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Date   : Sun, 11 Jan 2004 16:11:52 GMT
From   : Pete Turnbull <pete@...>
Subject: Re: IDE Interface for BBC

On Jan 10, 10:25, Mike Tomlinson wrote:
> In article <040110060610@...>, Jonathan Graham Harston
> <jgh@...> writes
>
> >  I don't know how hard it is
> >to find suitable SCSI/SASI drives for the Acorn card,
>
> Depends on which Acorn card you're talking about.  [I realise you -
JGH
> - probably know this, but am including it for completeness and so it
> gets archived]
>
> Hard disks for the BBC B used a SASI interface attached to the 1MHz
bus,
> and an Adaptec SASI-to-MFM converter board to which an MFM hard drive
is
> connected.  I think Acorn's SASI card used all-TTL logic, and quite a
> lot of it too, but I also had a Watford SASI card which reduced the
chip
> count to about 8 using PALs.  The Adaptec converter board was about
4" x
> 6", was packed with chips, and had, if I remember right, an 8086 CPU
and
> its own firmware in ROM which in theory made it more powerful than
the
> machine it connected to.

Actually the Adaptec is a SCSI card, unlike most of the other similar
cards (eg the Xebex S1410 series) which were around at the same time,
and I think that's one of the reasons Acorn used it.  Unlike other
cards, it stores the geometry information on the drive (so the host
doesn't need to be programmed with heads/sectors/cylinder/RWCC/etc
information) and implements the Mode Sense and Mode Select commands.
 It's not a very full command set compared to later devices, and isn't
quite the Common Command Set (which came much later, in fact it wasn't
ratified until SCSI-2).  It also supports the ATN line, which is what's
missing from a SASI interface (the early Acorn 1MHZ Bus host adaptors
don't, though).  The Adaptec manuals (and also Acorn's) do refer to it
as a SCSI device, not SASI: "The Adaptec 4000/4010/4070 Hard Disk
Controller interfaces ... to any ANSI X3T9.2 SCSI standard host adapter
interface".

Adaptec ACB4000 series cards were used in a few other contemporary
devices, such as Sun workstations, where they were handled by the same
SCSI driver as other embedded-SCSI drives.

So I'd guess there's a possibility you could find a SCSI drive to work
with the Acorn interface, but it would have to be one that didn't rely
on CCS, only on basic SCSI-1 ops, nor on the ATN line being
implemenmted -- and that almosat certainly means an older SCSI-1 drive.
 Also you'd have to be confident you could do a low-level format on it,
because Acorn's software expects 256-byte sectors, which is unusual.

BTW, the Adaptec boards are a bit bigger than 4" x 6" -- they are
designed to have exactly the same footprint as a 5.25" drive, so
they're 5.75" x 7.75"

> Acorn's FileStores (Econet fileservers) came in two flavours.  The
basic
> Filestore used floppy discs and a hard disc was an add-on option. The
> earlier E01 had a 1MHz bus and used the same hard discs with SASI
board
> and Adaptec converter as were connected to the BBC B.  The later E01S
> used SCSI drives.  The SCSI interface in the E01S I imagine would
have
> been very similar to the Master Domesday interface, especially as it
was
> based on the Master circuit.

I remember the day Laurie Hardwick proudly showed me the first
Filestore running two real embedded-SCSI drives.  Wow, a total of a
massive 60MB!  I think that was on an EO1 (not E01S) but it might have
been a development unit.  The "S", by the way, stood for "stacking".
 There was no provision for adding additional hard drive units to the
early Filestrores (E01 and E20), and the reason for the change was make
that possible.  IIRC one of the differences is the addition of the ATN
line to the SASI/SCSI interface.  I can't remember if they also moved
the host adaptor out of the E20 into the E01  -- if they did, the
external expansion bus would have been SCSI on the E01S instead of the
(modified) 1MHz Bus on the E01.  Anybody got one to check?

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York
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