Date   : Tue, 25 Jul 2006 12:30:32 +0100 (WET-DST)
From   : Peter Coghlan <PCOGHLAN@...>
Subject: Re: What types of

>
>> I'm looking for a 3.5" floppy drive for my Beeb and was wondering if anybody
> 
>Just any 3.5" drive. Plug in. Use. Eight quid from your nearest PC
>box shifter.
> 
>DFS 1.20 has some issues with drives stepping faster than it
>expects. DFS 1.21 fixes this.
> 

DFS 1.20 does not have any such issues or issues regarding timing
and 3.5" drives noted elsewhere. In fact, the DFS is unaware and
unconcerned if the drive has stepped the heads faster than it might
expect (if it had such expectations, which it doesn't). As long as
the heads are at the required track and settled by the time the 8271
starts reading data, the DFS could not be anything but happy about
how they got there.

DFS 1.21 (which it should probably be clarified, does not come from
Acorn) is presumably the result of running a particular patch against
DFS 1.20. The result of this patch is that "Drive error 10" (drive not
ready) errors are no longer flagged. Instead the command that was being
executed when the error occurred is retried indefinately until the error
goes away.

This appears to be an attempt to work around an issue with newer floppy
drives which suppress index pulses while they are stepping the heads.
Missing index pulses result in the disk controller support circuitry in
BBCs sending a not ready signal to the 8271 which then aborts whatever
command is in progress and issues an error 10.

To reiterate, this is nothing to do with drives stepping faster than
expected or timing problems of any kind and is to do with missing
index pulses from the drive.

Regards,
Peter.