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

Peterson, James F (jfpeterson@space.honeywell.com)
Wed, 16 Sep 1998 11:06:35 -0400

Mike,
I believe you are correct, it is still a transmission line. The comments in
the EETimes article sound like they're coming from a marketing type.
But that said, I do believe that the settling times on this new VME
backplane could be improved, and the loads could be more consistent. Every
board would now see the same load. Currently, the board in slot 1 really
sees a different load than the board in the middle of the backplane. Also,
if the board in slot 1 was the driver, the board in slot 2 sees a very ugly
signal compared to the board in slot 21....
A major drawback on this approach is the huge routing resources needed to do
a star topology, instead of a daisy-chain, on a 21 slot VME BUS!
Best regards,
Jim Peterson
jfpeterson@space.honeywell.com <mailto:jfpeterson@space.honeywell.com>
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
813-539-2719

----------
From: Mike Mayer [SMTP:Mike.Mayer@cp.artesyn.com]
Sent: Wednesday, September 16, 1998 10:06 AM
To: si-list@silab.Eng.Sun.COM
Subject: [SI-LIST] : New VME Backplane -- Star Layout

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
http://www.artesyn.com/cp

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