Date   : Fri, 25 Aug 2006 22:11:49 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: writing and reading on 3.5 floppy

On Aug 25 2006, 12:07, Mike Howard wrote:

> I'm no expert on the workings of floppys & drive selection. I believe
> though, that the twist in a PC cable forces the drive to drive 0
(zero)
> from drive 1 (default). Therefore, connecting a floppy connector to
the
> end of a twin BBC cable (no need to remove the BBC flat edge
connector)
> leaves the 3.5" drive as drive one (as there is no twist in the BBC
cable).

Actually the twist causes the drive to behave as drive 1 instead of
drive 2, not 1 instead of 0.  Therefore a drive set up for a PC,.
connected on the end of the cable beyond the twist, will appear as
drive 1; that is to say it would respond to DS1 (DRIVE SELECT 1) on the
interface, not DS0 (nor DS2, or DS3).  If the drive selects on the
drive were not altered, but it were moved to before the twist, it would
appear as drive 2, ie it would respond to DS2.  That's no use to a
Beeb, though, because a Beeb only uses DS0 and DS1 -- "drive 2" and
"drive 3" in DFS are simply the other sides (the upper surfaces) of
drives 0 and 1, and have nothing to do with what any other system might
call drives 2 and 3.

The twist actually doesn't swap one drive select line into the place of
another.  In fact it swaps drives selects with motor control lines, one
of which is entirely a figment of IBM's imagination, so it's even worse
than it seems.

Clear as mud?  Take a look at the interface specs, especially the
*original* before IBM screwed it up for PCs.  For example, try reading
Michael Haardt's handy "Floppy User Guide", from about page 12 on.


-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York