Very Interesting problem. I'll be looking forward to
seeing if anyone has a really clever solution to this.

It is not clear to me that you can do this without an
impedance mismatch. Four possible alternatives come to
mind, none of which are particularly attractive!!!

1. From each driver, immediately fan out to 16 individual
    traces, one for each other board. The obvious problems
    are (1) area and connector pins (2) the driver is now
   driving 16 parallal 50 Ohm traces (hard to do) (3) the
   unfilled slots would probably need to be terminated in
   some way (although you probably have this problem now)

2. Think of the cable tv analogy --- a 75 Ohm cable going to
   an indeterminate number of vcr's, tv's, cable boxes and
   fm receiver inputs. That is done with an antenna (read
   passive impedance) matching circuit. Problem is probably
   building one with wide enough bandwidth.

3.  Think of the Ethernet analogy --- a 50 Ohm trace along the
   backplane terminated at each end. Here you probably need a
   buffer at each connector, either on the backplane or on the
   card itself. Or, finally

4.  Think of the other network analogy where all nodes feed to
   a single router which conditions the signal and feeds all
   the other nodes. This would require additional circuitry
   on the mother board or one aditional card.

These are inefficient, brute force solutions for connecting
all your nodes without reflections. I hope someone has an
easier solution!!

Doug Brooks



At 01:07 PM 2/14/98 -0500, you wrote:
>We have a product that is a shelf with a backplane into
>which plugs 2 power supplies and 8 modules. The 8 modules
>are all in parallel to one another and are capable of
>talking to one another through the backplane.  A single
>module contains 2 boards that are each 50 ohm boards.
>These two board are also in parallel with each other.
>
>16 connectors in the backplane for 16 PCBs each
>50 ohms. For any type of configuration, a single
>50 ohm board is capable of talking to as many as
>2*N + 1  other 50 ohm boards where N = # modules in shelf
>and  1 = the other board in its own engine.
>
>In other words,
># modules in shelf      # of other 50 ohm boards one board
>                                  talks to through the backplane
>                          dependent upon configuration of
>                          shelf.
>
>1 module                         1 other 50 ohm board
>2 modules                        3 other 50 ohm boards
>3 modules                        5 other 50 ohm boards
>  ...                   ...
>8 modules                       15 other 50 ohm boards
>
>2 connectors of the same module are about 3/4 inch apart.
>2 connectors of different modules are about 5/8 inch apart.
>
>        Module #1          Module #2
>      |<-------->|<------>|<-------->|
>          3/4"      5/8"
>
>      0          0        0          0
>      0          0        0          0
>      0          0        0          0
>      0          0        0          0
>      ...        ...
>
>What should the impedance of the backplane be?
>50 ohms?  100 ohms?  800 ohms?
>
>
>Regards, Doug
>
>Private emails welcome
>
>mailto:[log in to unmask]
>
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****************************************************
Doug Brooks, President              [log in to unmask]
UltraCAD Design, Inc.           http://www.ultracad.com

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