Date   : Wed, 12 Oct 2005 15:29:44 +0100
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: Twin 3.5" disc drives for the Master

Raf wrote:
> Hi people,
> 
> Could anyone out there help me please with this.
> I am not a good technician but I would be very grateful
> to anyone who could build me one, or preferably two, twin
> 3.5" disk drive units, stacked or side-by-side, ready to connect
> to my Master(s) and which will work properly in DFS, and as drive :0 and 
> :1 in ADFS. A friend in the UK has "tried" for me but did not succeed, 
> recently. The twisting did not respond???

Twist as in twisted cable? Did your friend check the drive ID jumpers?

In the PC world (going from memory here), every floppy drive gets set up 
as drive 1 (i.e. each physical drive thinks it's the second drive in the 
system). The cable twist between drive A: on a PC and the controller 
then re-routes the motor and selection control signals so that the PC 
can talk to the drive 'past' the twist in the cable as drive A: and the 
drive 'before' the twist (if fitted) as drive B:, despite them both 
being the same.

If I'm remembering this right, the PC then has to use ID 2 to talk to A: 
and ID 1 to talk to B:, with ID 0 and 3 being unused (the PC can only 
address two drives using this scheme, despite the FDC being able to 
address four, as ID 0 or 3 would end up the same either side of the 
cable twist)

In the rest of the world, a straight-through cable is used, and each 
drive in the system is jumpered with a different ID.

Strictly speaking, the last drive in the chain should be terminated 
(sometimes by fitting resistor packs, sometimes just using appropriate 
jumpers on the drive). Often you can get away without doing this, though 
- but not always, and some controllers may be more picky than others.

It wouldn't surprise me if your friend finds that the two drives show up 
as second and third units, with nothing as the first or fourth drive 
(map that to Acorn's surface numbering scheme obviously - presumably :2, 
:3, :4 and :5)

With a twisted cable *and* drive jumpers set up, you'd probably end up 
addressing both drives at once, and nothing will work!

The hard part tends to be finding documentation for the jumpers in the 
drives you're using. Sometimes you're lucky and the jumpers are marked 
with enough info to guess their function (ID jumpers can be pretty 
obvious, for instance). Often the manufacturer's datasheet's the only 
way to fnd out (particularly on 3.5" drives where there isn't the space 
on the PCB to properly label things)

That's my understanding anyway, but the coffee hasn't kicked in here yet :)

cheers

Jules