Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!ico!vail!rcd
From: rcd@ico.isc.com (Dick Dunn)
Newsgroups: comp.sys.ibm.pc
Subject: engineering margins (or, living on the edge...)
Message-ID: <1989Nov10.214341.4071@ico.isc.com>
Date: 10 Nov 89 21:43:41 GMT
Distribution: na
Organization: Interactive Systems Corporation
Lines: 58

Just because yours works...

It never ceases to amaze me how cavalier some people are about ignoring the
margins for proper operation engineered into products--or for that matter,
how much time they'll spend rationalizing their decisions to "live on the
edge" by doing so.

Yes, a 20-MHz 386 will probably run just fine at 25 MHz.  It may even run
at 30 MHz.  Yes, 100-ns DRAMs may run at 80 ns.  Yes, most MFM drives can
be run as RLL instead.  So why is the 20-MHz processor marked as 20
instead of 25, etc.?

It's so that it will be reliable!  That means a lot more than just saying
that it will work in your machine today...it means that it will work in any
properly designed machine, both now and years from now when some of the
components have drifted in value a bit.  It means that it will work in a
machine which just happens to have gotten the worst-case mix of parts at
the edges of their rated tolerances.  It means that it will work if the
machine gets warm or cool.  It means that it will work in an aging machine
with worst-case components in a room at 85 F.

The fact that you can run your 20 MHz chip at 25 really doesn't mean squat.
Individual examples of pushing tolerances don't warrant generalizations.
It may work that way forever, or it may fail tomorrow.  If it works, you're
lucky, not clever.  If it works for a year and then fails at the demo in
front of your biggest customer, you were unlucky...and stupid.  The
engineers who designed the chip and worked out the manufacturing process
planned for enough margin to allow things to work with normal variations.
You threw away the safety margin.  Now, maybe it's worthwhile to accept the
risk of system failure to get extra speed or capacity--that's a decision
you may want to make...just make it with your eyes open.  Don't think
you're getting something for nothing.

BUT WAIT, you say:  What about those motherboards we've been hearing about
that advertise 25 MHz but come with a 20 MHz processor?  Simple--there are
bad engineers who design boards.  Running a CPU at 25% over its rated clock
speed is just plain sloppy.  It's a way to make a cheaper, poorer product.
Some of them will fail eventually; the failure rate will be low, although
it will be a lot higher than a board built with a proper 25 MHz processor.
If you get such a board, you're taking a higher chance that you'll have to
return the board or get them to exchange the processor for the right part,
leaving you in the meantime with a flaky or inoperative system.  Again,
that may be what you want, trying to get the most speed possible within a
limited budget and accepting a greater chance of problems as part of the
compromise...just go into it aware of the tradeoff.

My personal view on the motherboard business is that I wouldn't buy one
from a company that put in a lower-speed-rating processor than the mother-
board clock speed, EVEN IF they agreed to upgrade the processor on request.
Why not?  Trust.  It's easy to see the discrepancy in the processor clock
speed since it's stamped on the chip package.  It's not easy to find out
where else on the motherboard they may have cheated on proper timing
tolerances--too many gate delays or whatever...but if I see it once, I'll
suspect it's happened more than once.  Speed isn't the only thing that
matters to me.
-- 
Dick Dunn     rcd@ico.isc.com    uucp: {ncar,nbires}!ico!rcd     (303)449-2870
   ...Keep your day job 'til your night job pays.