Re: Re[4]: Installation 1.3 failed (SOLVED: tape & ultra scsi)
From: JIM_BURT_at_nass-fx@nass.usda.gov
> Amidst all my problems, I had long ago turned off my Exabyte tape drive
> residing at the end of my scsi daisy chain, EXPECTING THE SCSI DAISY CHAIN
> WOULD ACT AS IF I HAD TERMINATED AT THE LAST DISK DRIVE.
> I NOW KNOW THIS IS NOT TRUE.
Don't do that. Active termination doesn't work when powered off. The
SCSI bus driver transistors in the device may shut down to a low-impedance
state, partially "shorting out" the bus. The powered-down bus driver
transistors can present a non-linear load to signals on their outputs,
causing much more noise than there would otherwise be on the bus.
> I tried making my scsi cables strait,
> And I would guess that scsi cable twists matter little so long as
> one does not form 360 degree curves.
I don't think twists in the wire matter one bit. The important thing
about transmission lines (that's what the SCSI bus conductors are at 5
MHz and up) is that they must be properly terminated at each end to
prevent reflection of the signal back from the end of the wire, and
they must have loads with the right impedance. This means you leave all
devices turned on, you remove the internal termination from devices that
aren't at one end of the bus or the other, and you use correct active
terminators at both ends of the bus. In many cases the SCSI controller
is one end of the bus and provides its own termination.
> I tried APS Technologies SCSI Sentry, a digital active terminator.
Active termination is a must. You can get ones without lights for half
the price of the SCSI sentry.
> I DISABLED "support for ultra speed".
You might get this to work with all-ultra-SCSI peripherals on the bus, and
make sure your terminator is the right one for ultra-SCSI.
I find some SCSI cables don't work with some controllers. My SGI is
horrible about this, and my Adaptec 1542 had some problems, too. The
Belkin "Space Shuttle" cable works great on everything, but it may cost
$40 for a cable. Most computer stores have this as their "premium" SCSI
cable.
Thanks
Bruce
--
Bruce Perens K6BP bruce@pixar.com 510-215-3502
Finger bruce@master.debian.org for PGP public key.
PGP fingerprint = 88 6A 15 D0 65 D4 A3 A6 1F 89 6A 76 95 24 87 B3
--
TO UNSUBSCRIBE FROM THIS MAILING LIST: e-mail the word "unsubscribe" to
debian-user-request@lists.debian.org .
Trouble? e-mail to templin@bucknell.edu .
Reply to: