Path: utzoo!yunexus!ists!jarvis.csri.toronto.edu!rutgers!ucsd!swrinde!gem.mps.ohio-state.edu!ctrsol!cica!iuvax!purdue!bu-cs!dartvax!eleazar.dartmouth.edu!jskuskin
From: jskuskin@eleazar.dartmouth.edu (Jeffrey Kuskin)
Newsgroups: comp.arch
Subject: Re: Surges
Message-ID: <17067@dartvax.Dartmouth.EDU>
Date: 17 Nov 89 21:52:15 GMT
Article-I.D.: dartvax.17067
References: <503@ctycal.UUCP> <15126@haddock.ima.isc.com> <7000@pt.cs.cmu.edu> <150@csinc.UUCP>
Sender: news@dartvax.Dartmouth.EDU
Reply-To: jskuskin@eleazar.dartmouth.edu (Jeffrey Kuskin)
Organization: Dartmouth College, Hanover, NH
Lines: 40

In article <150@csinc.UUCP> rpeglar@csinc.UUCP (Rob Peglar x615) writes:
>In article <7000@pt.cs.cmu.edu>, lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes:
>
>> 
>> This is well-known as one of Seymour Cray's concerns. In the Cray-1,
>> he was worried that standing waves could develop in the copper ground
>> plane of a circuit board. His answer was to make a machine with power
>> demand that was (I believe) completely independant of the data
>> flowing through it. 
>
>This was a worry one machine before the Cray-1 - the Cyber 7600.
>
Quoting from my Computer Architecture textbook:

"Cray-1 modules are 6 inches wide.  The distance across the
board is about a nanosecond which is just about the edge
time of the electrical signals.  Unless due precautions are
taken, when electric signals run around a board, standing
waves can be induced in the ground plane.  Part of the
solution is to make all signals paths in the machine the
same length.  This is done by padding out paths with foil
runs and IC packages.  All told, between 10 and 20 percent
of the IC packages in the machine are there simply to pad
out a signal line.  The other part of the solution is to
use only simple gates and make sure that both sides of
every gate are always terminated.  This means that there
is no dynamic component presented to the power supply.
... The final result is that there is just a purely
resisitive load to the power supply."
 
From:  "The CRAY-1 Computer System," by Richard M. Russell.
       CACM, January, 1978.
 
Maybe someone could comment on to what extent similar
issues are important today as microprocessors approach
the 50-100 MHz. range.
 
-- Jeff Kuskin, Dartmouth College
 
E-Mail:  jskuskin@eleazar.dartmouth.edu