Well, the point I was making was that financial
pressures sometimes make things that started out well
designed not well built. Interesting you brought up
Muntz - although his televisions were made from pure
evil (running tubes in parallel to get the 6.3v, fer
instance), he did make cheap TV's that less wealthy
people could buy, that worked fairly well as long as
you were in a big city, and until it broke. I have the
schematics for the Xerox/Diablo HyType II printer here
- made by a company with quite considerable
engineering and manufacturing know-how - and what you
describe happened in this design, someone took out too
many bypass caps, then someone ECO'd them back in. And
this was not a "cheap" printer. Obviously, someone
tried to shave every nickel. I know that in super-high
volume products, engineers do have to fight for every
single part - because this cost board space, stocking,
etc, and so on. A 0.005 cent resistor actually costs
quite a bit more more on the board. But this is a
classic engineering/cost engineering problem, common
in any kind of engineering/manufacturing endeavor. If
engineers had their way, blank check, to make a
"perfect" product, no one would be able to afford any
of the "goodies" we have today. But, I state the
obvious.
I marvel at how well the HP2644A I have is built. But
really, did it have to be built so well that it would
be working 27 years later, long after it was obsolete?
The original purchaser paid for a lot more than they
used. HP used to make the best stuff. But if they
still made stuff that way, they would be out of
business. Such is the world we live in.
--- Eric Smith <eric_at_brouhaha.com> wrote:
> > I asked how come there were only a few
> > bypass caps instead of the traditional one per
> chip,
> > and the chief engineer (who wrote The famous
> > Microcomputer Design book, Don Martin of Martin
> > Research), who definitely knew how to build
> things,
> > told me that the board would start with them all
> in.
> > Then they would be removed until the board stopped
> > working. Then they'd put that one back in!
>
> The Earl "Madman" Muntz approach. Just because you
> think he knew
> how to build things didn't make that good
> engineering practice.
> You should have *more* bypass caps than you need
> under normal
> conditions, because there will be real-world
> conditions that
> don't match your lab bench.
>
> There are definitely ways to determine a reasonable
> minimum number
> of bypass capacitors as an engineering exercise, but
> this is MUCH
> more complicated than taking them out until the
> product breaks, and
> adding one back in. In particular, it's not just
> the total count
> of bypass capacitors that's important. They have to
> be in the
> right places. Just because they're conceptually all
> in parallel
> doesn't mean that they can be anywhere on the board.
> The power and
> ground traces (or planes) have resistance and
> inductance, which is
> part of the reason that you need bypass capacitors
> in the first place.
> So the bypass capacitors must be physically near the
> components that
> have large current fluctuations.
>
> "One per chip" or "one per two chips" are rules of
> thumb that can be
> used with some kinds of logic chips to get
> reasonably bypassing
> without having to do detailed analysis. Yes, you
> can get by with
> less, but not simply by randomly removing some and
> hoping for the
> best.
>
> How do I know this is a problem? From personal
> experience with
> two companies that left out bypass capacitors
> despite the
> objections of the hardware engineer, because it
> still seemed to
> work OK in the lab. In both cases, I had to debug
> the resulting
> problems, which were blamed on software. At first I
> believed it
> was the software, but eventually I discovered that
> adding the bypass
> capacitor back into the circuit fixed the problem.
> Once I'd finally
> proven this to management, it was ECO'd back in. In
> one case, a
> bunch of inventory had to be reworked (expensive!),
> and in the other
> case, the inventory was scrapped. But the worst
> part was the units
> already in the field. RMAs cost a *lot* of money.
>
> > Seriously,
> > they would look at the power supply and use just
> > enough with a little extra margin.
>
> You can't determine how many bypass capacitors you
> need
> by looking at the power supply.
>
> > It's my understanding that electrommagnetic
> deflected
> > vector displays take very, very high-power
> deflection
> > coils and drivers, and this is where the real
> money is
> > in these units. I don't know if the Imlac is
> > electrostatic or electromagnetic deflection
> > (electromagnetic, I suspect).
>
> That has *nothing* to do with using lots of RC
> delays in
> the design of the digital logic of a processor. I
> won't
> go so far as to say that doing so is always wrong,
> but
> you would need an awful lot of justification for
> doing it.
> From what I've seen, the Imlac was in fact poorly
> designed,
> just as others have asserted. They might have
> reduced
> their manufacturing cost, or they might not have.
> But
> they decreased the reliability of the machine
> considerably.
> Usually a tradeoff like that winds up being a false
> economy
> for the company -- penny wise, pound foolish.
>
> Eric
>
>
>
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Received on Sat Sep 14 2002 - 22:13:00 BST