UNIX-pc interference (Was Re: external disk RF signal)

[Gil Kloepfer Jr.]

In article <1399 at mtunb.ATT.COM> jcm at mtunb.UUCP (was-John McMillan) writes:
>Wrapping them in duct tape?  O brother!  What is the attenuation of duct
>tape and how was it grounded?  I'm sure EE's will respond to the issues
>here, so I'll skip on.

Well, it seemed like a good idea at the time (embarassment)...  FYI: it
was grounded to the 3B1's case and had a grounded copper wire going all
the way up the outside of the cable (don't ask...).  It helped a LITTLE,
not much.

>RF isn't the source of all evils.
[Case studies about RF propogated through AC line deleted]

I thought about this once and tried using a good isolation transformer
to see what would would happen.  No luck.

I also did some experiments last night on this subject which I think might
be of interest to the net.  I stand corrected on the source of the
interference -- the cables *ARE* the source.  Furthermore, what seems
to be happening is that they are, for some reason, leaking miscellaneous
signals (*NOT* disk signals) from inside the 3B1 and the cables are some
kind of antenna for them.  Since I'm far from an EE, I couldn't comment
more except for observation.  When the 2nd disk cables are disconnected
and wrapped in aluminum foil with the 3B1 diag booted, the interference
is minimal.  As soon as the aluminum foil is removed and the cables are
extended, the interference gets bad.  No disk activity is in progress
because the diag is booted.  When you hook the cables up to things (like
the other hard disk and its power supply), *NOW* the AC line becomes
even more of an antenna and the TV gets completely blasted.

>So: neither underestimate the effects of RF propagation from unshielded
>cables, nor depend on the INTERNAL isolation of your equipment -- in
>either direction!  At home, use a spike-arrester with built-in noise
>filters to minimize line-carried interference.  The TV interference
>on your neighbor's set may be RF, but on YOURS it may be house wiring.

One other interesting reminder -- for those of you out there hooking
peripherals or other stuff to your 3B1 -- BUY THAT $5 OUTLET TESTER
FROM SEARS OR RIPOFF SHACK and USE IT!  You may someday save your computer!
ESPECIALLY if you don't have some kind of electrical isolation between
these peripherals.  I speak from experience (blush).

-------
Gil Kloepfer, Jr.          U-Net: {decuac,boulder,talcott,sbcs}!icus!limbic!gil
ICUS Software Systems      Voice: (516) 968-6860 [H]   (516) 746-2350 x219 [W]
P.O. Box 1                 Internet:  gil at icus.islp.ny.us
Islip Terrace, NY  11752   "Life's a ...  well, you know..."