Adding a hard disk to the BBC Micro

Acorn did offer a hard disk upgrade for the BBC Micro, but it cost around £1000 - somewhat out of the range of hobby users. Torch offered something similar too.

However a group of users in the Midlands (UK) put together some instructions on how to roll your own hard disk upgrade, and also provided a number of utilities. These have become known as the "Tubby Utils" after Mike Tubby (G8TIC) who was one of the authors.

Time passes...

Please remember that this documentation is dated 1987. We're in the days of ST-506 disk drives of around 20MB in size. The hard disk system you'll learn about is a host adaptor hanging off the 1MHz bus which talks an early dialect of SCSI to an Adaptec ACB 4000 "SCSI to ST-506" bridge board.
If you want a more "up to date" solution, then some options are:

Using other Bridge Boards

Acorn's Hard Disk upgrades used the Adaptec ACB 4000 "SCSI to ST-506" bridge board. If you use one of these, then you'll have fewest problems - for example, the *VERIFY command in ADFS 2.03 will just work (it uses SCSI command $2F, which not many other boards support). However you may find yourself with a different bridge board - for example, Torch used Xebec S1410 bridge boards.

The challenge here is that as far as the ADFS is concerned, the drive is just a sequence of sectors (addressed solely by sector number). But the drive itself is organised as Cylinders/Heads/Sectors. Something has to do the translation - and that's where problems can arise. The Adaptec board is clever, and it stores the drive geometry on a 'hidden' area of the drive. At power-up or BREAK, the Adaptec controller loads the drive geometry from this hidden area.
Many other bridge boards will at power-up or BREAK default to the geometry for the (then) industry standard 5MB drive , and rely on "something else" to set the drive geometry correctly. Something in the Torch software did this for the Xebec S1410 bridge board - but I never had the rest of the Torch system to find out what. A possible solution to the problem is a !BOOT file to set the drive geometry - but this !BOOT file must be located entirely within the first 32 sectors of the drive in order to guarantee that it's found whilst the controller is using the wrong geometry!
Before I obtained an Adaptec ACB 4000 board, I used a Xebec S1410 bridge board. To make life much easier, I patched the firmware to change the geometry which it uses as default to exactly match the geometry of my drive.

Patching the Xebec S1410 firmware

The firmware on my board was in a 2732 EPROM labelled 104778D. The drive geometry is stored at addresses $0FF6 to $0FFD, with an EPROM checksum at $0FFE. I found files named XEBEC_D on my hard disk which were a ROM dump and a manual partial disassembly.
I also found files named XEBEC_F on the hard disk. On this version, the drive geometry is stored at addresses $0FE5 to $0FEC, with an EPROM checksum at $0FED. I've no idea where I got these files from.
The standard 5MB drive geometry is stored as:
0099Cylinders($0099 = 153)
04Heads($04 = 4)
0080Reduced Write Current Cylinder($0080 = 128)
0040Write Precompensation Cylinder($0040 = 64)
0BECC Length($0B = 11)
153 cylinders * 4 heads * 33 sectors * 256 bytes per sector = 5MB.
Change to suit your drive.
My recollection was that I'd disabled the checksum routine, but it appears that I was wrong. SCSI command $E4 is 'perform controller diagnostics', and this reports on whether the checksum is correct. I've obviously recalculated the checksum after changing the drive geometry bytes so that SCSI command $E4 reports 'checksum OK'. Not that anything in real life will worry about this...

The Tubby Utils

I've scanned the documentation for the Tubby Utils. This documentation is well worth a read if you're considering a do it yourself hard disk upgrade.

I didn't have much luck OCRing the scanned document, so they are all PDF documents. If anyone wants to OCR them, or type them in by hand...

Introduction

There was originally a 2 page introduction. The second page was a list of information being sought, and details of a pending change of address. Since this was almost 20 years ago, I've not included that page.
And yes, I scanned it upside down!

Manual

The real documentation on how the hard disk system can be put together.

Program Listings

The utility programs. You probably don't want to type them in, most are linked below.

'Missing' pages

After the program listings, there were two pages giving contact details for the group members, details of suppliers of component parts, etc. As these are nearly 20 years old, I've not scanned them ("Hello, do you still have for sale the E-Type Jaguar you advertised for £500 in Exchange and Mart in 1973 ?").

Artwork

The artwork for etching your own host adaptor, and its circuit diagram. Because the board layouts have been printed, photocopied, and then scanned they are probably not directly usable.

The Tubby Programs

More help

If you're interested in the above, and you aren't subscribed to bbc-micro@cloud9.co.uk - the BBC Micro Mailing List then subscribe immediately.
Andrew Benham (G8FSL), Southgate, London N14 4XD, United Kingdom

Mail Andrew Benham (adsb@adsb.co.uk)