Date   : Tue, 10 Feb 2004 18:21:55 GMT
From   : Pete Turnbull <pete@...>
Subject: Re: Discs

On Feb 10,  9:16, Richard Gellman wrote:
> Jules Richardson wrote:
>
> > > Disc drives themselves come in different flavours too. PC 5.25"
floppy
> > > drives generally don't work with BBC discs, although they can be
used,
> they
> > > have a different track stepping,
> >
> > For clarity, you presumably mean that standard PC disk formats use
a
> > different geometry (sector size, tracks, sectors per track etc.)
when
> > formatted on a PC using a *standard* format utility? The physical
floppy
> > *drive* hardware is the same, and assuming you get an necessary
jumpers
> > set up correctly, a drive sourced from a PC will work with a BBC.
>
> Something more subtle than that. Typically the PC drives use a
slightly
> different drive stepping distance to the BBC ones. While track 0 will
still
> read fine, and possibily track 1, subsequent tracks tend to be
misaligned.

Sorry, Richard, but that's rubbish.  If you've had a problem, you have
a faulty or misaligned drive.  Or perhaps you've been trying to read
disks written with a double-stepped 80-track drive, in a real 40-track
drive.  The track-to-track spacing, mechanical setup, etc, is EXACTLY
the same for all normal 40-track and all normal 80-track 5.25" drives.

There *are* such things as drives with different track spacing -- once
upon a time it was possible to get 5.25" 80-track drives with 100 track
per inch spacing instead of the standard 96 tpi.  However, they were
for a very few specific CP/M and specialist systems, not PCs, Beebs,
Apples, etc.  All 40-track and 35-track drives are 48 tpi.  That's why
a PC with a suitable controller can read, write, and format BBC disks
and many other formats.  The drives are identical (modulo jumper
settings).

> I think the bottom line on this one is not to expect "foreign" drives
> to work, at least not without tweaking.

This, however, is fair comment.  Drives intended for PCs are often
jumpered differently to the SA400 standard, and may need changed.  Some
simply may not be usable, because they have the minimum circuitry to
work on a PC and don't have the jumper options.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York