Re: [SI-LIST] : RE: [SI-LIST]: Long bus or star?

Shayle Hirschman (shayle@mho.net)
Mon, 29 Mar 1999 13:46:04 -0600

How about if each leg of the star is short enough to not be considered a
transmission line relative to the signal's rise and fall time?

Would reflections then add, acceptably, while the signal is still
transitioning? Or would this lead to too large an amplitude due to multiple
simultaneous reflections from each leg?

*************************************************************
Shayle I. Hirschman, Senior Engineer
Managing Director
Digital Design Solutions
http://www.digital-designs.com
shayle@mho.net
Phone 901/759-1802 Fax 901/759-2324

At 10:58 AM 3/29/99 -0800, you wrote:
>>Chris Bobek wrote:
>
>
>>I have an address bus that is 75ohm, and 19" long. There are 8 loads >that
>>are daisy-chained along the trace. The edge from the driver (which
>>is at one end) is 3ns.
>
>>I was wondering if I would be better off with a star pattern, where >each
leg is short (less than 4"). Would this work and not require
>>any termination?
>
>
>>Thank you very much,
>
>
>Star patterns are a nightmare. There are so many reflections in
>the system that it easily becomes unstable. If the star is not
>perfectly balanced your signal integrity will degrade rapidly.
>Just the process variations for each agent can be enough to degrade
>your signal integrity to an unacceptable level. Furthermore, if
>you have a GTL-like bus a star wont work because it would require
>too many pull-up resistors. Star topologies can be made to work,
>but not without a lot of pain. They should be avoided for a high
>speed bus unless all other options have failed.
>
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