Date   : Mon, 09 Feb 2004 21:51:31 GMT
From   : Pete Turnbull <pete@...>
Subject: Re: Discs

On Feb 9, 17:11, 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.

Correct, with some caveats.  Most older PC 40-track drives are fine,
but 80-track drives are often high density, and those won't always be
amenable to use at single- or double-density (the write current is
different, and the rotational speed is usually different).  However,
some such drives do have jumpers to fix the speed at 300 rpm (instead
of 360 rpm) and fix the write current to the correct value for SD or DD
disks (the media is the same for SD and DD, different for HD).  The
reason I say "most" older drives are OK is that the Beeb expects to see
a READY signal on pin 34 of the interface, whereas PC drives often
generate a DISK CHANGED signal instead.

> Caveat - I'm not sure about the 40/80 track switching for reading a
40
> track disk in an 80 track drive. For a couple of the Watford drives I
> have, it seems extra circuitry was added to stock drives to
(presumably)
> double-step the heads in hardware when the drive was selected as a 40
> track unit. I guess the floppy controller IC in the BBC just doesn't
> have the ability to do this under software control?
>
> > BBC Disc drives are 40-track, 80-track or 40/80 switchable. Some
will claim
> > there is no such thing as an 80-track only drive, but I have seen
them and
> > can confirm the existence of these bizarre units.

They were in fact the standard before High Density came along, and were
widely used outside the IBM PC world.

> See above; aren't those standard high-density floppy drives without
any
> double-stepping logic on the drives themselves? i.e. they're the norm
as
> far as the world of floppy drives as a whole is concerned; the
oddities
> are the 40/80 hardware-switchable drives which are perculiar to just
the
> BBC. Maybe I just totally confused myself (and everyone else :-)

I suspect you did.  They're *not* HD drives.  Standard 80-track drives,
except for PC HD drives, are 80-track, 96 tpi, 300 rpm, use 300 Oersted
media, and typically use a data rate of 125kb/s (for single density, FM
recording) or 250kb/s (for double-density MFM).  HD drives are
80-track, 96tpi, but in a standard PC environment run at 360 rpm, use
600 Oersted media, and typically use a data rate of 300kb/s (for double
density MFM) or 600kb/s (for HD using MFM).  Some of them can run at
300 rpm as well.  Most HD drives can also be told to switch write
currents, so they can use 300 Oersted media, in order to read IBM-style
40-track DSDD ("360K") media -- and are double-stepped in software in
order to complete the picture.

That difference between 300 Oersted and 600 Oersted, by the way, is why
you can't reliably use HD disks in place of SD/DD, or vice versa for
5.25" disks.  But you can sometimes get away with using 3.5" HD instead
of DD or vice versa, because 3.5" DD disks[1] are 600 Oersted and 3.5"
HD disks [2] are 720 Oersted, not terribly different.  I'd still not
use the "wrong" disk for any data I cared about, however.

[1] commonly but incorrectly referred to as 720K, because that's the
system-level data capacity under a particular formatting scheme; the
correct term is 1MB, as that's the raw capacity.

[2] commonly but incorrectly referred to as 1.44MB which is wrong
partly because it's the result of a particular format, and partly
because it results from mixing 1024 and 1000 to get a megabyte (18
sectors of 512 bytes makes 8K per track, * 160 tracks = 1474560 bytes
or 1440KB, not 1.44MB).  You can argue that a megabyte is 1000 x 1000
bytes, or that it's 1024 x 1024, but IMHO only an idiot would argue for
1000 x 1024.  Anyway the correct term is 2M because that's the raw
capacity.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York