[SI-LIST] : New VME Backplane -- Star Layout

Mike Mayer ([email protected])
Wed, 16 Sep 1998 09:05:53 -0500 (CDT)

There is an article in the latest EE Times (September 14, 1998 p. 57)
about the star-layout VME backplane. It is ligh on technical details,
so I am having trouble understanding some of the quoted comments from
the developers of the backplane.

Background: the a 21-slot VME backplane is about 18 inches long. The
traditional backplane is routed straight across, with each slot
forming a stub. The star configuration routes each slots backplane
signals individually to the center of the backplane where they all
connect together.

In the article the developer says this makes every board see a lumped
load instead of a transmission line. "We think no impedance is the way
to go" "Don't solve the problem: eliminate it. Don't fix the
transmission line; get rid of it."

What I don't understand is the view from a driver in slot 2, for
instance. The driver there will see 2 inches of trace on the board,
the connector, then ~6.5 inches of trace to the star at the center of
the backplane, Traces from the star go to the other 20 slots. How can
the 6.5 inches of trace on the backplane not be a transmission line?
And how can the star be anything but a poor termination? Drivers are
typically 74F series parts, so the rise time is probably around 2nS
(although the spec does not disallow faster rise times).

Mike Mayer                               Artesyn Communication Products, Inc
                                         Madison, WI

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