Date   : Sun, 21 Dec 1997 22:59:29 +0000
From   : Mike Tomlinson <jasper@...>
Subject: Re: Hard Drives

In article <9712211241.ZM7537@...>, David Lomas
<davidl@...> writes
>
>Mmm. Does that mean that the 1MHz bus card _might_ drive a SCSI hard disc,
>rather than going through the second adapter to a MFM drive?

It's possible.  I understand that hardware-wise, SASI is almost
identical to SCSI, but the software protocols may differ.  A visit to
comp.periphs.scsi and a scan of the FAQs there might be useful.

SASI was proprietary, but formed the basis of the eventual SCSI-1
standard.

As far as I can tell, the 1Mhz bus converter is a simple form of SCSI,
with no processor, memory etc. unlike any other SCSI card.  Therefore,
all the work of setting up registers and transferring data is going to
have to be done in software.  The more I think about it, the more
impressive it is that Acorn managed to fit hard disc control, floppy
control *and* a filing system into the ADFS ROM.  Then again, that might
be why they used an intermediate control card from Adaptec - reducing
the ADFS's function to saying "get me sector 0", "write this stuff to
sector 32767", etc.  instead of having to do all the work of running the
disc controller itself.  The Adaptec card had an 808x (8085?) CPU, ROM
and RAM of its own.

> Or did your
>comment above that mean the 1MHz bus card was actually a SASI card?

I understand it is.  I might be wrong.

>Any more details on it anyway (e.g. web addresses etc.)?

Mark Usher has mentioned this URL:

http://www.nvg.unit.no/bbc/doc/HardDrive-v1.0.zip

but I too would dearly like to know more about this.  There's been the
odd bit of speculation on this list but what would be really interesting
would be some reminisces and/or concrete information from people at
Acorn directly involved with designing and building the thing.

-- 
Mike Tomlinson