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.
"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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