On Friday 02 August 2002 23:59, Terry Koyama wrote:
>All,
>
>Finally solved the problem. It was the SCSI Cable.
>Before trying a different SCSI cable, I
>transferred the SCSI controller and tape drive to
>a Windows 2000 machine. I updated the firmware
>and ran all of its diagnostic tools. Everything
>ran fine. I even made a few backups with no
>problem. Remember, this was with the original cable.
>
>I then transferred the SCSI controller and tape drive
>back to the original Linux machine (with the original
>cable. Still the same problem. So I asked a
>friend to borrow his SCSI cable and poof...it worked!
The original's warranty, probably written in Rhode Island Red or
Plymouth Rock chicken blood, ran out. Those things need a warranty
extension via the usual chicken sacrifice from time to time.
Really stubborn cases will require a goat. :-)
On a more serious note, scsi cables are by nature a 'transmission
line', with their perfomance and history firmly buried in classic
transmission line theory. And as such they must be terminated in
their characteristic impedance at the physical ends, and *only* at
the physical ends of the cable. This impedance is typically for
flat cables, about 120 ohms and is implicite in the scsi spec
primarily because that was the characteristic of the flat cable
available at the time (and still is).
The usual resistive terminator package gives about 132 ohms as its a
quasi-parallel combination of a 330 ohm going to ground, and a 220
ohm going to the termination power supply, usually a diode drop
below the 5 volt line, usable but not perfect, and totally blown to
hell by someones plugging in the term packs swapped end for end,
thereby reducing the standing voltage for a logic 1 from 3.0 volts
to only 2 volts which totally destroys the noise margin of a TTL
circuit.
A lashup that uses 'active' terms will come quite a bit closer since
the buildout resistor there is usually 120 ohms on the money.
The length spec allows many meters of cable in a properly setup
system, but if the last device is a foot from the end of the cable
just because the connector location was handy, that last foot of
unterminated cable is going to ring like the liberty bell did, once
for every signal transition. Folks who don't understand that can
send for the chickens and the voodoo spells, but they'll be wasting
their money.
I've also had relatively poor luck with the so-called high quality,
round (because they are all rolled up) cables, and have learned to
replace them on sight if there is any hint of scsi bus problems.
One of the toolkits a techie needs is a roll of scsi cable in the
correct width and a bag of connectors to match, which in 10 minutes
will make you a better cable than 99% of what you can buy at
Staples et all, and for 20% of the cost of the blister-packed
versions.
Then, when all good design practices have been put in place, is the
time to sacrifice that goat. :-)
[...]
--
Cheers, Gene
AMD K6-III@500mhz 320M
Athlon1600XP@1400mhz 512M
99.09% setiathome rank, not too shabby for a WV hillbilly
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