Hi,
> I am in the process of doing a re-design on a multi-board system that
I
have working right now. We are redesigning to convert to USB2.0 and to
add
[SNIP]
Thanks,
> Theron Hicks
Just some anecdotal evidence. We had a fairly simple multi-card PCI setup where we had to use very short line-lengths to an on-card PCI bridge for each card because the signals with simple daughter cards were unusable. Known impedance and proper termination is critical even at the PCI's 33/66 MHz. 200 MHz would be many times worse.
As a straw man, consider PCI. It runs a bit faster than 40 ns, but it doesn't get anywhere near 18 slots.
Usual practice with multidrop back plane busses is to make that stub as short as possible and live with it. (no termination) It screws things up, generally by looking like a small cap which reduces the effective impedance of the backplane. (Same math as a row of memory chips on a bus.)
Sometimes with things like this, you can gain a factor of 2 by putting the master card in the middle and splitting the bus into two. Or you split it into 4 and interlace the cards on each side.
One thing to consider is putting terminators at each end of the backplane and using something other than LVTTL.
I expect you will be doing lots of simulations. Please let us know what you decide to build and/or how well it works.
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