Re: [SI-LIST] : Star connection termination

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From: Bob Perlman ([email protected])
Date: Sat Nov 27 1999 - 11:13:58 PST

Hi -

>Massimo Polignano writes:
> Does anybody out of there have any suggestion about the following basic,
> but really important and frequent, problem: how a star-point (a point from
> which many traces going to other ICs start) must be series-terminated?
> Do one have to put a resistance equal to the characteristic impedance of
> the trace before the point from which the traces start? Or is it better to
> insert a resistance on each line coming up from the star-point? In the
> latter case, what is the best value for the resistances?

I gave a presentation on series termination at the 1999 PADS
International Users Conference earlier this month. Among other
things, I discussed the star topology. You can find the
presentation, a companion paper, and three AVI files that illustrate
wavefront travel at:


One other thing to note: to me, "star" means branching out to
multiple loads at the end of the net, and placing a single series
termination at the driver. It has been pointed out in other
messages that one may also star at the driver end of the net,
provided that each branch gets its own series termination. This
alternate form of star can work very well, but has at least one
gotcha: the driver has to be strong enough to drive multiple
transmission lines in parallel.

Take care,
Bob Perlman

Bob Perlman
Cambrian Design Works
Digital design and signal integrity consulting

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