver DIP 12 and the 
inverting input 104 is connected to the second differential data input 26. 
The output 106 of the differential amplifier 100 is connected to the 
control input 116 of the switched current source 110. The voltage source 
input 112 of the current source 110 is connected to the voltage supply 
V.sub.CC (via the first voltage supply pin V.sub.CC1 20 of the receiver 
DIP 12) and the output 114 of the current source 110 is connected to the 
emitter 120. The emitter 120, in turn, is connected to ground (via the 
first ground pin GND.sub.1 22 of the receiver DIP 12). Depending on the 
relative values of the signals received by the differential data inputs 
24, 26, the output 106 of the amplifier 100 causes the current source 110 
to vary its current output, thus varying the amount of optical radiation 
generated by the emitter 120. 
The optical radiation generated by the emitter 120 is directed across the 
optical cavity 44 to the differential output circuit 42 illustrated in 
FIG. 3. The output circuit 42 comprises a photodetector 160 and a 
single-ended amplifier 170, having an input 172, an output 174, and an 
enable input 176. The photodetector 160 is connected between the voltage 
supply V.sub.CC (via the second voltage supply pin V.sub.CC2 30) and the 
input 172 of the single-ended amplifier 170. The output 174 of the 
single-ended amplifier 170 is connected to the data output 34 of the 
receiver DIP 12 and the enable input 176 is connected to the enable output 
36. Note that the output circuit 42 may take on a differential design 
rather than the single-ended design illustrated in the figure. 
In operation, the line receiver 10 is connected to a data communications 
line and functions as an interface between respective driving and 
receiving data circuitry. The receiver DIP 12 may be soldered or otherwise 
connected to a printed circuit board that contains the data circuitry or 
circuitry that connects to the data circuitry. The input differential 
amplifier circuit 40, and in particular the non-inverting and inverting 
inputs 102, 104 of the differential amplifier 100, receives two input 
signals A and A from the driving data circuitry via the first and second 
differential data inputs 24, 26 of the receiver DIP 12. The resulting 
output signal from the differential amplifier 100, which is dependent upon 
the values of the two input signals A and A, swings between the supply 
voltage V.sub.CC and ground. The output 106 of the differential amplifier 
100 controls the switched current source 110 to drive the emitter 120 
either to the on state (i.e., conducting) or to the off state (i.e., not 
conducting). Thus, the state of the emitter 120 depends on the values of 
the two input signals A and A. 
When the current source 110 drives the emitter 120 on, the emitter 120 
generates optical radiation. The optical radiation is directed across the 
optical cavity 44 and received by the photodetector 160 of the 
differential output circuit 42. The optical radiation impinges on the 
photodetector 160 which, in response thereto, drives the single-ended 
amplifier 170. The resulting output from the single-ended amplifier 170 is 
transmitted to the receiving data circuitry (via the data output 34 of the 
receiver DIP 12) upon receiving the appropriate enable signal from the 
receiving data circuitry. Note that the enable signal is transmitted to 
the enable input 176 of the single-ended amplifier 170 via the enable 
output 36 of the receiver DIP 12. 
When the current source 110 drives the emitter 120 off, the emitter 120 
does not generate any optical radiation. As a result, the photodetector 
160 does not drive the single-ended amplifier 170 and the receiving data 
circuitry receives no signal. 
While there has been described what is believed to be the preferred 
embodiment of the invention, those skilled in the art will recognize that 
other and further modifications may be made thereto without departing from 
the spirit of the invention, and it is intended to claim all such 
embodiments that fall within the true scope of the invention. For example, 
although the embodiment shown here has only two inputs, for a possible 
total of four states, the invention is not limited to two inputs or four 
states and can handle any number of inputs and states.