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From: hull@hao.UCAR.EDU (Howard Hull)
Newsgroups: comp.sys.amiga
Subject: Re: PALs,jumpers and expansion on the Amiga 1000.
Message-ID: <883@hao.UCAR.EDU>
Date: Sun, 6-Sep-87 12:00:18 EDT
Article-I.D.: hao.883
Posted: Sun Sep  6 12:00:18 1987
Date-Received: Sun, 6-Sep-87 21:12:27 EDT
References: <8709010613.AA05631@cogsci.berkeley.edu> <1367@ihlpm.ATT.COM> <2141@tekigm2.TEK.COM>
Organization: High Altitude Obs./NCAR, Boulder CO
Lines: 65
Keywords: Two on the Bus and one inside
Summary: "When the going gets weird, the Weird go pro..." H.Thompson

In article <2141@tekigm2.TEK.COM>, phils@tekigm2.TEK.COM (Philip E Staub) writes:
> In article <1367@ihlpm.ATT.COM> nevets@ihlpm.ATT.COM (Steven R Ringwood) writes:
> >In the end I used wire-wrap wire, a solding iron, and an old alligator clip.
> ...
> >
> >Did it work, for me YES.
> >
> I'm glad to know that Bryce's fix worked for you, but personally, I'm a bit
> surprised to find it to have been effective with such small wire. Presumably

I am reminded of the time I had to determine an ECO on a flakey async counter
board at a field site.  I knew it had to be a layout problem, as the wire-wrap
prototype had worked ok.  I had told the layout artist "Make sure you assign
enough pins for the grounds.  You need at least two, including the ones on
the end of the connector" instead of '...and make sure you assign both sets
of the end pins to ground and grid your ground conductors'  (This was an all
digital board).

Looking at the board, there was only one 20 mil ground etch running all the
way around the outside of the board to ONE pin at each end.  There were two
other pins about a cm in from each end that were added to access a couple of
trapped IC pins that needed grounds.  Being in the field as I was, I got out
the braid, copper flashing, and mica.  I tried bunches of things with the
ground system, and while most things helped, I couldn't get it to do better
than one fault per hour (groan).

So by then I had gone on to the question of the bypass caps and looking for a
problem there.  Then I accidently soldered a 0.1 ufd stacked ceramic cap
*between the interior and exterior ground pins* - that's ground to ground,
y'all.  At that point, the problem vanished!  I rechecked by replacing the
cap with braid strap, and the circuit faulted within 30 seconds of turnon.
By then I was out of time and had to rush to make a flight.  At any rate,
there were no further reports of trouble with the system while we worked on
the problem at the home plant.  Now that sure would have made a funny looking
ECO, wouldn't it...  That's why they call those people layout ARTISTS.  Of
course, we had to redo the layout to kill the spook.

It's clear that in the relam of DC vs. 20 MHz., the same board looks quite
different to the sensing and referencing electronic elements.  The high
frequency grid resonances can play an important part in data reliability,
and the pin to pin DC drop can displace the amplitude of the ringing (which
some people, in my opinion, mistakenly call "noise" - it's not noise, it's
really only "aberration").  The result of too large a grid (even using 200 mil
lands) will be a flakey design which will not function reliably if produced
in mass quantity.  Indeed, memory and IC counter manufacturers carefully
tailor the rise time of their products so that they are in concert with the
set-up, propagation delays, and desired speed of the product line, but not so
fast as to generate multi-megaHertz ringing responses in boards that employ
good practice and proper layout techniques.

For designers to be assured of the correct function of high-speed digital
boards, they must either provide a good ground grid or go to multilayer,
and correctly utilize an interior ground sheet.  Even that implementation
requires some artistry.  There is an Intel App Note in the Memory Products
section about the former method, and a good essay in a back-issue of DTACK
Grounded (Hal Hardenberg) about the latter.  Unfortunately, I am away from
work and my references right now, but I can respond to mail requests from
any who want the references (unless your site is behind RUTGERS or CMU -
both of which are zero probability addresses from here, it seems).

						Best Regards,   Howard Hull
[If yet unproven concepts are outlawed in the range of discussion...
                 ...Then only the deranged will discuss yet unproven concepts]
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