Re: GS Crashing: copying files and Apple hi-speed SCSI (?)

[dempson@actrix.gen.nz (David Empson)]

In article <32j5tl$n8v@eis.calstate.edu>,
Greg R. Achenbach <gachenb@eis.calstate.edu> wrote:
> Boy, there is some really good info coming through on this topic.  I just 
> realized that this might be good to put in the FAQ.  

An excellent suggestion.

> Let me summarized what I think I have gleaned from the posts, and then 
> have people correct me if I am wrong.
> 
> 1)  It seems that proper SCSI function depends on 3 things - termination, 
> termination power, and magic.
> 
> 2)  The RamFast provides both termination and termination power.  The 
> Apple HS SCSI provides termination, but not termination power.  The 
> original Apple SCSI provides neither.

I have heard that recent Apple HS SCSI cards have a diode on them to
provide termination power.  This could be added to old cards.

(I don't think this is a good idea, though - SCSI termination requires
up to 500 mA, which is all the power that is officially available to
the slots on the 5V rail.)

> 3)  Drives vary in whether they provide termination, termination power, 
> and how much magic is necessary to get them to run.

Correct.  Nearly all SCSI drives have an option for internal
terminators.  Some older drives may have the termination resistors
soldered on, but all recent drives have sockets for them.

> If the above are true, then it would be great to have info in the FAQ on 
> how to tell if a drive is terminated and supplying termination power.

Termination is easy to tell, if you have the bare drive mechanism.
Look for a row of three yellow or black termination resistor packs (9
pins each, in a single line), probably socketed, or three empty
nine-pin sockets.

Termination power is not easy - it depends on the drive mechanism.
With my Quantum, I had to use a continuity tester to find where the
termination power was coming from (I haven't had a close look at
surface-mount capacitors, fuses and diodes before, and I didn't know
which components were which).

Some drives have a jumper which selects whether they supply terminator
power.  If the drive has an option for it, it _should_ involve a power
diode (to prevent the TERMPWR line backfeeding into the drive's 5 volt
rail), and there may also be a fuse and a capacitor, as in my case.

I haven't looked closely at other mechanisms yet to see how they do it.

> But, if it is not supplying termination power, I would like to have
> detailed instructions on how to modify the drive so that it does.

I don't know the correct component types, but I can probably find out.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand