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

Hall, Stephen H ([email protected])
Mon, 29 Mar 1999 10:58:11 -0800

>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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