Patent Publication Number: US-6711027-B2

Title: Modules having paths of different impedances

Description:
RELATED APPLICATION 
     This application is a continuation in part of application Ser. No. 09/911,634, filed on Jul. 23, 2001, pending, and this application is a continuation in part of application Ser. No. 09/911,635, filed on Jul. 23, 2001, pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present inventions relate to semiconductor chip modules and, more particularly, to layouts for paths for circuit boards and modules. 
     2. Background Art 
     Modules include circuit boards, such as printed circuit boards (PCBs), that have chips (integrated circuits) on one or both sides of the modules. Examples of memory modules include single in line memory modules (SIMMs) and dual in line memory modules (DIMMs). The modules are positioned on motherboards, which may also support a controller, such as a memory controller (which may be integrated with other chipset functions or in a processor). 
     Traditional bus design, such as that used by current synchronous dynamic random access memories (SDRAMs) and double data rate (DDR) SDRAMs, involves the use of relatively long stubs (called a stubbed bus) as described in the following example in which two modules and a controller are on a motherboard. A bus of conductors extends a portion of the motherboard from the controller to the first and second modules. When the bus is adjacent the first module, relatively long stubs from the bus extend to the first module and to package connections of chips in the module. Likewise, when the bus is adjacent the second module, relatively long stubs from the bus extend to the second module and to package connections of chips in the second module. Package connections are used to interface with the world outside the chip. Examples of package connections include pins and balls (such as for ball grid arrays or flip chip arrangements). 
     Electrical reflections occur as a result of the stub. Electrical reflections from relatively long stubs tend to slow the maximum rate at which voltage switches may occur. Some systems have been used with very short stubs extending to chips from busses in the modules. 
     Changes in impedance can cause undesirable reflections. When changes in impedances are necessary, it may be desirable to increase or decrease impedances in steps. In the case in which a conductor splits into two sections, it is often desirable to raise the impedance of the two sections following the split to be about twice that of the section before the split. However, achieving this ratio is not always practical and a lower ratio may be used. 
     A path typically at least one termination device (called a termination), such as a termination resistor, to allow electrical energy to dissipate. Terminations may be positioned on the device originating the signal (for example, the controller), on the motherboard, on the module, on the die (chip), and/or in the package. Terminations are positioned between the conductor to be terminated and a reference voltage node, for example, having a ground voltage, a power supply voltage, or some other voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The inventions will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the inventions which, however, should not be taken to limit the inventions to the specific embodiments described, but are for explanation and understanding only. 
     FIG. 1 is a schematic plan view representation of a system including a motherboard, a controller, and two module connectors (not including the modules). 
     FIG. 2 is a schematic plan view representation of the motherboard of FIG. 1 with the modules inserted in the connectors. 
     FIG. 3 is a schematic front view representation of a second module of FIG.  2 . 
     FIG. 4 is a schematic cross-sectional side view representation of a second module of FIG. 3 taken along line  4 — 4 . 
     FIG. 5 is a schematic front view representation of a first module of FIG.  2 . 
     FIG. 6 is a schematic cross-sectional side view representation of a first module of FIG. 5 taken along line  6 — 6 . 
     FIG. 7 is a schematic representation showing multiple lines and package connections which are represented by a single line and package connections in FIGS. 1-6. 
     FIG. 8 is a schematic representation of an alternative to FIG.  6 . 
     FIG. 9 is a schematic representation of an alternative to FIG.  6 . 
     FIG. 10 is a schematic representation of a termination package of FIGS. 2 and 3. 
     FIG. 11 is a schematic plan view representation of a system including a motherboard, a controller, and two module connectors (not including the modules). 
     FIG. 12 is a schematic plan view representation of the motherboard of FIG. 11 with the modules inserted in the connectors. 
     FIG. 13 is a schematic plan view representation of a system including a motherboard, a controller, and two module connectors (not including the modules). 
     FIG. 14 is a schematic plan view representation of the motherboard of FIG. 13 with the modules inserted in the connectors. 
     FIG. 15 is a schematic cross sectional side view representation of the system of FIG.  14 . 
     FIG. 16 is a schematic front view representation of module  1  of FIG.  14 . 
     FIG. 17 is a schematic representation of impedances of the system of FIG.  14 . 
     FIG. 18 is a schematic partially plan and partially front view of a system like that of FIG. 14 with the addition of error correction code (ECC) chips and buffer chips included on the modules. 
     FIG. 19 is a schematic representation of on die terminations in the buffers of FIG.  18 . 
     FIG. 20 is schematic representation of off die terminations for the buffers of FIG.  18 . 
     FIG. 21 is a schematic representation of the system of FIG. 14 including a clocking path. 
     FIG. 22 is a schematic representation of the system of FIG. 21 including an additional clocking path. 
     FIG. 23 is a schematic plan representation of a system including a motherboard, a controller, connectors and modules inserted therein. 
     FIG. 24 is a schematic plan representation of a system including a motherboard, a controller, connectors and modules inserted therein. 
     FIG. 25 is a schematic representation of a routing path of the system of FIG.  24 . 
     FIG. 26 is a schematic representation of routing paths of the system of FIG.  14 . 
     FIG. 27 is a schematic cross sectional side view representation of module  1  of FIG. 14 in which a short term card is used in place of module  2 . 
     FIG. 28 is a schematic plan representation of the short term card of FIG.  27 . 
     FIG. 29 is an alternative to FIG.  28 . 
     FIG. 30 is a schematic cross sectional side view representation of an alternative embodiment for the system of FIG.  14 . 
     FIG. 31 is a schematic cross sectional side view representation of an alternative embodiment for the system of FIG.  14 . 
     FIG. 32 is a schematic representation of a selectively enabled on die termination circuit that may be used in the systems of FIGS. 30,  31  and  34 . 
     FIG. 33 is a schematic representation of a selectively enabled on die termination circuit which is an alternative to that of FIG.  32  and may be used in the systems of FIGS. 30,  31  and  34 . 
     FIG. 34 is a schematic representation of a system employing a layout of FIG.  30 . 
     FIG. 35 is a schematic plan view representation of a single sided system similar to the dual sided system of FIG.  14 . 
     FIG. 36 is a schematic cross sectional side view representation of a single sided module. 
     FIG. 37 is a schematic cross sectional side view representation of a single sided module. 
     FIG. 38 is a schematic representation of a system having three modules and a controller. 
     FIG. 39 is a schematic plan view representation of a system including a motherboard, a controller, and two module connectors with modules. 
     FIG. 40 is a schematic front view representation of module  1  including more than one chip per path on a module side. 
     FIG. 41 is a schematic cross-sectional side view representation of the module of FIG. 40 and a second module showing spaced chips. 
     FIG. 42 is a schematic representation of a routing path for the system of FIGS. 40 and 41. 
     FIG. 43 is a schematic representation of routing paths for the system of FIGS. 40 and 41. 
     FIG. 44 is a schematic representation of side view of a module having stacked chips. 
     FIG. 45 is a schematic representation of side view of a module in system similar to that of FIG. 30, but with only one of the chips in a path having the on die terminations enabled. 
     FIG. 46 is a schematic representation of front view of a module having paths with different impedances and lengths for different chips. 
     FIG. 47 is a schematic representation of a system similar to that of FIG. 34, but without loop through sections. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description describes multiple inventions which are claimed in different patent applications. The same description is provided in each application. However, the various inventions are not restricted to the details of the figures or written disclosure. Indeed, the inventions may be practiced through details different from those shown in the figures and described herein. 
     1. OVERVIEW: A SIMPLE EXAMPLE 
     FIGS. 1-10 provide a schematic representation of a relatively simple system illustrating features of the inventions. Note that FIGS. 1-38 are somewhat transparent (schematic) rather than being strictly plan, front, side, or cross-sectional views in that some objects below the surface are sometimes shown. Further, the figures are not intended to show correct relative sizes or shapes of objects in the drawings. FIG. 1 illustrates a system  10  having a motherboard  12 . A controller  14  is associated with motherboard  12 . Module connectors  20  and  22  are on motherboard  12 . Module slots  16  and  18  are formed in module connectors  20  and  22 . When modules  1  and  2  are inserted in the connectors, module slots  16  and  18  are filled. In the case in which memory modules are to be inserted in module connectors  20  and  22 , controller  14  includes memory controller functions. (The inventions are not restricted to use with memory modules.) Controller  14  may be in a chipset (e.g., a north bridge/hub) or it may be in a processor chip or group of chips or elsewhere. 
     A connector connection is an electrically conductive interface between a module connector (e.g., module connector  20  in FIG. 1) and a conductor on a path. For example, conductor connections C 1  are interfaces between module connector  20  and conductors on path  1 . There are N parallel connector connections in C 1  and N parallel conductors in path  1 . In this disclosure, connector connections are designated with the letter “C” followed by a number (e.g., C 1 , C 2 , etc.). Various devices may be used for connector connections including, for example, pin through holes, pads, and balls. 
     Modules typically include fingers, such as fingers  62  in FIG.  5 . The fingers interface with connector contacts on the module connectors. The connector contacts are designated with the letter “M” followed by a number (e.g., M 1 , M 2 , etc.). “M” is chosen to indicate it interfaces with the module. The connector contact may be a spring or other element. The inventions are not restricted to any particular connector contacts. As an example, in FIG. 5, finger  64  interfaces with a connector contacts M 8  which is coupled to conductors  44 . 
     A package connection is an electrical conductive interface between the module and a chip package. In this disclosure, a chip includes a die and a package of some sort connecting the die to the outside world and perhaps also protecting the die. (Note that in common usage, the term chip is sometimes used synonymously with the term die.) There are a variety of ways in which the package connection may occur. Examples of items used in the package connections include pins and balls (such as for ball grid arrays which may be used in flip chip arrangements). Package connections are designated with the letter “P” followed by a number (e.g., P 1 , P 2  etc.). 
     Routing path  1  and routing path  2  are paths that may be on the surface of and/or inside the motherboard. They may be data paths and may be unidirectional or bi-directional. Path  1  is shown with solid lines and path  2  is shown with dashed lines. Paths  1  and  2  are in an arrangement that may be referred to as a split ring. Clocking and control signals are not illustrated in the simplified FIGS. 1-10. Path  1  extends from controller  14  to connector connections C 1  and connector contacts M 1  on module connector  20 . Note that there are N parallel conductors in path  1 . Accordingly, there are N connector connections designated as connector connections C 1  and N connector contacts designated as connector contacts M 1 . The term “extend” does not imply that the path is in a straight line. 
     Path  1  continues at connector contacts M 2 . Note there is a gap between M 1  and M 2 . As shown in FIG. 2, this gap is filled by conductors as shown on module  1  between fingers that interface with M 1  and fingers that interface with M 2 . (In FIGS. 1 and 2, M 1  and M 2  are on opposite sides of module connector  20 , but that is not required. M 1  and M 2  could be on the same side of module connector  20 .) Path  1  continues from M 2  to connector connections C 2  to connector connections C 3  and connector contacts M 3 . Path  1  continues from a connector contacts M 4  to a connector connections C 4 . Again, there is a gap between M 3  and M 4  which is filed with conductors on module  2 . 
     Likewise, path  2  (shown in dashed lines) extends from controller  14  to C 5  to M 5 , from M 6  to C 6  to C 7  to M 7  and from M 8  to C 8 . As shown in FIG. 2, gaps between M 5  and M 6  and between M 7  and M 8  are filled with conductors on modules  1  and module  2  as shown. As with the case of path  1 , path  2  includes N parallel conductor lines and C 5 , C 6 , and C 7  each represent N connector connections. Likewise, M 6 , M 7  and M 8  each represent N connector contacts on module connectors  20  and  22 . 
     The conductors on Module  2  between M 3  and M 4  are referred to as conductors  58 . With reference to FIGS. 2 and 10, conductors  58  include conductors  58 - 1 ,  58 - 2 , . . .  58 -N. Termination package  30  includes N terminations T 1  T 2 , . . . TN (Ts). Terminations Ts are shown as resistors. In practice, they may be one or more transistors, discrete resistors, or other devices. 
     For high speed signaling, it is often desirable that there a ground conductor associated with every signal conductor on the motherboard, or there is at least one ground conductor for every two signal conductors, although the inventions are not limited to either of these ratios. Referring to FIG. 10, the N conductors  58  each are terminated to a ground plane, although the inventions are not so limited. Conductors (e.g., N or N/2) from the ground plane couple to connector connections C 4  (of which there are, for example N or N/2). Conductors  32  (of which there are, for example, N or N/2) are at a reference node Vref, which may be power supply voltage, ground voltage or some other voltage. Termination package  42  and conductors  44  may be similar. 
     Since the reference lines  32  and  44  are used whether or not there are on module terminations, the on module terminations of termination packages  30  and  42  reduce the number of connector contacts (M) and connector connections (C) by exactly or essentially N for each module for each two paths. In the case of four paths, it would be reduced by exactly or essentially 2N per module. This results in exactly or essentially a ¼ reduction in the number of connector contacts (M), corresponding fingers, and connector connections (C) for the paths (which may be data paths). 
     FIGS. 3,  4 ,  5 , and  6  provided additional details regarding the illustrated embodiment. Of course, the inventions are not restricted to these details. Package connections P 1  and P 2  each represent N package connections on path  1  and package connections P 3  and P 4  each represent N package connections on path  2 . In FIGS. 4 and 6, path  1  (called  60  on module  1 ) and path  2  are slightly set back from chip  26  and  38  respectively. This is not required in practice, but is provided for convenience in making the figures. Very short stubs  54  and  52  protrude from package connections P 1  and P 3  into chips  26  and  38 , respectively. These stubs are extremely short in comparison to the stubs of traditional stubbed busses. Note that in FIG. 3, the path  1  is shown with front and back vertical lines for ease of illustration and understanding. In practice, the lines may be staggered as shown in FIG. 3 or line up one behind the other. 
     FIG. 7 illustrates that conductors  60  include N conductors  60 - 1 ,  60 - 2  . . .  60 -N. Short stubs  54  include N short stubs  54 - 1 ,  54 - 2  . . .  54 -N. Package connections P 1  include package connections P 1 - 1 , P 1 - 2  . . . P 1 -N. In FIG. 7, chip  26  includes a die  26 A and a package  26 B. 
     The position of connector connections C 1  and C 2  are selected for ease of illustration. In practice, they may be positioned more like that shown in FIGS. 8 or  9 . Further, as shown in FIG. 9, a chip may have more than one set of package connections and short stubs ( 54  and  66 ) per conductor. 
     2. EXAMPLES WITH ADDITIONAL PATHS 
     As shown above, by using on module terminations, the number of connector connections, connector contacts, and corresponding fingers on the modules can be reduced by a factor of exactly or essentially ¼. The following examples of FIGS. 11-14 will apply this to a system with more chips. The number of module connections and corresponding fingers is limited by the pitch of the module connections and the length of the connector. In standard computer motherboards, there is a certain amount of space for module connectors. Accordingly, for a given pitch and length, the number of module connections and corresponding fingers is limited. The reduction of ¼ can be useful where connections and fingers are in shortage. The inventions are not limited to these details. 
     In FIGS. 11-17, bi-directional routing paths  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8  each include N conductors. Connector connections (C), connector contacts (M), and package connections (P) shown in FIGS. 11-17 represent N connector connections, connector contacts, and package connections. 
     FIG. 11 shows a system  90  without modules  1  and  2  and FIG. 12 shows system  90  with modules  1  and  2 . System  90  includes a motherboard  94  and a controller  92 . Module connectors  116  and  118  are on motherboard  94  and include each a front side closer to controller  92  and a back side farther from controller  92 . Modules  1  and  2  are inserted in module slots  16  and  18  in connectors  116  and  118 , respectively. Module  1  includes one set of chips I 1 , I 2  . . . I 16  and module  2  includes another set of chips I 1 , I 2  . . . I 16 . Paths  1 ,  3 ,  6 , and  8  are shown in solid lines and paths  2 ,  4 ,  5 , and  7  are shown in dashed lines. Path  1  includes conductors from controller  92  to C 1  to M 1  and from M 5  to C 5  to C 14  and M 14 . As can be seen by comparing FIGS. 11 and 12, module  1  couples M 1  and M 5  and module  2  couples M 14  and “on module terminations” (OMT), which are terminations for path  1 . Note that the OMT may be similar to or somewhat different from that of FIG.  10 . 
     Path  2  includes conductors from controller  92  to C 21  to M 21  and from M 13  to C 13  to C 6  and M 6 . As can be seen by comparing FIGS. 11 and 12, module  2  couples between M 21  and M 13  and module  1  couples M 6  and an OMT, which are terminations for path  2 . The route of paths  3 ,  4  . . .  8  can be seen in FIGS. 11 and 12. The paths may be terminated on a single end (e.g., merely the OMT) or may be terminated on dual ends (in OMT and also in controller  92 ). The OMTs may be located in a variety of places (e.g., FIGS. 16,  36 , and  37 ). 
     A problem with the routing of system  90  is that modules  1  and  2  are not interchangeable unless it does not matter if they are rotated 180 degrees. This is because in module  1 , the OMTs are coupled to chips I 2 , I 4 , I 5 , and I 7 , but in module  2 , the OMTs are coupled to chips I 10 , I 12 , I 3 , and I 15 . If for some reason rotation is unobtainable, module  1  must be different than module  2 , which causes additional expense. If error correction code (ECC) chips are added to modules  1  and  2 , the ECC chips have to be in between chips I 4  and I 5  and I 12  and I 13  to keep rotation possible. A disadvantage of having to rotate modules, is that they might be inserted in the wrong orientation. It may be preferred to have the modules and module slots keyed, so the modules cannot be inserted into the module slots in the wrong orientation. 
     System  100  shown in FIGS. 13 and 14 solves this problem with FIGS. 11 and 12. System  100  is like system  90  except for slightly different routing as shown in the figures and described as follows. In FIGS. 11 and 12, path  1  goes between C 5  and C 14 . By contrast, in FIGS. 13 and 14, path  1  goes between C 5  and C 18 . Similarly, in contrast, in FIGS. 13 and 14, path  3  goes between C 7  and C 20 , path  6  goes between C 10  and C 21 , and path  8  goes between C 12  and C 23 . In comparison with system  90 , the routing of system  100  is referred to as a swizzle routing because the points of connector connection swizzles from the front side to the back side of module  2 . Note that in system  100 , the OMTs are in the same places in module  1  and module  2 . Therefore, module  1  could be inserted into slot  18  and module  2  could be inserted into slot  16  and system  100  would perform the same. Modules  1  and  2  and slots  16  and  18  can be keyed so they cannot be rotated. Therefore, in this example, only one type of module needs to be on stock and it cannot be inserted the wrong way. 
     Note that system  100  provides only one example of a routing in which the OMTs are in the same position. A variety of other routings will provide the same result. For example, through an additional swizzle in module connectors  116  and  118  could cause each OMT to switch from the front to the back side of modules  1  and  2 . The relative position of the OMTs could also be changed. 
     FIG. 15 provides a cross sectional representation of FIG. 14 looking from the right hand side into chips I 8  and I 16  in modules  1  and  2 . Part of path  8  is shown in module  1  and part of path  7  is shown in module  2 . Package connections P 5  and P 6  provide the signal on path  8  to chips I 8  and I 16  of module  1  through very short stubs  140  and  142 . Package connections P 7  and P 8  provide the signal on path  7  to chips I 8  and I 16  of module  2  through very short stubs  144  and  146 . Note that paths  7  and  8  may be essentially on the surface of modules  1  and  2  or may be under the surface of modules  1  and  2 , but are shown inset for convenience in drawing. Paths  8  and  7  do not have to extend as high up into modules  1  and  2  as is shown in FIG.  15 . For example, they might extend just to the level of the package connections or above that level. OMTs could have been illustrated in FIG.  15 . 
     In the case of chips I 7  and I 15  in module  1 , path  7  has a loop that looks similar to path  8  in FIG. 15 except as follows. In FIG. 15, path  8  is looped between two connector contacts M 12  and M 4  and is coupled to package connections P 5  and P 6  between M 4  and M 12 . The loop for path  7  between I 7  and I 15  does not go between two connector contacts. Rather, in the case of I 7  and I 15 , path  7  has a loop between I 7  and I 15  that couples package connections for I 7  and I 15 , but it loops between M 11  and some interface to the OMT. This may be similar to or different from that shown for path  1  in FIG.  3 . The paths in the module each couple to package connections. See FIG. 26 showing portions of routings for path  1  and path  2 . 
     FIG. 16 shows a front view of module  1  of FIG.  14 . FIG. 16 illustrates OMTs coupled between chips I 2 , I 4 , I 5 , and I 7  and connector connections Cpath 2  Vref, Cpath 4  Vref, Cpath 5  Vref, and Cpath 7  Vref, respectively. Connector connections Cpath 2  Vref, Cpath 4  Vref, Cpath 5  Vref, and Cpath 7  Vref interfaces with connectors that carry references signals (power, ground, or some other voltage depending on the implementation) for the signals on paths  2 ,  4 ,  5 , and  7 , respectively. Merely as an example, there may be a 1:1 or 2:1 ratio of signal to reference conductors. Since connector connections Cpath 2  Vref, Cpath 4  Vref, Cpath 5  Vref, and Cpath 7  Vref are used whether or not there is OMTs, there is a substantial savings (e.g. ¼) in fingers, connector contacts etc. (Compare discussion for FIG. 10.) 
     FIG. 17 provides an electrical impedance diagram for path  1  in FIG.  14 . ZoMB 1  and ZoMB 2  are impedances for the motherboard traces of path  1  in the positions shown. ZoMod 11 , ZoMod 12 , and ZoMod 13  are impedances in module  1  in the positions shown, and ZoMod 21 , ZoMod 22 , and ZoMod 23  are impedances in module  2  in the positions shown. (Note there are N of each of these.) CI 1  and CI 9  represent capacitances for chips I 1  and I 9  of module  1 , and CI 10  and CI 2  represent capacitances for chips I 10  and I 2  of module  2 . It may be desirable to make the impedance of ZoMB 2  equal ZoMB 1 . It may be desirable to make the effective impedances of the combined CI 1 , ZoMod 11  and ½ of ZoMod  12  equal to that of ZoMB 1 . Likewise, it may be desirable to make the effective impedances of the combined CI 9 , ½ of ZoMod  12 , and ZoMod 13  equal to that of ZoMB 1  and so forth with module  2 . To compensate for CI 1 , the impedance of ZoMod 11  and perhaps ZoMod 12  may be higher than ZoMB 1 . Likewise, the other impedances in modules  1  and  2  may be higher to compensate for CI 9 , CI 10 , and CI 2 . It may not be practical to get the impedances exactly equal because of size or other expense constraints. Merely as an example, the impedances ZoMB 1  and ZoMB 2  might be 39 ohms and the impedances ZoMod 11 , ZoMod 12 , and ZoMod 13  and ZoMod 21 , ZoMod 22 , and ZoMod 23  might be 63 ohms. Various other values might be used such as 30 and 60 ohms, 33 and 63 ohms, and 50 and 100 ohms, to mention only some of the possibilities. 
     In FIG. 17, the impedance may increase as the path gets closer to the chip loads. For example, the impedance ZoMod 11  could increase as it gets closer to chip I 1 . ZoMod 11  could be higher than impedance in connector  116 . Merely as an example, ZoMod 12  could be higher than that of ZoMod 11  and ZoMod 13 . 
     The paths of systems  90  and  100  and systems described below may be terminated on a single end or both ends (in the chipset as well as on the OMT). The capacitance C illustrated in controller  92  for path  1  may be about 2 pF. However, controller  92  and other controllers shown in the figures are not limited to the details illustrated. 
     3. SYSTEMS INCLUDINGS ECC CHIPS AND/OR BUFFER CHIPS 
     FIG. 18 illustrates a motherboard  200  which may be similar to that of FIG. 14 except that it also includes ECC chips and buffer chips buffer M 1  and buffer M 2  on modules  1  and  2 . 
     The ECC chips connect through connector connections C 27  and C 28  and may be of a well known type. The ECC chips may be positioned in different locations from those shown. 
     The buffer chips buffer M 1  and buffer M 2  may received address and/or command signals from controller  202  on a path including conductors  204  (which has M conductors). The path splits from conductors  204  to conductors  206  and  208 , with conductors  206  coupling to connector connections C 25  and conductors  208  coupling to connectors connections C 26 . Buffers M 1  and M 2  may in turn provide the address and/or command signals on P conductors to chips I 1 -I 8  and ECC chips if present. (Of course, the inventions are not restricted to the use of a particular number of chips per module.) 
     The ECC chips and the buffer chips buffer M 1  and buffer M 2  may be terminated on the motherboard, on the module, on die, and/or in the controller. (They may be single or dual terminated chips). Rather than terminate at the end of single path after two ECC chips (actually four chips if consider ECC chips on other side of modules are included) as shown in FIG. 18, there could be one path to the ECC chip(s) on module  1  with an OMT on module  1  and another path to the ECC chip(s) on module  2  with an OMT on module  2 . 
     In FIGS. 18-20, C 25  and C 26  represent multiple connector connections, and P 27  and P 28  represent multiple package connections. Conductors  204  represent multiple conductors with ZoMB 1  representing corresponding impedances. Conductors  208  represent multiple conductors with ZoMB 2  representing corresponding impedances. There are multiple conductors between C 25  and P 27  with corresponding impedances ZMod 1 , and there are multiple conductors between C 26  and P 28  with corresponding impedances ZMod 2 . There are multiple Cb 1 s and Rb 1 terms in buffer M 1  and multiple Cb 2 s and Rb 2 terms in buffer M 2 . Controller  202  is not restricted to the details shown in FIGS. 19 and 20. In FIGS. 19 and 20, signal regeneration for distribution to I 1 -I 8  and ECC, etc. is not shown. 
     FIG. 19 shows an example of how the path ( 204  and  206 ) between controller  202  and buffer M 1  can be terminated on the die of buffer M 1 , and how the path ( 204  and  208 ) between controller  202  and buffer M 2  can be terminated on the die of buffer M 2 . More particularly, buffer M 1  includes termination resistors Rb 1 terms and buffer M 2  includes termination resistors Rb 2 terms. Because of the split to  206  and  208 , it may be desirable that the impedances ZoMB 2  are twice the impedances ZoMB 1 . It may be desirable that the impedances ZoMod 1  are twice the impedances ZoMB 1 , and the impedances ZoMod 2  are twice the impedances ZoMB 1 . It may be desirable that the effective impedances of Cb 1  and Rb 1 term are twice the impedances ZoMB 1 , and the impedances Cb 2  and Rb 2 term are twice the impedances ZoMB 1 . However, in practice these goals might not be practical because of size or other cost constraints. Therefore, ZoMB 2 , ZoMod 1 , ZoMod 2 , and the effective impedances of Cb 1  and Rb 1 term and the effective impedances Cb 2  and Rb 2 term might be, merely as an example, less than twice ZoMB 1 . For example, ZoMB 1  merely as an example, might be 39 ohms and the other impedance values might be 63 ohms. Of course, the inventions are not restricted to these impedance values and other values might be used such as 30 and 60 ohms, 33 and 63 ohms, and 50 and 100 ohms, to mention only some of the possibilities. The impedances other than ZoMB 1  and ZoMB 2  do not have to equal each other. For example, the impedances for Rb 1 terms do not have to equal that of ZoMod 1 . 
     FIG. 20 shows an example of how the path ( 204  and  206 ) between controller  202  and buffer M 1  can be terminated off the die of buffer M 1 , but on module  1 , and how the path ( 204  and  208 ) between controller  202  and buffer M 2  can be terminated off the die of buffer M 2 , but on module  2 . More particularly, buffer M 1  includes a capacitive load Cb 1  and buffer M 2  includes a capacitive load Cb 2 . Module  1  includes conductor impedances ZoMod 11  and ZoMod 12  and termination resistors Rmod 1 term. Module  2  includes conductor impedances ZoMod 21  and ZoMod  22  and termination resistors Rmod 2 term. It may be desirable that the impedances ZoMB 2  are twice the impedances ZoMB 1  (because of the split to  206  and  208 ). It may be desirable that the effective impedances of Cb 1 , ZoMod 11  and ZoMod 12  be twice the impedances ZoMB 1 , and resistances of RM 1 term be twice ohms of the impedances ZoMB 1 . It may be desirable that the effective impedances of Cb 2 , ZoMod 21  and ZoMod 22  be twice the impedances ZoMB 1 , and resistances of RM 2 terms be twice the ohms of the impedances ZoMB 1 . The loads Cb 1  and Cb 2  may not be significant so that ZoMod 11 , ZoMod  12 , ZoMod 21 , and ZoMod  22  may be close to the same as RMod 1 terms and RMod 2 terms. However, in practice these goals might not be practical because of size or other cost constraints. Therefore ZoMB 2 , Rmod 1 terms, Rmod 2 terms, and the effective impedances Cb 1 , ZoMod 11  and ZoMod 12  and effective impedances of Cb 2 , ZoMod 21  and ZoMod 22  might be, merely as an example, less than twice ZoMB 1 . For example, ZoMB 1  merely as an example, might be 39 ohms and the other values might be 63 ohms. Of course, the inventions are not restricted to these impedance values and other values might be used such as 30 and 60 ohms, 33 and 63 ohms, and 50 and 100 ohms to mentioned only some of the possibilities. The impedances other than ZoMB 1  and ZoMB 2  do not have to equal each other. For example, the impedances ZoMod 11 , ZoMod  12 , ZoMod 21 , and ZoMod  22  may be different than RMod 1 terms and RMod 2 terms. 
     The above described schemes (e.g., 39 ohms for ZoMB 1  and others being 63 ohms) may have two advantages. First, it reduces impedance mismatch. Second, the higher impedance values can be higher or lower depending on physical layer PCB routing feasibility. An advantage of having the termination on the module external to the die is termination does not have to be added to the die thereby reducing the silicon thermal junction temperature risk. This is at the expense of some signal integrity reduction in comparison to on die termination. 
     In some embodiments of FIGS. 18-20, a p-channel push current mode driver may be used in controller  202  and elsewhere, although the inventions are not so limited. For example, such a driver may include a node at which the conductors  204  meet the controller  202 . A p-channel field effect transistor (pFET) has a signal (such as a data signal D#) couple to its gate. The pFET is coupled between the node and a current source. The current source is coupled between the PFET and a power supply for controller  202 . An Ro between the node and ground may be greater than 5 times the impedances of conductors  204 . However, this ratio may be lowered to improve the matching at the expense of voltage swing for the equivalent driver current. An advantage of this I/O type is primarily the ability to decouple the receiver voltage supply from the driver voltage supply. Other advantages are its high speed capability and its ability for the Ro to be high or low in comparison to the impedance of the path of conductors  202 . Of course, the inventions are not limited to these details. 
     4. CLOCKING 
     FIGS. 21 and 22 illustrate a clocking system that may be used in some embodiments of the inventions. The inventions are not, however, restricted to the details of the clocking schemes of FIGS. 21 and 22. Portions of modules  1  and  2  of FIG. 14 are used for purposes of illustration. 
     Referring to FIG. 21, clocking for chips I 1 , I 9 , I 2 , and I 10  in modules  1  and  2  are shown. Similar clocking can be duplicated for the other chips in modules  1  and  2 . Data paths  1  and  2  are shown as in FIG. 14 to provide a context. Path  1  is coupled to package connections P 21 , P 22 , P 23 , and P 24 . Path  2  is coupled to package connections P 41 , P 42 , P 43 , and P 44 . A clock signal Clk is provided by controller  92  on a path to chips I 1 , I 9 , I 2 , and I 10  in modules  1  and  2 . The clocking path includes connector connections C 60  and then splits to go to package connections P 51  of chip I 1  and P 52  of chip  19  and to package connections P 53  of chip I 2  and P 54  of chip I 10  of module  1 . The split clock path merges and proceeds to connector connections C 61  of connector  116  and travels to connector connections C 62  of connector  118 . The clocking path then splits to go to package connections P 55  of chip I 1  and P 56  of chip I 9  and to package connections P 57  of chip I 2  and P 58  of chip I 10  of module  2 . The split clock path merges and proceeds to connector connections C 63  of connector  118  and travels to controller  92 , wherein it may terminate. 
     The clock signal may be differential and therefore there may be two conductors and corresponding connections in the clock path. A low or full voltage swing clock may be used. 
     The clock signal of FIG. 21 provides timing for the data signals on paths  1  and  2 . Through this technique, a single clock signal can be used for eight chips. It may be preferred, however, to have one clock signal for reading through path  1  and writing through path  2  and another clock signal for reading through path  2  and writing through path  1 . FIG. 22 provides such a system. In FIG. 22, clock signal Clk 1  is the same as clock Clk in FIG.  21 . Paths  1  and  2  are not shown to avoid clutter in the drawings, but paths  1  and  2  in the system of FIG. 22 may be the same as in FIG.  21 . FIG. 22 also adds another clock path to carry a clock signal Clk 2 , which is provided by controller  92  on a path to chips I 1 , I 9 , I 2 , and I 10  in modules  1  and  2 . The clocking path for Clk 2  includes connector connections C 71  of connector  118  and then splits to go to package connections P 55  of chip I 1  and P 56  of chip I 9  and to package connections P 57  of chip I 2  and P 58  of chip I 10  of module  2 . The split clock path merges and proceeds to connector connections C 72  of connector  118  and travels to connector connections C 73  of connector  116 . The clocking path for Clk 2  then splits to go to package connections P 65  of chip I 1  and P 66  of chip I 9  and to package connections P 67  of chip I 2  and P 68  of chip I 10  of module  2 . The split clock path merges and proceeds to connector connections C 74  of connector  116  and travels to controller  92 , wherein it may terminated. 
     In FIG. 22, data is written through path  1  synchronously with Clk 1  and read through path  1  synchronously with Clk  2 . Data is written through path  2  synchronously with Clk 1  and read through path  2  synchronously with Clk  1 . Accordingly, reading and writing for eight chips can occur with only two clock signals (which may each be differential signals). 
     It may be desirable if the impedances of the clocking and data paths are matched such that clocking and data signals have close to the same switching speed through these paths. The impedances can be increased (e.g. 25 to 50 ohms or 40 to 60 ohms or some other values) with the splits and return to the original value when the paths merge. As mentioned, the data paths of FIG. 17 may also have stepped increases in impedance with loaded sections near the chip loads having even higher impedance. There may be a further higher impedance of for example 65 ohms (in the 25 to 50 ohms case) for sections that correspond to the loaded sections in the data paths to match the data paths. That is, the clock paths including sections that correspond to the loaded sections of data paths and have corresponding increases in impedances in the sections corresponding to the loaded sections to obtain good matching. As mentioned, although doubling impedances may be desirable for one standpoint, it may not always be practical from an overall system viewpoint and other impedance values may be used. 
     5. MULTIPLE MODULE SYSTEMS 
     The following discussion and figures describes and shows systems with two sets of modules on different paths. These systems have particular application to the server environment, but are not restricted to this environment. As illustrated, there are two modules per set. However, there could be three modules or more per set and/or more than two sets. 
     Referring to FIG. 23, a system  300  includes a motherboard  304  which supports four modules connectors  312 ,  314 ,  316 , and  318  into which modules  1 ,  2 ,  3 , and  4  are inserted. Modules  1  and  2  are in one set and modules  3  and  4  are in another set. Paths  1 ,  2 , . . .  8  (which may be bi-directional data paths) are provided to modules  1  and  2  as shown and terminate on motherboard  304  (motherboard terminations (MBT)). For example, path P 1  travels between controller  308  and a MBT by way of chips I 1  and I 9  in module  1  and chips I 1  and I 9  in module  2 . Path  1  may do a short loop through in module  1  between chips I 1  and I 9  in a manner similar to shown in FIGS. 15 and 26, so as to provide the signals on path  1  to package connections of chips I 1  and I 9 . Likewise, path  1  may do a short loop through in module  2  between chips I 1  and I 9  in the same manner and then terminate outside module  2  in a termination package or other termination resistors supported by motherboard  334 . Paths  2 ,  3  . . .  8  may also have a similar short loop through arrangement in modules  1  and  2 . 
     Paths  9 ,  10  . . .  16  are provided to modules  3  and  4  as shown and terminate with MBT. Paths  9 ,  10  . . .  16  may also have a short loop through arrangement in modules  3  and  4  similar to that of FIGS. 15 and 26. 
     In the illustrated embodiment, chips I 1 -I 16  are designed to receive N/2 data bits and paths P 1 -P 16  each have N/2 lines. For example, if in the system of FIG. 14, N is eight, then N/2 may be four in the system of FIG.  23 . In that case, the systems of FIGS. 14 and 23 would have the same number of data lines (16×4=8×8). (ECC chips may add additional lines.) However, the N in FIG. 23 does not have to be the same as the N in FIG. 14, and N/2 does not have to be four. The paths of FIG. 23 do not have to have N/2 lines. 
     Referring to FIG. 24, a system  330  includes a motherboard  334  which supports four modules connectors  342 ,  344 ,  346 , and  348  into which modules  1 ,  2 ,  3 , and  4  are inserted. Modules  1  and  2  are in one set and modules  3  and  4  are in another set. Paths P 1 , P 2 , . . . P 4  (which may be bi-directional data paths) are provided to modules  1  and  2  as shown and terminate on motherboard  334  (MBT). Paths P 5 , P 6 , . . . P 8  (which may be bi-directional data paths) are provided to modules  3  and  4  as shown and terminate on motherboard  334  (MBT). The paths may terminated on the module or on die, but that may require one of the modules to be different than the other(s) or to have a selectable terminations on die or on the module (described below). 
     In the illustrated embodiment of FIG. 24, chips I 1 -I 16  are designed to received N data bits and paths  1 - 8  each have N lines, which is twice as many as the paths of FIG.  23 . However, since there are also one half the number of paths in FIG. 24 as in FIG. 23, the number of lines in FIG. 23 is the same as in FIG. 24 (N×8=N/2×16) as in FIG.  23 . In FIG. 24, the paths provide data to four chips. For example, path  1  provides data to chips I 1 , I 2 , I 10 , and I 9 . There are many ways in which this can be done. FIG. 25 illustrates one way. Referring to FIG. 25, path  1  passes adjacent to chip I 1  and is coupled to a package connections P 21  of chip I 1 , either directly or through a via. Path  1  extends to chip I 2  of module  1 , where it is coupled to package connections P 44 , and so forth with path  1  coupling to package connections P 43  of chip I 10  and P 22  of chip I 9  in module  1 . The path does not have to have this particular layout. For example, it does not have to extend above the chips or extend in straight lines or at 90 degree angles. 
     The routing of FIG. 25 can be compared to that of FIG.  26 . FIG. 26 illustrates a portion of the routing of paths  1  and  2  in module  1 . Path  1  in module  1  includes a short loop through section to couple to package connections P 21  and P 22 . 
     The ECC, buffering, clocking schemes, short term card, and selectable on die or on module terminations described herein may be used in connection with the systems  300  and  330  and other systems described herein. Systems  300  and  330  are not restricted to using only two modules per path, but could use three or more. 
     6. TERMINATION CARD (DUMMY MODULE) 
     The system of FIG. 14 may be used with two modules or with a module and termination card, which is an example of a dummy module. A dummy module includes a circuit board (substrate), without the chips of an active module, that is used in place of the active module. A reason to have a dummy module is because it is less expensive than the module, but it completes the circuit as does the module. Referring to FIG. 27, an example of a termination card is short termination card  360  which in the illustrated embodiment fits into slot  18  of system  100  (see FIGS.  14  and  15 ). Short termination card  360  is referred to as short because it is not as tall as the modules  1  and  2 . Short termination card  360  is designed to pass or terminate signals from the paths or other signals (e.g., ECC, buffering, clocking schemes or other features mentioned in this disclosures) that may be present just as if it were module  2 . 
     FIG. 28 illustrates a schematic plan view of short term card  360  having first and second sides  362  and  364  connected to connector  118 . First side  362  includes finger groups FG 1 , FG 2  . . . FG 8 . Second side  364  includes finger groups FG 9 , FG 10 , FG 11 , and FG 12 . In either FIG. 27 or FIG. 28, there may be additional finger groups if needed (for example, in the case in which there are additional paths or if the module with chips include ECC chips or a buffer). 
     As can be seen through comparing FIG. 28 with module  2  in FIG. 14, in those paths for which there are no on module terminations, short termination card  360  fills the gap between connector contacts (e.g., between M 16  and M 24  and FG 1  and FG 9 ). In the case in which an OMT is involved, it is not necessary to pass conductors through the module. Rather, a module could be on the same side as the path is received. For example, in FIG. 14, in module  2 , conductors of path  1  pass between M 18  and the other side of the module to couple to an OMT. Short termination card  360  could have a similar routing with the OMT on the same side as that of module  2  (as shown in FIG.  27 ), or it could have the OMT on the same side as M 18  and not have to pass conductors across the module. 
     FIG. 27 shows possible locations of OMTs on module  1  and short term card  360 , although the inventions are not restricted to these locations. Any suitable locations on the module is acceptable for the OMTs. Note that in the case of module  1 , the OMT is for a path other than path  8 . In the case of short term card  360  and module  1 , only one of the OMTs is shown. 
     FIG. 29 shows an alternative short term card  368  in which the OMTs are on the second side  364  coupled to finger groups FG 2 , FG 4 , FG 5  and FG 7  by conductors. 
     In some embodiments, some OMTs could be on side  362  and other OMTs could be on side  364 . This could be with a routing like that shown in FIGS. 14,  28 , and  29 , or with a different routing in which some of the OMTs on the module where on one side and some on the other. 
     7. SHORT STUB CREATED FROM A PATH LOOP IN THE MODULE AND SELECTABLE ON DIE TERMINATIONS 
     FIGS. 30-34 illustrate two independent aspects of the inventions (1) a short stub created from a path loop in the module and (2) selectable on die terminations. These aspects are presented together, but they may be used independently. 
     Note that in FIGS. 14 and 15, path  8  loops between package connections P 5  and P 6 . By contrast, in FIG. 30, path  8  in a system  380  extends only partly into module  1  and couples to a short stub  386  which in turn couples to package connections P 5  and P 6  either directly or through a via. Path  8  has no loop in module  2  but rather extends all the way to package connections P 11  and P 12  (or to a via that connects to P 11  and P 12 ). FIG. 31 shows a route for path  7  in which a short loop in module  2  connects to a short stub  388  and no loop is in module  1 . The loop in paths  7  and  8  can be longer or shorter than shown (note that in FIG. 31, the loop in path  7  is shorter than that of path  8  in FIG.  30 ). 
     In a separate invention, in FIGS. 30 and 31, modules  1  and  2  each have selectable on die terminations. In the example of FIGS. 30 and 31, the on die terminations are disabled in chips I 8  and I 16  of module  1  and chips I 8  and I 16  of module  2 . The on die terminations are enabled in chips I 7  and I 15  of module  1  and I 7  and I 15  of module  2 . The chips with the enabled on die terminations may be the same as those in the module with OMT in FIG.  14 . Accordingly, for some paths, module  1  will have enabled on die terminations and module  2  will have disabled on die terminations. For other paths, module  1  will have disabled on die terminations and module  2  will have enabled on die terminations. 
     A circuit  400  in FIG. 32 is one example and a circuit  440  in FIG. 33 are examples of circuits that can be used selectively enable or disable on die terminations. Various other circuits could be used and the inventions are not restricted to these details of circuits  400  and  440 . For example, the R-termination elements could be pull down rather than pull up. Circuits  400  and  440  include an R-termination network  404  which are illustrated in FIG.  34 . Referring to FIG. 32, R-termination network  404  includes X number of R-termination elements R-term  1 , . . . R-term X. Depending on the implementation, X may be as low as less than 5 to more than 100. Each element includes transistors T 1 , T 2 , and T 3 . The R-term elements are controlled by an active R-term on/off selection circuit  408  through multiplexers  412 - 1  . . .  412 -X and a linearized active R-term network bias circuit  410 . In the illustrated circuit  400 , the “1” value of multiplexers  412 - 1  . . .  412 -X is tied to Vcc (but it may be provided by bias circuit  410 ). The “0” value of multiplexers  412 - 1  . . .  412 -X is provided by bias circuit  410  (but it may be tied to ground). That is, in the illustrated circuit  400 , the “0” value might not be at ground to control how much transistors T 1  and T 2  are turned on. Some feedback could be used to compensate for temperature, process variations etc. Configurable driver  414  includes pre-driver swing control circuit  416  and driver bias circuit  418 . In FIG. 32, network  404  is between power and data node  430  and driver  414  is between data node  430  and ground. That is, the termination is to the power supply voltage node. Alternatively, network  404  could be between node  430  and ground and driver  414  could be between the power supply node and ground. Note that the system can have only one or more than one power supply and ground voltage values. 
     In FIG. 34, system  380  includes a controller  384  which is coupled to modules  1  and  2 . Modules  1  and  2  and connectors  116  and  118  are similar to those in FIG. 15, except that the chips include selectable on-die terminations instead of OMTs. Path  8  is illustrated. In FIG. 34, in module  1 , chips I 8  and I 16  have on die terminations disabled and in module  2 , chips I 7  and I 15  are on die terminations enable. Accordingly, in each of chips I 8  and I 16 , selection circuit  408  select each of multiplexers  412  to provide the “1” value to turn off each R-term element (R-term  1  . . . R-term X). Drivers  414  may also be turned off. By contrast, in chips I 7  and I 15 , driver  414  is on and selection circuit  408  selects at least one of the R-term elements of R-termination network  404 . The number of R-term elements selected and perhaps the “0” value are controlled to give a desired impedance level, described next. There may be one or more than one selection circuit  408  and one or more than one bias circuit  410 . 
     Referring to FIG. 33, circuit  440  is similar to circuit  400  except as shown. For example, network bias circuit  410  controls the gates of FETs T 1 - 1  and T 2 - 1  . . . T 1 -X and T 2 -X. Multiplexers  412 - 1  . . .  412 -X have inputs tied to power and ground. The drains of T 1 - 1  and T 2 - 1  . . . T 1 -X and T 2 -X are tied together. 
     Referring to FIG. 34, impedance ZoMB 1  represents the impedance on path  8  on the motherboard between controller  384  and connector  116  and impedance ZoMB 2  represents the impedance on the motherboard on path  8  between connector  116  and connector  118 . Impedances ZoMod 11  and ZoMod 13  are impedances of path  8  in module  1  and ZoMod 12  is the impedance of stub  386  (see FIG.  30 ). Impedance ZoMod 2  is the impedance of path  8  in module  2 . It may be desirable if the impedances of ZoMod 11 , ZoMod 12 , and ZoMod 13  were larger than that of ZoMB 1  and ZoMB 2 . Merely as an example, assume ZoMB 1  and ZoMB 2  were 39 ohms. The impedance of ZoMod 11 , ZoMod  12 , and ZoMod 13  might be 63 ohms to, for example, compensate for chip capacitance. Of course, other values (such as those listed above) could be used for tradeoffs with expense, board layout, and performance. It may be desirable if the impedances of the enabled on-die terminations (R-termination networks  404 ) in chips I 7  and I 15  were about twice that of ZoMB 1  and ZoMB 2 . For example, if ZoMB 1  and ZoMB 2  are 39 ohms, then about 78 ohms might be chosen for network  404 . Of course, other values might be chosen and the inventions are not restricted to these values. ZoMod 2  may be the same as that of ZoMB 1  or it could be higher, for example to compensate for chip impedance. 
     In each of the systems described herein, an attempt can be made to increase impedance gradually or through steps. For example, in FIG. 34, the impedance might increase from ZoMB 1  to MoMod 11  to ZoMod 12  and then decrease from ZoMod 12  to ZoMod  13  to ZoMB 2 . The trace sections near the chips may have higher impedance than those trace sections further from the chips. 
     As illustrated, system  380  is a dual termination system in that controller  384  includes an R-term element  446  and a driver  414 . There may be a separate R-term element for each line of path  8 . Rather then use a single R-term element for termination, more than one element may be used. 
     The on-die termination feature is not restricted to use with the short stubbed systems of FIGS. 30 and 31. It may be used in connection with systems with short loop through configurations (such as in FIG.  15 ). In that case, in FIG. 34, the impedances in module  1  would be like those shown in module  1  of FIG.  17 . Further, the circuit of FIG. 32 could be employed in connection with terminations in the chip package or on the modules. 
     8. SINGLE SIDE EMBODIMENTS 
     FIGS. 11-34 illustrate systems and features thereof for dual sided modules (chips on both sides of the module). The invention, however, is not restricted to use with dual sided modules, but rather could also be used with single side modules. FIG. 35 illustrates routing for a system similar to that of FIG. 14 except that single side modules are used rather than dual sided modules. FIGS. 36 and 37 show to routing paths for path  8  to terminate on an OMT through package connections P 80 . Various other positions of the OMT could be used. FIG. 35 shows connector contacts being on both sides of the connector module slots ( 16  and  18 ). Alternatively, they could all be on the front side of the module slots. 
     9. ADDITIONAL INFORMATION AND INVENTIONS 
     The illustrations described above show only two modules. However, more than two modules per path may be used. For example, FIG. 38 shows a system  480  with a controller  482  which includes modules  1 ,  2 , and  3 . Module  3  is a module that from a path point of view acts as a bridge between modules  1  and  2 . Accordingly, in any of the FIGS. 1-37, modules  1  and  2  may be the same, with module  3  acting as a bridge between modules  1  and  2 . Alternatively, there could be different routings for modules  1  and  2  when there is a module  3 . 
     In some embodiments of the inventions, a path does not go to two modules, but stays on only one module and has on module termination on that module. For example, FIG. 39 shows a system  490  which includes a motherboard  494  and paths as shown. The ECC and/or buffer chips may be used in any environment described herein. 
     The modules may be removable received into the module slots or may be more permanently received by the connectors. 
     The above described embodiments of the inventions may be used in modules with more than one chip on a path on a side of a module. For example, in FIG. 40 chips I 1  and I 17  are in path  1 , chips I 2  and I 18  are in path  2 , etc., on the same side of module  1  There may be chips I 25 -I 32  (not illustrated in FIG. 40) on the other side of module  1 . FIG. 41 illustrates a side view of module  1  of FIG. 40 as well as a module  2  in a system  500 . Chips I 24  and I 32  in module  1  receive the signal on path  8  through short stubs  520  and  522  and package connections P 75  and P 76 . Chips I 24  and I 32  in module  2  receive the signal on path  7  through short stubs  524  and  526  and package connections P 77  and P 78 . System  500  in FIG. 41 is like the system of FIG. 15 except with the addition of the extra chips. Additional chips may be used with other embodiments described in this disclosure. FIG. 42 illustrates path  1  in an embodiment of system  500  in an arrangement similar to that of FIG.  25 . In FIG. 42, path  1  is coupled to package connections P 21 , P 81 , P 82 , P 44 , P 43 , P 83 , and P 84  and on to and from module  2 . FIG. 43 illustrates an alternative embodiment of system  500  in an arrangement similar to that of FIG. 26 in which portions of paths  1  and  2  are shown. 
     FIG. 44 illustrates that chips may be stacked. For example, chip I 24  is stacked on chip I 8  and chip I 32  is stacked on chip I 16  of module  1 . Stubs  556  and  558  are relatively short in comparison with respect to the long stubs of the prior art. However, they may be slightly longer than short stubs  140  and  142 . Note that in FIG.  44  and the other figures, the relative size and lengths of objects in the figures is not intended to show actual relative sizes and lengths, but is rather provided for convenience in drawing. Further, FIG. 44 is schematic in nature. For example, the positions of stubs  140  and  142  is for convenience and not intended to imply an actual position relative to other stubs such as stubs  556  and  558 . Connections P 91  and P 92  are used to provide the signals from stubs  556  and  558  to chips I 24  and I 32 . Connections  564  and  566  couple stubs  556  and  558  to path  8 . Of course, there may be other stubs (not illustrated) to couple path  8  to chips I 24  and I 32  in addition to stubs  556  and  558 . Chips I 8  and I 24  may both be TSOPs (Thin small-outline packages) in which case stubs  140  and  142  may be in different positions than that shown. The inventions are not restricted to being used with any particular type of packaging technology and the drawings should not be interpreted restrictively in this regard. 
     FIGS. 40 and 44 illustrate ways in which additional chips may be included with the module. Other ways may be used. For example, both spaced chips (as in FIG. 40) and stacked chips (as in FIG. 44) may be used on the same module. The arrangements of FIGS. 40 and 44 may be used with on module terminations, on die terminations, or on motherboard terminations, and may be used in connection with any of the various inventions described in this disclosure. 
     As shown in FIGS. 24,  25 , and  42 , the path travels one chip in a horizontal direction. In other embodiments, a path could travel more than one chip in a horizontal direction and/or have various other paths (e.g., zigzag) to couple to additional chips. The path can also travel more than one chip in the vertical direction. 
     FIG. 45 illustrates that in a system like that of FIGS. 30 and 34, except that only one chip per path has the on die terminations enabled. For example, in FIG. 45, the on die terminations of chip I 7  are enabled (on) and the on die terminations of chip I 15  are disabled (off), so that only one chip (I 7 ) on path  8  has its on die terminations enabled. If only one chip has its on die terminations enabled, then the impedances of the enabled terminations might be chosen to be only ½ of the impedance values in the case in which two chips in parallel have on die terminations enabled. 
     Several embodiments of the invention have include loop through sections in the paths such as the short loop through sections of FIG.  15 . However, several of the inventions described herein, may be used in connection with a longer stubbed bus without short loop through sections. For example, FIGS. 46 and 47 illustrate embodiments with longer stubs. FIG. 47 illustrates a system  600  similar to that of FIG. 34, but without loop through sections. For example, path  8  includes a relatively long stub  602  between M 50  and chips I 8  and I 16  of module  1 . Stub  602  has an impedance ZoMod 1 , which may be chosen to be higher than the impedance of ZoMB 1  to compensate for the loading of chips I 8  and I 16 . The value of ZoMod 2  may be the same as ZoMB 1  and ZoMB 2 . In some embodiments, the on die terminations of chip I 7  are enabled, but the on die terminations of chip I 15  are disabled so that only one chip on the path has its on die terminations enabled. 
     In some embodiments, a module has routing paths of different impedances. The different routing paths may be interleaved, but do not have to be interleaved. The routing paths may also different lengths. For example, in FIG. 46, chips I 1 , I 3 , I 5 , and I 7  are coupled to paths  1 ,  3 ,  5 , and  7  and chips I 2 , I 4 , I 6 , and I 8  are coupled to paths  2 ,  4 ,  6 , and  8 . Paths  2 ,  4 ,  6 , and  8  may be like stub  602  in FIG.  47 . Paths  1 ,  3 ,  5 , and  7  may be like path  8  in FIG. 47 between M 23  and chips I 17  and I 15 . Chips I 1 , I 3 , I 5 , and I 7  have the on die terminations enabled (on) and chips I 2 , I 4 , I 6 , and I 8  have the on die terminations disabled (off). The connector contacts are represented as dots M 41 , M 42  . . . M 48 . The portions of paths  1 ,  3 ,  5 , and  7  between connector contacts M 41 , M 43 , M 45 , and M 47  and chips I 1 , I 3 , I 5 , and I 7  have an impedance of ZoMod 2  (like that in FIG.  47 ). The portions of paths  2 ,  4 ,  6 , and  8  between connector contacts M 42 , M 44 , M 46 , and M 48  and chips I 2 , I 4 , I 6 , and I 8  have an impedance of ZoMod 1  (like that in FIG.  47 ). As mentioned in connection with FIG. 47, ZoMod 2  may be the same as that of path  8  on the motherboard. ZoMod 1  may be greater to compensate for the capacitive loading in the chips with on die terminations disabled. 
     In FIG. 46, paths  1 ,  3 ,  5 , and  7  are illustrated as being longer than paths  2 ,  4 ,  6 , and  8 . The reason for this is paths  2 ,  4 ,  6 , and  8  are chosen to be short to reduce loading effects. Paths  1 ,  3 ,  5 , and  7  could also be short (or they could all be the same length, but longer as in FIG.  16 ), but there may not be enough room on the module to make all the paths short so only paths  2 ,  4 ,  6  and  8  are shown as being short. Because chips on paths  1 ,  3 ,  5 , and  7  are not terminated, there is no particular reason to make them short. The length referred to in this paragraph is the length of the path on the module (between the chips and the edge of the module having fingers), not necessarily the total length of the path. FIG. 46 has high and low impedance paths interleaved. That is not necessary. For example, the following non-interleaved pattern could be used: low, high, low, high, high, low, high, low. 
     In the case of on die terminations, when a continuity card (e.g., a short termination card similar to that in FIG. 28) is used, the terminations may be on the continuity card or the BIOS may sense that a continuity card is being used and enable the on die terminations of a chip(s) on another module. 
     The actual impedances mentioned in this disclosure will of course typically not be exactly the impedance values mentioned, but may be within a desired tolerance of the mentioned amount. 
     As mentioned, there are various ways in which the routing between modules may occur and the inventions are not limited to the routings shown in FIGS. 11-14. For example, in FIG. 11, C 5  may be coupled to C 13 , C 6  may be coupled to C 14 , C 7  may be coupled to C 15 , etc. In this case, C 21  and M 21  may be moved to be in alignment with M 14 , C 22  and M 22  may be in alignment with M 16 , etc. 
     As noted, various features of different illustrated or described embodiments can be mixed and matched with various other features illustrated or described herein. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. 
     If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Accordingly, it is the following claims including any amendments thereto that define the scope of the invention.