Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!hao!hull
From: hull@hao.ucar.edu (Howard Hull)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Trouble with right mouse button
Summary: Check the 8520 chips too
Keywords: Paula 8520 Mouse hazard
Message-ID: <1485@ncar.ucar.edu>
Date: 26 Feb 89 19:55:37 GMT
References: <2073@pembina.UUCP>
Sender: news@ncar.ucar.edu
Reply-To: hull@hao.UCAR.EDU (Howard Hull)
Distribution: na
Organization: High Altitude Observatory/NCAR, Boulder CO
Lines: 96

I just the other day frazzed an Amiga 1000.  Here in Colorado, static
electricity generation is an extreme winter hazard.  I am always very
careful about static electricity, having developed a number of "peculiar"
habits, such as touching another person in some unrestricted part of
their anatomy or touching a light switchplate or a lamp, or the finger
stop on my dial telephone before handing them any electronic parts or
circuit boards, even those in protective packaging.  I never reach for
the keyboard on approaching the Amiga without touching the desk lamp
first.  At work we use static mats on the floor and benches to provide
grounding and to suppress generation of static charge.  A wrist strap
is absolutely essential when working on exposed electronic assemblies,
but usually turns out to be overly restrictive in the home computing
environment.  I do not normally regard a well engineered microcomputer
(such as the Amiga) as an "exposed electronic assembly" though.  It is
apparent that some care has been taken in the packaging of the Amiga to
reduce the hazards associated with inadvertent static electricity generation
and discharge.

The EMI shield is very solid protection for most of the machine, and the
keyboard has evidently been treated with a somewhat conductive coating or
is made of a material that has slight conductive properties (or so it seems
to my experience).  On occasion I have snapped an arc to the #1 mouseport
shell.  This is usually a surprise considering the precautions I take, but
I have done it three or four times in four years of working with my Amiga.
There has never been a problem from it, though, since the shell is grounded
to the EMI shield inside the machine.

However, the Amiga does have an unexpected point of vulnerability.  The
shield provided at the plug end of the mouse is not manifested in the
construction of the mouse provided with the Amiga 1000 computer.  I was
on the telephone when I noticed my printer was going ape on a DPaintII
print-out, so I lunged for the mouse.  Ker-Snapo!  A centimeter long
arc jumped from my hand to the longitudinal seam in the mouse.  This was
followed immediately by the GREEN screen, so I knew I had big trouble on
the docket (failure to gain memory access).  Disassembly of the mouse
revealed that there is a connector header with the crimp ends of eight
mouse wires within a few millimeters of the tongue and groove casing fit.
It must take about ten thousand volts to get to it, but I had taken on
easily twice that (somehow) in the lunge.

Wondering what might have failed, I grabbed my A1000 schematic set
and began looking for what might have taken it tough.  As in your case,
I was astonished to see the P0X, P0Y, P1X, and P1Y lines going directly
to the 8364 MSI chip (Paula) without a buffer IC in between.  There was
a 220 ohm resistor in series with each line, and there was a 0.047 uF
bypass capacitor at the MSI chip pin.  Since Paula is directly addressed
on the blitter bus, I felt certain that I had a dead Paula chip.  I have
two Amiga 1000 machines, so I was well prepared to find out what had died.
Realizing that I may very well have a propagative failure [a propagative
failure is a failure in one or more parts that results in destruction of
the swap test object immediately upon power-up; propagative failures can
ruin every test part you swap while trying to determine a single-point
failure], I nonetheless began to swap stuff from one computer to the
other.  A reboot not only did not show a green screen, the LED didn't flash.
Changing Paula had no effect.  I finally had the machine stripped down to
almost nothing with still no result when I decided maybe it was better to
plug the parts one at a time into the working machine rather than vice-versa.
You'd be amazed at how few parts need to be in the Amiga 1000 and still allow
it to boot to the point that it can yet send the boot audio to the monitor.
In my case, the Michigan Insider was gone, and I had removed the internal
disk drive and the Paula chip.  It would not boot without the A1000 daughter
board, though.

The dead chip turned out to be one of the [infamous] 8520 pair.  The 8520
wiring goes nowhere near the mouse port, according to my schematic set.
I did have a Canon PJ1080A printer plugged into the parallel port, but
all of the peripherals were plugged into the same AC common strip.  The
modem is isolated (except for stray capacitance).  It was the parallel
port 8520 that got nailed.  I am thus disposed to the notion that the
0.047 uF bypass capacitors that protect Paula had in fact been instrumental
in the destruction of the 8520 (U6P).  The 8520 must have a weak substrate
diode protect structure to be vulnerable to this kind of incident (elevation
of the common potential above that of the I/O pins).  Paula, the mouse, and
the sixteen cent SN74LS157 mouse mux chip survived ok.

What to do?  Well, don't grab your mouse when you're running around the house
with an extra Coulomb or two of miscellaneous kitty-cat ions.  What I did was
bend one end of a paper clip to just the right length and stick it in the
little hole along side the forth pin up from the tail of the mouse (the black
wire - it's connected to the shield part of the mouse plug), arranging the
clip so that it lies parallel to the cover seam.  That way, the next time
I arc it, it will dump to the Amiga RFI shield, and the ground elevation
will be confined to the mouse proper.  That's not to say it will avoid all
damage.  It's just that the damage will be likely restricted to the mouse.
As soon as I get some new 8520 chips, I'll test this...  (Gutsy-est move I
ever saw, man...)

Until I have a chance to test it, note this DISCLAIMER: What you do to your
mouse is your own business, and none of mine.  This is just my best guess
of what to do to get an edge up on the situation.  If you try this on your
Amiga and get unsatisfactory results, I am here and now disclaiming any and
every responsibility for whatever happens that displeases you.  In the mean
time, I advise CBM to DO SOMETHING about this - please get the mouse some
underware.
						Howard Hull
						hull@hao.ucar.edu