Date   : Wed, 10 Oct 2007 22:32:09 +0100
From   : pete@... (Pete Turnbull)
Subject: BBC drives

On 10/10/2007 17:09, Paul J wrote:
> On 10/10/07, *Jules Richardson* <julesrichardsonuk@... 
> <mailto:julesrichardsonuk@...>> wrote:
> 
>  > The quick answer is that "it might work", depending on whether you 
> have a PC
>  > that'll read FM-density data or not.
> 
> As far as I am aware, that is correct if you are using a PC 5.25 floppy 
> drive, but not if you are using a BBC 5.25 floppy drive. The interface 
> for newer PC drives is different to the older BBC drives, although the 
> cables and connectors are the same.

That's not so.

The interface (and the data encoding) is the same regardless of the 
drives, though a PC interface has some differences in the way drive 
selects are used compared to a BBC Micro or other standard floppy 
interfaces (it's the PC one that's odd, BTW).  All PCs use drive selects 
1 and 2 for the first and second floppies, and use a twist in the cable 
to switch drives selects and the Motor On signal around.  A BBC Micro 
(and other machines using standard floppy controllers in the way 
originally designed) uses drive selects 0 and 1 for the first and second 
drives.  However, the read, write, and other control signals are the 
same.  So whether a PC can or cannot read/write FM (single density) as 
opposed to MFM (double density) has nothing to do with the origin of the 
drive, or how the interface is wired, but is determined solely by the 
floppy controller chip and the completeness with which its support 
circuitry is implemented.

However, the differences do mean that to use a BBC drive on a PC, or 
vice versa, you may need to be able to change some drive links (like the 
drive select).  To use PC drives on a Beeb if they're not jumpered to 
provide a READY signal you may need to add that (some PC drives put a 
Disk Changed signal on pin 34 instead of Ready), and to use PC drives on 
a Beeb you need to be able to enable Drive Select 0 instead of 1 (at 
least for Drive 0) and *not* use a cable with a twist.  Some 80-track 
5.25" drives for the PC world are jumpered to run at 360 rpm instead of 
300 (and use a data rate of 300bps instead of 250bps) and that too has 
to be changed to use them on non-PC architectures like the Beeb.  Some, 
though, run at 300 rpm if the High Density signal is not enabled.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York