Date   : Mon, 09 Feb 2004 22:22:44 -0000
From   : "Chris Thornley" <C.J.Thornley@...>
Subject: Re: Discs

Hi,
I found these article which might help. They show how to modify HD TEAC 5.25
drives to operate at 300rpm and how to change signals like ready and disk
change so they work with older computers.

http://www.oldskool.org/disk2fdi/525HDMOD.htm
http://www.buchty.net/casio/fz1-diskdrive.html
http://nemesis.lonestar.org/computers/tandy/hardware/model16_6000/floppyfix.
html

Chris


               />      Christopher J. Thornley is cjt@...
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-----Original Message-----
From: Majordomo List Manager [mailto:majordomo@...] On Behalf Of
Pete Turnbull
Sent: 09 February 2004 21:52
To: bbc-micro@...
Subject: Re: [BBC-Micro] 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