Date   : Fri, 03 Nov 2006 11:12:02 +0000 (WET)
From   : PCOGHLAN@... (Peter Coghlan)
Subject: DFS on 3.5inch - Is this supposed to work?

>
>I've recently undertaken the move to using 3.5inch double density disks 
>(as opposed to high density disks with the extra hole), which works very 
>well with a standard PC floppy drive. Indeed I can format disks using 
>either the Master 128 ADFS formatter on the Welcome disc, or using my 
>A4000 or RiscPC, and the disks work on either system absolutely fine.
>
>My initial thought was that a modern PC floppy drive wouldn't be able to 
>handle the 200K single density format of DFS. But when I tried 
>formatting one, it worked. I've even stored files on them without any 
>spurious errors.
>
>Now my question is, is this working perfectly, or is there some sort of 
>half-way format that I've created where single-density data is being 
>recorded in a double-density mode or some such silliness? I.e. if I put 
>these discs into a PC 3.5in drive connected to my model B (being set up 
>soon), is the humble 8271 going to suggest orifices to which the idea my 
>be suitably inserted?
>

Hi Richard,

Formatting double density disks in a 3.5in drive with media sensor (ie one
that can tell the difference between DD and HD disks) on a Master running
ADFS should work fine and produce "standard" double density disks rather
than some wierd half-way format.

Doing the same except running DFS on the master should also work fine and
produce equally "standard" single density disks which should then be usable
on a model B with an 8271 controller and a suitable 3.5in drive. As far
as the drive is concerned, double density and single density are the
same thing and if it can do one, it can do the other - it doesn't even
have to know which density the controller is speaking. (This is not
the case for high density where the drive has to do things differently
to when it is processing double or single density).

There is a possible gotya in that the 8271 controller may not like the
PC HD drive if the drive suppresses index pulses during seeks. If this
is the case, the result will be "drive error 10" or "disk error 10" or
"drive not ready" or somesuch errors when attempting to use files that
are not near the start of the disk. If you come upon this problem,
possible solutions are to find a 3.5in drive that does not suppress
index pulses during seeks (possibly an older one or a DD only one), hack
the drive so that it doesn't suppress index pulses during seeks or use a
hacked version of the DFS which retries indefinately when it encounters
an error 10.

(I put "standard" in quotes because as far as I know, the standard disk
formats for the bbc micro were all 5.25in.)

Regards,
Peter.