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

Mellitz, Richard (richard.mellitz@intel.com)
Mon, 29 Mar 1999 12:42:38 -0800

Star patterns work :-). The image that comes to mind in the one of a clown
balancing on one leg on a high wire, with a stick in each hand and mouth and
spinning plates on top of sticks. Star or comb routing is an exercise in
delicate balancing.

... Rich

-----Original Message-----
From: Scott McMorrow [mailto:scott@vasthorizons.com]
Sent: Monday, March 29, 1999 2:43 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : RE: [SI-LIST]: Long bus or
star?

<< File: Card for Scott McMorrow >> Steven,

Actually, if you take a look at the Xeon AGTL+ bus you will
see that one valid topology for it is a modified star. All
legs on
the bus have Termination resistors with a value of 75 ohms.

Regards,

Scott

"Hall, Stephen H" 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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