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From: padpowell@wateng.UUCP (PAD Powell[Admin])
Newsgroups: net.arch
Subject: Re: Cray vs ICs, continued
Message-ID: <88@wateng.UUCP>
Date: Wed, 15-Jun-83 08:12:39 EDT
Article-I.D.: wateng.88
Posted: Wed Jun 15 08:12:39 1983
Date-Received: Thu, 16-Jun-83 06:59:33 EDT
References: <3018@utzoo.UUCP>
Organization: U of Waterloo, Ontario
Lines: 27

The whole area of high performance digital logic borders on black art,
with a healthy dab of applied mathematics, mechanical engineering, and
love of experiment.

For instance, if you send a signal down a line, it will keep on going
until it comes to a discontinuity.  Whereupon really ugly things will happen.
The usual analogy is that of a wave in a bathtub.  Start the wave at one end,
and watch what happens as it hits the other: reflections, etc.  To keep
this crud to a minimum in electronic circuits, it is neccessary to terminate
a line with the same impedance.  Lines are usually designed to be purely
resistive, so terminating them with a resistor is called for.  Now think
about all those little chips, about 3mm across, needing all those huge
resistors...  So the cunning chip designers put the resistors on the chip,
making the chip slightly larger, and increasing its power consumption.

Unfortunately, this is only the first step. Due to other problems, it is usually
not possible to run a single line for a signal.  It becomes neccessary, for
distances over about 4 inches, to use two lines, and drive them differentially.

The design of the hardware for this monster has to be seen to be believed.
Strangely enough, I have the feeling that it is one of the last areas to
be impacted by the CAD market...  After all, who would invest the hundreds
of man years neccessary to design a system which would automatically lay out
a board?  Answer: IBM, Japan Inc., ITT, and other people...  Hot topic
for people interested in wierd and wonderful CAD applications.

Patrick Powell, U. Waterloo