Date   : Sat, 27 May 2006 15:23:33 +0200
From   : D.G.van der Pol <danielg@...>
Subject: Re: Back in circulation

Op 17-mei-06 om 11:52 heeft Ian Wolstenholme het volgende geschreven:

> The problem [briefly] is that practically every 50-pin SCSI disc drive
> I have tried connecting up to the Acorn Winchester host adapter (apart
> from "known" good ones like the Rodime RO652) does not work.  At best
> the Beeb ignores it, looking at the floppy drive when you try to mount,
> or at worst I get all kinds of whining noises coming out of the 
> various PSUs
> running the show and the ribbon cable between the host adapter and the
> SCSI drive starting to melt.
I had this once in a RISC-PC, it is caused by the fact that two devices 
on the SCSI-bus are supplying termination power. Because the 5V is 
never exactly 5V, there will be an increddible current flowing through 
the ribboncable, which heats and melts. This is not so much a problem 
of SCSI-drives, it is just poorly set up.

> I think the low level format/256 bytes per sector issue is a bit of a 
> red
> herring because if ADFS won't attempt to mount the drive in the first
> place, it doesn't really matter how it is going to be formatted.
ADFS can not mount an unformatted harddrive, the formatting program 
does not need the disk to be mounted, it just needs to send it 
SCSI-commands to format and data to build a valid ADFS-volume. After 
that is done, the disk can be mounted by ADFS.
The 256 bytes/sector is absolutely an issue. SCSI uses block-transfers 
for data. If a harddisk is formatted with 512bytes/sector, all 
block-transfers expect 512 bytes. ADFS does only cope with 256 bytes, 
so there are two possibilities: Rewrite ADFS to support 512 bytes 
transfers. This would probably double the amount of memory space needed 
by ADFS.
Or try to find an harddisk that allows a low level reformat with 256 
bytes/sector. I found documentation about a few drives that support 
sector sizes from 256 til 1280 bytes per sector. But I never could lay 
my hands on such drive.

> I've never had any difficulty with ST506 drives and a separate 
> Winchester
> controller board (although admittedly the controllers support 256 bps).
> With that setup, ADFS tries to mount the drive, finds it isn't 
> formatted and
> reports a disc error and then you can go in and format it.
That is the way it should work with SCSI-drives as well.

> With SCSI drives, missing out the Winchester controller, the result is
> invariably nothing or flames.
My experience is a bit different: I can sent commands to the drive, 
have the led come on, send it to sleep etc. But an effective low-level 
format never resulted in something else than 512bytes/sector.... And 
after that melding wire in the RISC-PC, I have never seen flames.... 
(By the way, the RISC-PC, Cumana SCSI-card & Syquest harddrive all 
survived the incident.)