Re: ARGH, weird SCSI I/O errors on second/third drive on Debian/PPC
Jessica,
Probably the best thing to do to ensure you don't have extra termination is to
temporarily disconnect your CD drive entirely. The most likely place where the
terminator would be to start with is on your CD drive. If you find this helps,
then figure out how to disable the CD drive's terminator and hook the drive
back up.
There is almost certainly some software that can tell you whether you are
terminated correctly. But I don't know what it would be called specifically.
The command:
dmesg | less
might be informative, in case the driver has reported any error messages. Try
doing that right after you boot. Also look in /var/log/syslog and
/var/log/messages
As for the terminator power options, your terminator does need to be powered.
There are different places where the power can come from though. One of the
wires on the SCSI cable is the power for the terminator. Again some CD drives
supply terminator power, but if you're disconnecting the CD for testing you
won't have that. I don't know whether your host bus adapter (probably the MESH
chip) supplies it.
I don't think there is a problem with having two devices supplying terminator
power, so it is probably best to enable it on your drive.
You might also try using a separate terminator, one that plugs into the cable.
There are devices called "active terminators" that provide a higher quality
termination than the regular passive terminators. Do a google search for "scsi
cable guy" and you'll find a good supplier of these. I forget his website but
that's how I always find him.
Also, importantly, the terminator must be on the last connection on the cable.
That means if your plugging your drive into the middle and the CD is at the
end of the cable, you should enable the termination at the CD drive rather than
your disk drive. Alternatively you can temporarily disconnect the CD and plug
your drive into the end of the cable.
What terminators do is absorb electrical signals that would otherwise strike
the end of the wires and reflect backwards and interfere with the normal
signals. The terminator has a resistor resistor on each wire whose resistance
is the same as the "impedance" of the wire. This has the effect of making the
wire appear to be infinitely long when viewed from the middle of the wire, so a
signal sent down the wire never comes back.
If you terminate a cable in the middle there will still be the loose end of the
cable that causes reflections.
If you terminate twice I think there will be absorption of some of the normal
signal that will degrade it.
However, it is my experience that Mac SCSI tends to work OK with no termination
at all (besides what is normally provided by the host bus adapter). I really
suspect your problem is not really termination, but I'm sorry to say I don't
know what else it could be. It would be best to eliminate termination as a
possibility.
--
Michael D. Crawford
GoingWare Inc. - Expert Software Development and Consulting
http://www.goingware.com/
crawford@goingware.com
Tilting at Windmills for a Better Tomorrow.
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