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

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

In article <32e6vs$8vp@paperboy.gsfc.nasa.gov>,
Bill North  <north@spso.gsfc.nasa.gov> wrote:
> In article <CuDLFI.9x4@actrix.gen.nz> David Empson,
> dempson@actrix.gen.nz writes:
> >In article <32bb82$flq@paperboy.gsfc.nasa.gov>,
> >Bill North  <north@spso.gsfc.nasa.gov> wrote:
> >> You don't say how many devices you have daisy-chained
> >> to the scsi card. If more than one, this could be a
> >> termination problem. Ignore the scsi card docs.
> >> Terminate both the first and last devices in the chain.
> >
> >That may help, but it definitely isn't correct.
> 
> Correct? Who's talking correct? With scsi, I'm talking magic. :)

Yes - "SCSI Termination ... Welcome to the Twilight Zone".  :-)

> >I have three drives
> >on my Apple HS SCSI card, and only the last one is terminated.  It
> >works fine, even if I turn off any one or two drives.  The drives are
> >a Quantum 240LPS, a SyQuest 44 MB cartridge drive, and an old Rodime 60
> >MB.
> 
> I also have three drives: a new Quantum 85, an old Quantum 80, and
> a Syquest 44. The 85 and 44 worked great in any combination with
> only the last drive terminated. Adding in the 80, I _must_ terminate
> both the first and last or disaster strikes. These are on the Apple
> HS scsi.

I've been investigating further.  It turns out that my Quantum 240MB
drive is supplying terminator power (it has a diode and fuse on the
TERMPWR line, using surface-mount components on the circuit board).
This explains why I have _never_ had any problems using this drive on
any SCSI bus I have plugged it onto, using either version of the Apple
SCSI card or the RamFAST SCSI card.  As long as I have the "correct"
termination, it works perfectly.

I'm not sure if my SyQuest supplies terminator power, but I know the
Rodime does (I usually have it switched off, though).

> >The original Apple SCSI card isn't terminated, so you should have a
> >terminator at the end of the chain, and if you have two drives, then
> >also place a terminator between the first drive and the SCSI card.
> 
> Aha! But the docs explicitly say to not terminate between the card
> and first drive if you have more than one drive. That's the part you
> should ignore. Terminate both first and last, not just last as the
> docs say. Do exactly as you say above.

No, don't do this unless you absolutely have to.  My SCSI bus does NOT
work properly if I do this!

If you have to put an extra terminator on, it doesn't matter where it
goes - it is just providing extra loading on the bus signals.  In
theory, it should work just as well if you put two terminators at one
end of the chain.

If it doesn't, then one of your drives has something non-standard
about its SCSI connection (e.g. the SyQuest 44 is notorious for SCSI
oddities - I have several friends who cannot get a SyQuest to work
properly with certain drives unless they are in a particular order).


In the case of the original Apple SCSI card, the terminator between
the card and the first drive is a compromise - it is as close as you
can get to that end of the bus.

One of the main purposes of the terminators is to prevent signal
reflection, by absorbing data pulses as they reach the end of the bus.
An extra terminator in the middle of the bus will increase the signal
loading on all pins, increase terminator power consumption, etc.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand