Date   : Sun, 23 Oct 2005 12:35:16 +0100
From   : Andrew Benham <adsb@...>
Subject: Floppy cables: the answer

Jeff Gaines wrote:

> There have been different views in here about un-twisting the cable,
> it just didn't work for me until I bit the bullet and un-twisted it.

Whilst looking for something else this morning, I nearly tripped over
a PC floppy drive cable.  So I grabbed the Avo and found out what the
twist does.

The twist affects 'pins' 10 to 16.

Pins 1-9 and 17-34 are unaffected by the twist - these go identically
to both drives.
Pin 10 from the controller goes to pin 10 on the drive connected to
the 'middle' connector, and to pin 16 on the drive connected to the
'end' connector.
Pin 11 from the controller -> pin 11 (middle) and pin 15 (end).
Pin 12 -> pin 12 (middle) and pin 14 (end).
Pin 13 -> pin 13 on both drives.
Pin 14 -> pin 14 (middle) and pin 12 (end).
Pin 15 -> pin 15 (middle) and pin 11 (end).
Pin 16 -> pin 16 (middle) and pin 10 (end).

So that's the cable. Now, what about the interface itself ?

I have some drive documentation from ~1979 on the Shugart SA400
floppy interface.  This is what the BBC Micro uses.

Pin 10   Drive Select 0
Pin 11   Ground
Pin 12   Drive Select 1
Pin 13   Ground
Pin 14   Drive Select 2
Pin 15   Ground
Pin 16   Motor On

Hmm.  The cable twist doesn't make sense.


Then I dug out my PC reference book which dates from 1991.

PC and XT Floppy Disk Controller Internal Connector

Pin 10    Motor Enable A
Pin 11    Ground
Pin 12    Drive Select B
Pin 13    Ground
Pin 14    Drive Select A
Pin 15    Ground
Pin 16    Motor Enable B


Now the penny drops.  The PC interface isn't SA400, and the twist makes
sense for this interface.

There are some other differences too:

Pin             SA400           PC
6               DS3             Unused

And I have documentation on a Matsushita JU364 3.5"
720k drive dating from about 1988 (and which works
with the BBC just fine) stating:

Pin             JU364           PC
4               In Use          Unused
34              Ready           Unused


So the answer is that the twisted cable won't work with
the BBC.


-- 
Andrew Benham         adsb@...       
Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill"