Date   : Tue, 25 Oct 2005 07:51:56 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Collected notes on floppy drive butchery?

On Oct 25 2005,  1:04, Richard Gellman wrote:

> I'm starting to think you just want to make me look like I'm talking
out
> of my a***.

No, I'm not :-)  I'm sorry if I seem to have picked on you but I didn't
want incorrect informastion to be propagated.

> Now, I have the Shugart floppy cable pinout in front of me right now,
> and I can see the following:
>
> Pin 10 - Motor Enable A
> Pin 12 - Drive Select B
> Pin 14 - Drive Select A
> Pin 16 - Motor Enable B

That's the *IBM PC* modification, *NOT* the Shugart or BBC/Acorn
pinout.

> Now, unless there's some kind of weird logic that has to be applied
to
> reverse sequencing, when I flip that around, I read the motor enable
> lines as directly swapped, and the drive select lines directly
swapped.

Yes, in an IBM PC.  NOT in a Beeb.

> To punctuate: There is no exchange of Motor Enable with Drive Select.

Yes there is, in a Beeb, or any other non-IBM PC type interface from
that era, such as a TRS-80, any of umpteen CP/M machines, etc; look at
the CORRECT pinout.

> And where's this "its 1 and 2 not 0 and 1" come from? The connector
> supports a maximum of two drives. On my pinout they are marked A and
B,
> but on almost every drive I have seen, the settings are for 0 and 1.
> Your statement implies that a third drive select line is present,
which
> it is not.

There are FOUR in the original floppy interface.  The IBM PC modified
that; ignoring one altogether, and re-using the lowest-numbered as an
extra MotorOn signal.  This was done to ensure that both motors could
be controled separately, such that both wouldn't be on at the same
time.  The original IBM 5150 PC power supply would have been too close
to the limit if hat happened.  Many machines only implement two lines
becasue many users only need two drives.  The Beeb is like that, but
the two drive selects supported by the Beeb are the original DS0 and
DS1.  The ones used by the IBM correspond to DS1 and DS2.

> The BBC Master may well support a DS2 line, but the floppy connector
> does not. Now if you want to personally refer to drive IDs using 1
and 2
> instead of 0 and 1, then thats your own business, but please refrain
> from criticising just because I use the more obvious numbering system
> (based on drive select jumper markings).

No, I was using the same counting scheme as you did originally,
starting at 0.  Originally, floppy interfaces had four select lines.
 Some manufacturers referred to them as 0...3, some as 1...4, but I
have consistently used the 0...3 scheme in my posts.  See my other post
with the diagram, for the pinout.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York