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

Scott McMorrow (scott@vasthorizons.com)
Mon, 29 Mar 1999 15:33:52 -0800

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All,

Long bus or star depends on required settling time and whether
the signal has to have a guaranteed monotonic transition at all
receivers.

Without perfect termination in a classical daisy chained bus, there
will be loads of reflections. There will also be loads of reflections
in a star topology when the edge rates are fast. However, due to
the symmetric balanced nature of the star (and it gets better the
more legs there are) the star will reach final state at all points
quicker than on a linear topology.

With drivers capable of reaching incident wave switching, a linear
bus terminated to Z0 on both ends is the best solution. And, it is
generally the only solution if monotonicity at all receivers is a
requirement. (Although there are also some clever non-perfectly
terminated solutions, also.)

No matter what, a solution, if it exists, depends on the driver strength
across corners, and the edge rate.

Scott

Chris Bobek wrote:

> >How do you propose to keep several legs ALL less than 12mm long?
> >(at least 60ps/cm, risetime of less than 300ps, line length
> >Tr/4 => 1.25cm) Note that this is a foolishly optimistic
> >calculation.
>
> Where did you get 300ps for the risetime? I was using 3ns.
>
> Here's a situation: Assume you have a trace that's 20" long. Obviously,
> this needs to be terminated for a 3ns edge. For the sake of argument,
> assume you calculate a "short" trace to be 3". That is, you can get by
> without termination (not counting ringing) for that "short" trace.
>
> Now, instead of laying out the board with this 20" trace that is daisy
> chained to each device, you supply a separate path to each device from the
> point-source that are each 3" long with no termination. Is this
> configuration considered "bad" or undesireable?
>
> What I realize here is that you will have a reflection that hits one of the
> devices at the end of the 3" trace. It will reflect all the way back to the
> other 4 devices, which are each 6" away. Because this length is considered
> "long" it will cause problems, no?
>
> So, is everyone basically saying that you should always *try* to daisy chain
> devices, no matter how long the chain is? (Incidentally, I made a decision
> to keep the daisy chain and terminate it).
>
> Thank you all for your help,
>
> Chris
>
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