Date   : Tue, 28 Apr 2009 22:48:53 +0100
From   : ken.lowe@... (Ken Lowe)
Subject: ADFS Drive Numbers

On Monday, April 27, 2009 7:16 PM, Pete Turnbull wrote:
> The winchester host adaptor (the little board) is just a bus buffer and
> address selector, essentially it's a 1MHz Bus to SCSI adapter.  It
> doesn't have an ID, AFAIR.

Agreed - the host adapter board has no device ID, although ISTR that it is 
used for certain control line encoding / decoding during some of the bus 
phases.

> I thought you could have two Adaptecs on the bus, but I may be wrong.
> ADFS has provision for 4 SCSI devices, but I'm not sure if they have to
> be on the same SCSI address or if it knows to use LUNs 0 and 1 on
> devices 0 and 1. For those who don't know, narrow SCSI supports 8
> devices (of which at least one is normally the host controller,  in this
> case the Beeb) and each device can theoretically have up to eight
> Logical Unit Numbers attached.  Here, the ACB-4000 is device 0 and the
> two winchesters are LUNs 0 and 1 on device 0.  It only supports those
> two LUNs.

IIRC, ADFS is hard coded to select SCSI device ID = 0. Unfortunately, most 
'modern' narrow SCSI drives (with embedded SCSI controllers) don't appear to 
support connecting multiple drives together in a way that they can be 
individually addressed by a uniquie LUN using the same SCSI device ID. This 
means that it's normally only possible to connect a single modern SCSI drive 
to the BBC via the host adapter as LUN 0 on device ID 0.

However, with some patching, the ADFS can be configured to address upto 4 
separate SCSI device IDs via a single host adapter. This is useful if you 
want to connect 4 separate SCSI drives to the BBC. I currently have a setup 
with 4 Fujitsu m268x SCSI drives connected to the BBC. Each drive has a 
uniquie SCSI ID (0, 1, 2 & 3), and can be accessed from the BBC as ADFS 
drives 0 thru 3, using only LUN 0 in each case. I use the Fujitsu drive, 
because it's still relatively easy to get hold of, and can be easily 
formatted to 256 byte sectors.

A lack of free space on the ADFS ROM made it quite difficult to patch, and 
I'm not sure that I caught every single drive switch. I'm sure that someone 
with the right skills could make a much better job of patching the ROM than 
my attempts. Happy to share my work with anybody who might be willing to 
improve what I've started...

-Ken