Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    A related copending United States patent application commonly owned by the assignee of the present document and incorporated by reference in its entirety into this document is being filed in the United States Patent and Trademark Office on or about the same day as the present application. This related application is Hewlett-Packard docket number 100111131-1, Ser. No. ______, and is titled “SIX-DROP BUS WITH MATCHED RESPONSE.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to data communication and more particularly to a transmission line structure for bi-directional communication between four sources/receivers.  
         BACKGROUND OF THE INVENTION  
         [0003]    In many communication systems, such as digital data sent between integrated circuits, a driver send electrical waveforms to a receiver. To accomplish this, the signal may have to propagate through a series of transmission lines. To minimize reflections, these transmission lines are often constructed such that their characteristic impedance (Z 0 ) is the same as the driver impedance, the receiver impedance, or both. For high-speed connections, it is desirable for the driver, receiver, and the transmission line to all have the same impedance. This helps produce a system where there are no reflections on the transmission line or its ends. For the simplest case of one driver connected to one receiver, matching the driver and receiver and transmission line is quite simple.  
           [0004]    Unfortunately, where a driver sends a signal along a transmission line to several receivers (or integrated circuits), producing a system with no reflections becomes more difficult. These systems (or busses) are typically called multi-drop busses.  
           [0005]    Multi-drop busses typically generate multiple reflections because of impedance mismatches at each transmission line branch or each receiver. These multiple reflections can combine in complex ways thereby making design of the whole system difficult and complex. Often, a design that has to deal with these multiple reflections will require segments of transmission lines with many different characteristic impedances. This further complicates the design and layout of the system.  
         SUMMARY OF THE INVENTION  
         [0006]    A four-drop bus has each driver or receiver terminated at the characteristic impedance of Z 0 . Each driver or receiver is connected to a segment of transmission line with a characteristic impedance of Z 0 . Two of these segments are connected at a first point. The other two of these segments are connected at a second point. The first and second points are connected by a central transmission line with a characteristic impedance of Z 0 /2.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is an illustration of a four-drop bus with matched response. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0008]    In FIG. 1, transmission line  101  has a characteristic impedance of one-half times Z 0 . This may also be written as Z 0 /2. Z 0  is an arbitrary characteristic impedance value that may be chosen with great latitude by the designer of the board or system by adjusting various board design parameters such as trace width, trace spacing, board layer thickness, etc., to fit a variety of constraints such as manufacturability, space, cost, or similarity to other impedances such as a driver impedance or termination impedance. Likewise, creating a transmission line with an impedance of Z 0 /2 can be done by adjusting various board design parameters such as trace width, trace spacing, board layer thickness, etc. Another way to create a transmission line of Z 0 /2 is two connect two transmission lines with characteristic impedance of Z 0  in parallel. Transmission line  101  ends at interface node  130  on one end and interface node  131  on the other. Transmission line  101  may also be referred to as the central transmission line.  
         [0009]    Connected to transmission line  101  at interface node  130  is transmission line  102  and transmission line  103 . Transmission lines  102  and  103  both have a characteristic impedance of Z 0 . The other end of transmission line  102 , node  150 , is connected to termination impedance  110  and receiver  120 . The other end of transmission line  103 , node  151 , is connected to termination impedance  111  and receiver  121 . The other terminal of termination impedance  110  and  111  are shown connected to drivers  140  and  141 , respectively.  
         [0010]    Connected to transmission line  101  at interface node  131  is transmission line  104  and transmission line  105 . Transmission lines  104  and  105  both have a characteristic impedance of Z 0 . The other end of transmission line  104 , node  152 , is connected to termination impedance  112  and receiver  122 . The other end of transmission line  105 , node  153 , is connected to termination impedance  113  and receiver  123 . The other terminal of termination impedance  112  and  113  are shown connected to drivers  142  and  143 , respectively.  
         [0011]    Alternatively, drivers  140 ,  141 ,  142 ,  143  may, in any combination, be replaced by a low impedance voltage source such as a power supply voltage or a termination supply voltage. Also, drivers  140 ,  141 ,  142 ,  143  may be controlled to always be driving a low impedance voltage or may themselves be controlled impedance drivers. In the case where drivers  140 ,  141 ,  142 ,  143  are controlled impedance drivers, termination impedances  110 ,  111 ,  112 ,  113  may not be needed.  
         [0012]    Transmission lines  101 ,  102 ,  103 ,  104 , and  105  may be of different and arbitrary lengths or delays. Assuming that drivers  140 ,  141 ,  142 ,  143  have sufficiently low impedance, termination impedances  110 ,  111 ,  112 , and  113  are preferably chosen to match the characteristic impedance Z 0 . If drivers  140 ,  141 ,  142 ,  143  are controlled impedance drivers, the controlled impedance of these drivers would preferably be chosen to match the characteristic impedance Z 0 .  
         [0013]    Using the four-drop bus shown in FIG. 1 will result in reflections that are the same independent of which driver  140 ,  141 ,  142 ,  143  is driving and which receiver  120 ,  121 ,  122 ,  123  is receiving. For example, if driver  140  drives a low impedance step voltage from zero to V in , all the termination resistors have an impedance of Z 0 , and drivers  141 ,  142 ,  143  are at a low impedance state to a termination supply, then the voltage at node  150  is a step from zero to V in /2. This step waveform propagates through transmission line  102  until it reaches interface node  130 . At interface node  130 , the load seen by transmission line  102  is equivalent to the characteristic impedance of transmission line  101  in parallel with transmission line  103 . This equivalent impedance is Z 0 /3. Calculating the reflection coefficient for this equivalent load yields:  
       Γ   =             1   3          Z   0       -     Z   0             1   3          Z   0       +     Z   0         =     -     1   2                               
 
         [0014]    Therefore, a step of −V in /4 will be reflected back down transmission line  102  toward node  150  and a step of V in /4 will be transmitted down transmission lines  103  and  101 . The wave reflected back down transmission line  102  is absorbed by the matched termination impedance  110  so this wave is not reflected at node  150 . Accordingly, node  150  has a final voltage of V in /4. Likewise, the V in /4 wave propagated down transmission line  103  is absorbed by the matched termination impedance  111  so this wave is not reflected at node  151 . Accordingly, node  151  has a final voltage of V in /4.  
         [0015]    The V in /4 wave propagated down transmission line  101  eventually reaches interface node  131 . At interface node  131 , the load seen by transmission line  101  is equivalent to the characteristic impedance of transmission line  104  in parallel with transmission line  105 . This equivalent impedance is Z 0 /2. Calculating the reflection coefficient for this equivalent load yields:  
       Γ   =             1   2          Z   0       -       1   2          Z   0               1   2          Z   0       +       1   2          Z   0           =   0                           
 
         [0016]    Accordingly, there is no reflection at interface node  131  and step waves of V in /4 are propagated down transmission lines  104  and  105 . The V in /4 waves propagated down transmission lines  104  and  105  are absorbed by the matched termination impedances  112  and  113 , respectively, so these waves are not reflected at nodes  152  or  153 . Accordingly, nodes  152  and  153  both have a final voltages of V in /4.  
         [0017]    Note that even though the voltage at each node is not the full swing voltage of V in , the voltage at each receiver node is the same and no reflections are observed at the receivers. This reduces the complexity of the system design and bus timing. Also note that this exercise could be conducted by driving the input waveform from any of the drivers  140 ,  141 ,  142 , or  143  and the outcome of a final voltage of V in /4 at each of nodes  150 ,  151 ,  152 , or  153  would result.  
         [0018]    Finally, note that due to design constraints or manufacturing process issues, the characteristic impedances of the transmission lines  101 ,  102 ,  103 ,  104 , and  105  the termination impedances  110 ,  111 ,  112 , and  113  may not be their exactly specified values of Z 0  or Z 0 /2. However, it should be sufficient that these impedances be approximately their specified values. A range of plus or minus 10% should be sufficiently approximate to satisfy most bus design requirements and still have sufficiently small reflections and final voltages that are sufficiently close to V in /4 for most applications.

Technology Category: h