Abstract:
A six-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 . Three of these segments are connected at a first point. The other three 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 / 3.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     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, Ser. No. 10/176,833, and is titled “FOUR-DROP BUS WITH MATCHED RESPONSE.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to data communication and more particularly to a transmission line structure for bi-directional communication between six sources/receivers. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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 
     A six-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 . Three of these segments are connected at a first point. The other three 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 /3. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a six-drop bus with matched response. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, transmission line  101  has a characteristic impedance of one-third times Z 0 . This may also be written as Z 0 / 3 . 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 / 3  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 / 3  is to connect three 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. 
     Connected to transmission line  101  at interface node  130  are transmission lines  102 ,  103 , and  104 . Transmission lines  102 ,  103  and  104  all 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 end of transmission line  104 , node  152 , is connected to termination impedance  112  and receiver  122 . The other terminal of termination impedances  110 ,  111 , and  112  are shown connected to drivers  140 ,  141 , and  142 , respectively. 
     Connected to transmission line  101  at interface node  131  are transmission lines  105 ,  106 , and  107 . Transmission lines  105 ,  106 , and  107  all have a characteristic impedance of Z 0 . The other end of transmission line  105 , node  153 , is connected to termination impedance  113  and receiver  123 . The other end of transmission line  106 , node  154 , is connected to termination impedance  114  and receiver  124 . The other end of transmission line  107 , node  155 , is connected to termination impedance  115  and receiver  125 . The other terminal of termination impedances  113 ,  114 , and  115  are shown connected to drivers  143 ,  144 , and  145 , respectively. 
     Alternatively, drivers  140 - 145  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 - 145  may be controlled to always be driving a low impedance voltage or may themselves be controlled impedance drivers. In the case where drivers  140 - 145  are controlled impedance drivers, termination impedances  110 - 115  may not be needed. 
     Transmission lines  101 - 107  may be of different and arbitrary lengths or delays. Assuming that drivers  140 - 145  have sufficiently low impedance, termination impedances  110 - 115  are preferably chosen to match the characteristic impedance Z 0 . If drivers  140 - 145  are controlled impedance drivers, the controlled impedance of these drivers would preferably be chosen to match the characteristic impedance Z 0 . 
     Using the six-drop bus shown in FIG. 1 will result in reflections that are the same independent of which driver  140 - 145  is driving and which receiver  120 - 125  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 - 145  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 three transmission lines  101 ,  103 , and  104  all in parallel. This equivalent impedance is 0.2*Z 0 . Calculating the reflection coefficient for this equivalent load yields:        Γ   =           0.2   ·     Z   0       -     Z   0           0.2   ·     Z   0       +     Z   0         =     -     2   3                                
     Therefore, a step of −V in /3 will be reflected back down transmission line  102  toward node  150  and a step of V in /6 will be transmitted down transmission lines  103 ,  104  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 /6. Likewise, the V in /6 waves propagated down transmission line  103  and  104  are absorbed by the matched termination impedance  111  and  112 , respectively, so these waves are not reflected at node  151  and node  152 . Accordingly, nodes  151  and  152  both have a final voltage of V in /6. 
     The V in /6 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 lines  105 ,  106 , and  107  all in parallel. This equivalent impedance is Z 0 /3. Calculating the reflection coefficient for this equivalent load yields:        Γ   =             1   3          Z   0       -       1   3          Z   0               1   3          Z   0       +       1   3          Z   0           =   0                            
     Accordingly, there is no reflection at interface node  131  and step waves of V in /6 are propagated down transmission lines  105 ,  106 , and  107 . The V in /6 waves propagated down transmission lines  105 ,  106 , and  107  are absorbed by the matched termination impedances  113 ,  114 , and  115 , respectively, so these waves are not reflected at nodes  153 ,  154 , and  155 . Accordingly, nodes  153 ,  154  and  155  all have a final voltage of V in /6. 
     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 - 145  and the outcome of a final voltage of V in /6 at each of nodes  150 - 155  would result. 
     Finally, note that due to design constraints or manufacturing process issues, the characteristic impedances of the transmission lines  101 - 107 , the termination impedances  110 - 115  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 /6 for most applications.