Date   : Tue, 06 Jul 2004 18:58:36 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Acorn Winchester unit

On Jul 6, 10:09, Jules Richardson wrote:
> On Mon, 2004-07-05 at 18:22, Pete Turnbull wrote:

> > Xebecs do as well, though if you don't initialise them they default
to
> > something like an ST506.
>
> For the benefit of those who don't know... ST506 is an interface
> standard as well as the model number of the original Seagate drive
which
> used this interface - just to make things nice and confusing! :-)

A lot of people confuse ST506 and ST412.  Neither is actually a
standard, they're the model numbers of Seagate drives that kind of
defined "the way to do it".  I specifically mentioned ST506 because
that was the first mass-produced small winchester, and it has one
particular feature: step pulses are not buffered.  That means the
controller must issue step pulses no faster than the drive can step
from one track to the next, which is quite slow (about as fast as a
floppy, in fact).  ST412 differs mainly in supporting buffered seeks,
so the controller can send a series of step pulses, faster than the
drive can handle an individual strack step, but the drive keeps a count
and ends up in the right place.  This is faster because the drive can
start stepping slowly, and once the heads have started to move, can
accelerate and turn the stepper motor faster (and then ramp down the
speed towards the end of the seek).

Using ST506 step rates is not only slow, but noisy and less friendly
for the drive (in the long term, becasue of vibration) -- but usig
ST412 rates on a drive that doesn't support them just results in lots
of errors.

> > dd won't change the sector size, though.
>
> Nope, but you could use it in place of a native formatter on the BBC
to
> set up a drive (albeit with 512 byte sectors) with the appropriate
data
> structures such that you could then drop it straight into a BBC and -
> assuming a bodged ADFS that discarded the latter 256 bytes per block
on
> a read - expect it to work. Saves having to write a new formatter on
the
> BBC side of things at least.

Yeah, OK, I didn't realise that's what you meant.  Given that a Beeb
doesn't need the amount of filestore my WinXP junk does, wasting half
the capacity of a drive is not a problem.  I think the smallest SCSI
drive I still have is about 100MB, and the Seagate one I have on my
Beeb (was a Rodime until it had a catastrophic crash a couple of years
ago) is 20MB, and little more than 1/3 full.

BTW, I think you've seen that drive, Jules, and it has a homebuilt,
home-etched 1MHz Bus PCB in it, which I made about 1985/6.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York