Date   : Tue, 06 Jul 2004 18:10:04 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Acorn Winchester unit

I accidentally emailed this to Johan instead of the list, so for what
it's worth (and thanks for pointing out my mistake, Johan) ...

On Jul 5, 22:58, Johan Heuseveldt wrote:
> On Mon 05 Jul, Mark Usher wrote:
>
> > There are usually parameters in one of the command blocks on the
SCSI unit
> > that you can set to the sector size. Just look at the SCSI spec, I
can't
> > remember off hand just at the mo which one. Usually drive specs
will tell
> > you if the drive has the capability or not, most of the older
SCSI-1 drives
> > do.
>
> And others seem - I feel - to be optimistic too, for connecting a
SCSI
> drive directly to the Host-Adapter, and let it work with ADFS.
>
> I don't want to spoil everyones appetite, but generally speaking this
> is /not/ possible.
>
> Why? Because SCSI drives won't allow a low level format, and there is
> no way to use a logical model of 256 bytes sector on top of the
physical
> 512 bytes per sectors /in/ the drive.

Hmm... The truth is not quite so absolute.  Many older drives permitted
exactly what Mark described, and most modern ones do, too, though
admittedly many just use the Format Unit to change the defect lists,
rather than changing sector sizes.

The ones in my RAID array certainly can change sector sizes, because
they can use sectors larger than 512 bytes (common sizes are 520 and
538 bytes, and yes they can be reformatted).  Adtron solid-state SCSI
"disks" specifically support 256-byte sectors.  SCSI Seagate Wren
drives supported 256-byte as well as 512-byte sectors.  At least one of
my more recent Seagate manuals lists the relevant entry in the Format
Device page as changeable.  There's a linux program to do an LLF (and
it reportedly takes a long time to do so, which is what I'd expect if
it really was doing a low-level format).  It's said to work on Seagate
and IBM drives, though I've never used it.  I have a low-level format
program that runs on my SGI machines, though I've never used it for
anything other than default parameters.

I suspect that SCSI drives that can support 256 bytes per sector may be
uncommon or even rare nowadays, but not non-existant.

> If it was /that/ simple to connect a SCSI drive to the host adapter
> directly, it would be widely in use for twenty years now! :-)

It was widely in use in Acorn, though admittedly that was using drives
that would now be regarded as very old!

> Another way is finding the original info about the -4000 board, as
this is
>quite intelligent. Perhaps a logical 256 bytes per sector is
supported?

That wouldn't help.  It doesn't connect to SCSI drives, it's a
SCSI-to-ST412 bridge.  It can format a drive with 256, 512, or 1024
bytes per sector, but it's irrelevant to the use of SCSI drives.  The
Xebec 1410A can handle both 256-byte and 512-byte sectors, but it is
also a bridge (SASI-to-ST506/ST412).

I have the manuals for the ACB4000 and X1410A, and the Adaptec one is
online at bitsavers:  http://www.bitsavers.org/pdf/adaptec/

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York