Abstract:
A method and apparatus for providing efficient and accurate electronic data transmission of information on a data bus in the presence of noise. Data signals are received on a plurality of input lines by a spacial derivative encoder. The spacial derivative encoder encodes the signals and transmits them to a receiver having a spacial derivative decoder. The spacial derivative decoder then decodes the signals. Minimal overhead is required as for n input lines only n+1 lines are needed to transmit each of the encoded signals.

Description:
RELATED APPLICATIONS 
     This application is a continuation application of Ser. No. 08/971,185, filed on Nov. 17, 1997 now U.S. Pat. No. 6,480,548, entitled SPACIAL DERIVATIVE BUS ENCODER AND DECODER. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to methods and apparatus for electronic data transmission, and in particular to a system and method for efficiently and accurately transmitting information on a data bus in the presence of noise. 
     2. Background Information 
     A data bus commonly consists of lines for transferring data words between devices within a computer. A data word is made up of data bits transmitted as varying voltages on the data bus lines. Noise, interference, or other events such as ground voltage shifts between transmitter and receiver alter the signal such that what is sent may not be what is received. The possibility of signal alteration, whether or not it actually occurs, creates a need for ways to improve signal integrity. Both hardware and software solutions have been considered in the development of accurate data transmission methods. 
     One method of improving signal integrity is to run a separate ground line between the transmitter and receiver. The ground line voltage is used as a reference voltage. The difference between the data signal received at the receiver and the ground line voltage at the receiver is used to determine the signal polarity. One problem with this method is that it is not precise—the margin of error inherent in measuring voltage levels at the receiver may either mask or exaggerate voltage level variations due to noise. In addition, since there is no signal being driven down the ground line, the ground line voltage may be affected differently by the interfering source, again potentially masking or exaggerating transmission errors. Many applications have a need, however, for highly accurate message transfer. 
     Another message verification method is to run a differential line for each data line. According to this method there are two lines for every channel between the transmitter and the receiver. The first line carries the signal and the second line carries the complement of that signal (for example, where the first line value is ‘1’, the second line value is ‘0’). The message being transferred is represented by the difference between the two signals. Any ground voltage shifts occurring during transmission equally affect both lines, so there is no net effect on the message because the difference between the two lines remains constant. One of the primary benefits of this method is that it requires no local reference value. The weakness is that it doubles the number of required transmission lines. With the continual push to manufacture devices with more elements and smaller overall size, there is a need to achieve a higher level of signal accuracy with minimal hardware overhead requirements. 
     SUMMARY OF THE INVENTION 
     A method is provided for electronically transmitting information on a data bus efficiently and effectively. The method of the present invention can be used in combination with a variety of operational modes, including but not limited to full duplex communications. By reducing noise generation at the transmitter and rejecting common mode noise at the receiver, the system of the present invention increases the data bandwidth on data buses. 
     The embodiments of the present invention are improvements over conventional systems and methods in part because fewer transmission lines are required and as a result noise generated at the transmitter circuit is reduced. Conventional differential transmission systems use 2*n lines for n bits, where the system of the invention requires only n+1 lines. In addition, power consumed by the transmitter circuit is decreased because fewer signals have to be transmitted. The reduced number of required lines allows devices employing the system and methods of the invention to maintain high quality data transmission while reducing the costs associated with manufacturing and maintaining more transmission lines. Another advantage of the present invention is that common mode noise is removed at the receiver. In one embodiment the system of the invention also compensates for crosstalk between the signals. 
     According to one embodiment of the invention, a method of encoding n signals is provided, using a reference voltage signal, wherein the differences between adjacent signals are determined and transmitted as n+1 encoded signals. A method of decoding m encoded signals is also provided, wherein the encoded signals are processed through an array of resistors, giving the original n signals. 
     Another embodiment describes a spacial derivative encoder, comprising one or more differential amplifiers, including a first and a second differential amplifier, one or more input data lines connected to the one or more differential amplifiers, a fixed reference voltage line, connected to the first and the second differential amplifier, and one or more output transmission lines connected to the one or more differential amplifiers, the number of output transmission lines equal to n+1, where n is the number of input data lines. A spacial derivative decoder is also provided, comprising one or more input transmission lines, one or more differential amplifiers, the number of differential amplifiers equal to m−1, where m is the number of encoded input transmission lines, a resistor network electrically located between the one or more input transmission lines and the one or more differential amplifiers, and one or more output data lines connected to the one ore more differential amplifiers, the number of output data lines equal to the number of differential amplifiers. In an additional embodiment the resistor network further comprises an array of resistors for averaging signals received on the one or more input transmission lines. Yet another embodiment provides a communications system, comprising a spacial derivative encoder and a spacial derivative decoder. 
     In another embodiment, signals are processed through a time-domain encoding scheme prior to differential encoding. This allows the system to account for the actual variation of a signal voltage, thereby compensating for signal dispersion and reducing base line wander due to frequency-dependent attenuation on lines. Time-domain encoding also reduces inter-symbol interference resulting from reflections or dispersion on the signal lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a transmitter transferring encoded data to a receiver over a signal line. 
         FIG. 2  is a schematic drawing of a spacial derivative encoder/driver  100  comprising two input data lines according to one embodiment of the present invention. 
         FIG. 3  is a schematic drawing of a spacial derivative encoder/driver  100  comprising four input data lines according to another embodiment of the present invention. 
         FIG. 4  is a schematic drawing of a spacial derivative decoder  200  comprising three input data lines according to one embodiment of the present invention. 
         FIG. 5  is a schematic drawing of a spacial derivative decoder  200  comprising five input data lines according to another embodiment of the present invention. 
         FIG. 6  is a representation of oscilloscope displays of an input data signal and a reference voltage signal in their original form, after they have been encoded, and after they have been decoded. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following Detailed Description of the Preferred Embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
       FIG. 1  is a block diagram of the basic process involved in transmitting data across a network. Input data is transferred to transmitter  100  from where it is then transmitted over one or more signal lines  130 . 0 – 130 . 4  to receiver  200 . Receiver  200  then transfers the data out. An accurate data transmission system outputs data from receiver  200  which matches that input into transmitter  100 . In one embodiment of the present invention data verification is accomplished through encoding the input data at transmitter  100  and then transmitting the encoded data to receiver  200  where it is decoded and verified before being transferred out. 
       FIG. 2  is a schematic drawing of a transmitter  100  comprising a spacial derivative encoder/driver according to one embodiment of the present invention. According to the embodiment shown, inputs to the encoder  100  include two fixed reference voltage lines  110 ,  112 , and two input data lines  120 . 0 – 120 . 1 . Data inputs are processed through differential amplifiers  150 . 0 – 150 . 2 . A differential amplifier is a dual-input amplifier that amplifies the difference between its two signal inputs. A differential amplifier eliminates or greatly minimizes many common sources of error. For example, drift errors tend to cancel out in a differential architecture. In addition, a differential amplifier is able to reject common-mode signals (that is, unwanted signals present at both of the amplifier inputs or other common points). 
     Amplifiers  150 . 0 – 150 . 2  have a differential output on transmission lines  130 . 0 – 130 . 2 . To accomplish spacial encoding, the system drives each output transmission line  130  with the difference between the signals on adjacent input data lines  120 . In the case of the first and last input data lines  120 . 0 ,  120 . 1 , the respective amplifiers compare the input data with input from fixed reference voltage lines  110 ,  112  and adjacent input data lines  120 . 1 ,  120 . 0 , respectively. Those skilled in the art will recognize that neither the spirit nor scope of the invention is exceeded by configurations incorporating a different number of input data lines. The number of input data lines that can be encoded in this way can be any number greater than zero.  FIG. 3  is another example, and illustrates the spacial derivative encoder of the present invention encoding four input data lines  120 . 0 – 120 . 3 . The first differential amplifier  150 . 0  compares reference voltage line  110  and input data line  120 . 0 , giving output on output transmission line  130 . 0 . The second differential amplifier  150 . 1  compares input data line  120 . 0  and input data line  120 . 1 , giving output on output transmission line  130 . 1 . The third differential amplifier  150 . 2  compares input data line  120 . 1  and input data line  120 . 2 , giving output on output transmission line  130 . 2 . The fourth differential amplifier  150 . 3  compares input data line  120 . 2  and input data line  120 . 3 , giving output on output transmission line  130 . 3 . And the fifth differential amplifier  150 . 4  compares reference voltage line  112  and input data line  120 . 3 , giving output on output transmission line  130 . 4 . 
     If the number of input data lines equals one, then this scheme becomes equivalent to a differential transmission line. Any number of data input lines greater than one increases the number of transmission lines by the same number. The number of required transmission lines can be determined by the simple formula n+1, where n is the number of input lines. As an example, four input data lines require five transmission lines, and eight input data lines require nine transmission lines. This provides significant savings in the number of transmission lines over conventional differential transmission, which requires two transmission lines for every one input data line (the conventional number of required transmission lines=n*2, where n is the number of input lines). The reduced number of transmission lines provides other benefits as well. Fewer lines means less power is consumed by the transmitter circuit. Also, since the transmitter is issuing fewer signals, the noise level at the transmitter is reduced. 
       FIG. 4  is a schematic drawing of a receiver comprising a spacial derivative decoder  200  according to one embodiment of the present invention. To decode the encoded signals received on data input lines  130 . 0 – 130 . 2  the following calculation is used for i=0 to (n−1), where n is the number of data input lines, Output(i) is the data output line reference, and IN(i) is the transmission line reference:
 Output( i )=average(IN(0) through IN( i ))−average(IN( i +1) through IN( n )) 
One skilled in the art will recognize that any number of input lines greater than one can be used without exceeding the scope or spirit of the invention. Differential output transmitted by encoder  100  is received by decoder  200  on transmission lines  130 . 0 – 130 . 2 . Resistor network  400  comprises a pattern of resistors RA 0 -RC 1  and averages the signals received on transmission lines  130 . 0 – 130 . 2 . In the embodiment shown resistor network  400  comprises two resistors for every input data line. It is to be recognized that  FIGS. 4 and 5  are exemplary only and that resistor network  400  is flexible enough to accommodate any number of input data lines.
 
     As an example of the operation of the spacial derivative decoder shown in  FIG. 4 , line  210 . 0  carries the voltage received from transmission line  130 . 0  to one input of differential amplifier  310 . 0 . Line  210 . 1  carries the average voltage from the remaining transmission lines  130 . 1 ,  130 . 2  to a second input of the differential amplifier  310 . 0 . Differential amplifier  310 . 0  performs the subtraction, transmitting the decoded signal on output data line  140 . 0 . Differential amplifier  310 . 1  generates the signal for output data line  140 . 1  by determining the difference between the average voltage of lines  130 . 0 ,  130 . 1  (received by differential amplifier  310 . 1  on line  210 . 2 ) and the voltage received on input data line  130 . 2  (received by differential amplifier  310 . 1  on line  210 . 3 ). 
       FIG. 5  illustrates a spacial derivative decoder comprising five input data lines. Input data lines  130 . 0 – 130 . 4  and differential amplifiers  310 . 0 – 310 . 3  are connected to resistor network  400 . Differential amplifier  310 . 0  determines the difference between the voltage on data input line  130 . 0  (via line  210 . 0 ) and the average voltage of the remaining input data lines  130 . 1 – 130 . 4  (via line  210 . 1 ). Differential amplifier  310 . 1  determines the difference between the average voltage of data input lines  130 . 0  and  130 . 1  (via line  210 . 2 ) and the average voltage of the remaining input data lines  130 . 2 – 130 . 4  (via line  210 . 3 ). Differential amplifier  310 . 2  determines the difference between the average voltage of data input lines  130 . 0 – 130 . 2  (via line  210 . 4 ) and the average voltage of the remaining input data lines  130 . 3 – 130 . 4  (via line  210 . 5 ). Differential amplifier  310 . 3  determines the difference between the voltage on data input line  130 . 4  (via line  210 . 7 ) and the average voltage of the remaining input data lines  130 . 0 – 130 . 3  (via line  210 . 6 ). The decoded signals are transmitted by differential amplifiers  310 . 0 – 310 . 3  on output transmission lines  140 . 0 – 140 . 3 , respectively. 
       FIG. 6  is a representation of oscilloscope displays of the transitions of an input data signal and a reference voltage signal through the various stages according to one embodiment of the present invention. The first display  610  shows a representation of an input data signal  612  and a reference voltage signal  611 , such as might be received by encoder  100  on lines  120 . 0 ,  110 , respectively. The data being transmitted is represented by the difference between the two signals  612 ,  611 . The second display  620  shows a representation of two encoded transmissions  621 ,  622  such as are transmitted by encoder  100  on transmission lines  130 . 0 – 130 . 4 . Note that the differential amplifiers  150 . 0 – 150 . 3  introduce a gain of one to enable more accurate interpretation of the signals at the receiving end. As can be seen from the display, the reference voltage signal effectively complements the input data signal. The third display  630  shows the decoded signals  631 ,  632  after they have been received and decoded by decoder  200 . The data message is reflected by the difference between the signals  631 ,  632 . 
     In another embodiment, signals are processed through a time-domain encoding scheme prior to differential encoding. Time-domain encoding enables the system to account for the actual variation of a signal voltage over time. Thus the system is able to compensate for signal dispersion and reduce the effect of base line wander due to frequency-dependent attenuation on lines. Time-domain encoding also reduces inter-symbol interference caused by propagation delays between the first- and last-arriving significant signal components resulting from reflections or dispersion on the signal lines. 
     The system of the present invention increases the data bandwidth on data buses by reducing noise generation at the transmitter and rejecting common mode noise at the receiver. The affect of purely random noise must still be considered, but a properly configured digital system minimizes this type of noise. 
     Although the present invention has been described with reference to the preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.