mark@mips.COM (Mark G. Johnson) (12/06/90)
There are a couple of documents that will be of great benefit to
fledgling backplane bus designers. The most important one is
"Motorola MECL System Design Handbook" by William Blood, available
from your local Motorola salesman. The next most important one
is "Design Guide for SPICE Simulation of Signetics Bipolar Logic".
These contain good introductions to the theory of transmission
lines and the problems of backplane bus operation in general.
If you can spend the money, I highly recommend the purchase of
a specialized simulator called TLC. It handles transmission lines
extremely well. Available from Quad Design Technology, Camarillo, CA.
Several connector vendors have written technical reports on the
electrical properties of their products; most contain a page or
two with an explicit SPICE equivalent circuit. One place to start
is the low priced "Metral" connector from DuPont, which is one of
the accepted standard connectors for FutureBus. As a general
rule, connector performance goes hand-in-hand with price; you
want speed, you pay buxx.
One of the things that becomes important at high speeds is crosstalk.
Just as you need to ground every other wire in a ribbon cable to
reduce crosstalk, you also need to provide large numbers of AC grounds
in the pin-matrix of a backplane connector to reduce crosstalk. There
is a tradeoff of pin-usage (some would say pin-wastage) versus
signal quality; electrically best would be a N grounds per 1 signal
pin (N >> 1) but that's expensive. A compromise is a checkerboard
pattern of AC grounds and signals, which uses up half the connector
pins for AC grounds. Or as another poster has indicated, you can
employ differential signalling: use two wires to send one bit.
Halves the width of the bus but vastly improves noise immunity.
--
-- Mark Johnson
MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086
(408) 524-8308 mark@mips.com {or ...!decwrl!mips!mark}