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Re: ARGH, weird SCSI I/O errors on second/third drive on Debian/PPC


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

     Tilting at Windmills for a Better Tomorrow.

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