Re: Quantum Fireball with Ramfast

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

In article <476mvo$a2q@blackice.winternet.com>,
Peter Handel <zelix@parka.winternet.com> wrote:
> In article <31OCT199519243614@jetson.uh.edu>,
> DAVID FRETZ / RACHEL FRETZ <st3b1@jetson.uh.edu> wrote:
> >Umm actually could just be a problem with the reselection of the
> >Quantum being on.  I have heard these problem mainly with Amigas
> >that had to turn off reselection to get working.  However I am not
> >sure if reselection is a SCSI-2 specific or not.  What I mean is
> >that both controllers are both SCSI-1 and the drive is SCSI-2 and
> >may not have to do reselection. 
> 
> Since the drive works on my Apple High Speed SCSI card, would this imply
> that the drive is just a regular SCSI drive, and not a SCSI-2 one?

No.  Current Quantum drives are definitely SCSI-2.

When talking about hard drives, SCSI-2 is generally upwards-compatible
from SCSI-1.  The SCSI-2 standard is "tighter" than the SCSI-1
standard, particularly in the defintion of the set of SCSI commands.
Most of the time, a SCSI-2 drive will work with a SCSI-1 controller.
The reverse combination is less likely to work.

"Reselection" is a feature of both SCSI-1 and SCSI-2.  It is not
supported by the RamFast, since the RamFast doesn't support the
arbitration phase, and has no SCSI ID of its own.

The Apple High-Speed SCSI card definitely supports this, and the
original Apple SCSI card should do so.

If reselection is enabled, the drive will disconnect from the host
while a command is being actioned.  When the command is complete (e.g.
a READ command is ready to transfer data) the drive will "reselect"
the host (i.e. wait for the bus to be free and try to connect to the
host, which involves sending the SCSI ID of the host) and transfer the
data.

While the command is being processed, the SCSI bus can be used for
other functions, e.g. the same host could send a command to another
drive, or a second host (sharing the bus) could send a command.  This
allows latency to be reduced, e.g. by setting up a read on each of two
drives - by the time the second command has been sent, the first drive
should be ready to transfer data.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand