Date   : Thu, 03 Apr 2008 22:15:37 +0100
From   : pete@... (Pete Turnbull)
Subject: Wires cut in floppy ribbon cable?

On 03/04/2008 19:59, Andy Ford wrote:

> Not sure if this will help, but here is a pinout of the standard 34 way 
> floppy drive connectors :)
> 
> http://www.allpinouts.org/index.php/Internal_Diskdrive

Not really.  That's the pinoout used by modern PCs, and isn't quite what 
the Beeb uses/expects.  Moreover, it's the PC interface side of the 
story, not the drive side. On a normal drive:

Pin 4 is normally DS3 (the drive selects are usually numbered 0 to 3)
Pin 6 is the In Use signal, some drives use this for the LED or gate it 
with other things to start the motor
Pin 10 is DS0
Pin 12 is DS1
Pin 14 is DS2
Pin 16 is Motor On (yes, there's only one thrue Motor On signal)
Pin 34 is Ready (only later 3.5" drives iuse it for Disk Changed)

Note that most signals are gated with the relevant Drive Select to 
become active.  note also that the Beeb (and most other machines) use 
DS0 and DS1 for the first two drives, whereas PCs use DS1 and DS2. 
That's why they're usually referred to as A and B rather than as 
numbers, in PC systems.  Note also that this is why a PC floppy cable 
can't be used for both drives on a Beeb without changing jumpers on the 
drive (the point of the twist in a PC cable is to avoid changing jumpers).

> Kris Adcock wrote:

>> On closer inspection, it seems that the first two wires of the old cable 
>> (the one with the stripe, and the one after) have been cut deliberately 
>> (there's a one-centimetre section very neatly clipped out).
>>
>> So what does the cutting do? An idiosyncracy of the drive?

Pin 1 should be 0V on any standard interface (Beeb, PC, whatever but not 
some Apples) but pin 2 is often the density select on PC drives, and 
it's likely that the clipped section was to prevent that being connected 
to anything.  Some drives are jumpered so that changing density also 
changes the speed; changing density (for SD or DD, which are the same, 
to HD) always involves changing the write current.  On a few really 
ancient drives changing the write current was in fact the original use 
of Pin 2, just as it is on the older 8" drive interface.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York