From: bredell@tdb.uu.se (Mats Bredell)
Subject: Re: Upgrades not limited to si's?
Reply-To: Mats.Bredell@udac.uu.se
Organization: Uppsala University Computing Center (UDAC)
X-Newsreader: TIN [version 1.1 PL6]
Date: Fri, 20 Nov 1992 11:57:55 GMT

In article <chips-091192131141@maverick.wr.tek.com>, chips@tek.com (Chip
Schnarel) wrote:
> 
> Look people,  do you really know what you're doing when you modify your
> machine?  Are you considering the consequences?  I'm not flaming those
> who've performed the upgrade, indeed I may perform it myself when my
> warranty expires.  But this whole-sale discussion of sticking a new crystal
> in anything should be couched in extreme caution.  (I'm NOT picking on
> Chewy, I'm talking about this whole thread.)
> 
> I've been waiting for someone with manufacturing experience to speak up,
> but since no-one has I'll stick my feet into my own mouth.
> 
> If you choose to upgrade your crystal speed, by all means go ahead, but
> before you make that decision, consider the following.
> 
> Electronic circuits are designed with a particular clock speed in mind. 
> Some margin is built into these designs.  The need for margin comes from
> two primary sources.  1) The speed of each component varies from machine to
> machine.  Margin assures that the worst possible combination of components
> still works.  2) The speed of all the components within a system varies
> with changes in temperature and powersupply voltage.  Margin assures that
> the machine still works under the worst possible combination of influences.
> 
> In manufacturing, the product is tested, typically over temperature.  On
> the old compact Macs this used to be a twenty-four hour burn in.  I don't
> know what they do now.  When you modify your Mac do you test it for
> twenty-four hours?  Do you test it over the whole temperature range?
> 
> If you don't, you might create a Mac that works fine right now, but gets
> flakey next summer when the room temperature is eighty degrees.  Or a Mac
> that gets flakey as the components age and the individual timings shift
> slightly.
> 
> This thread began with a discussion of Apple designing to 25MHz but
> shipping at 20MHz.  Maybe so, maybe not.  Who knows?  I do know that the
> CPU in my Mac is labeled -20 not -25.  Same with the FPU.  So at least two
> critical components are not rated for this speed.  Also, the manufacturing
> process tests at the operating speed, not the design speed.  No
> (production) Mac IIsi is tested at 25MHz until you do it yourself.  Let's
> say you modify your machine and it works.  It may be that you have removed
> most of the margin and that it works right now, but if anything else
> changes, you have no margin in reserve.  Your machine will become flakey or
> might just die.
> 
> It's your machine.  Do what you want with it, but make your decision from a
> position of knowledge.
> 
> Chip
> 
> BTW - Rosin core solder is hydroscopic.  That means that over time it soaks
> up water and becomes slightly conductive.  At 50MHz this can (and HAS in my
> experience) effect operation.  I hope that you have all cleaned the rosin
> off with alcohol before you replace the board.

The CPU is almost like an elevator: the sign says "Maximum x persons", but of
course they have put in some margin. If you enter x+1 persons into the
elevator it will probably work, but don't complain if it doesn't.

It's the same with computers. If you upgrade the computer will probably work,
but perhaps not always. The next time you get a bus error, it could actually
be the speed-up problems that are causing it. Don't come yelling at Apple or
your software dealer if htis happens ...

/Mats
-- 
Mats Bredell                                   Mats.Bredell@udac.uu.se
Uppsala University Computing Center (UDAC)     Ph:  +46 18 187817
Department of medical systems                  Fax: +46 18 187825
Sweden                                         Think straight - be gay!