Re: Daisychaining 5.25 " Drives

["Michael J. Mahon" <mjmahon@aol.com>]

Don Bruder wrote:
> In article <C12AD6B1.239DE%av261@lafn.org>,
>  Robert O Greenawalt <av261@lafn.org> wrote:
>
>
> > I have several of the beige plastic Apple 5.25" disk drives that work on my
> > Apple //e computer.
> > Who knows the limit of daisychaining such drives with one controller card?
>
>
> The limit is exactly two. You're more than welcome to plug as many as 
> you like into the daisy-chain, but only the first two will operate. The 
> rest will be inert.
>
> The controller card is only capable of addressing two drives, since the 
> "drive select" function of the card is a one-bit toggle - When the 
> signal on the drive-select line is "low", the card talks to drive 1, 
> which says "Oh, it's for me!" and the signals stop there. When the 
> signal is "high", Drive 1 says "Oops, not talking to me - route the 
> signal and data lines to the "next drive" connector." Any drives beyond 
> the first two in the daisy-chain are effectively nonexistent as far as 
> the card (and therefore, the rest of the machine) is concerned. 

Operationally, this is correct.  But the disk drive select signals
are decoded--one line for drive 1 and another for drive 2.

You can't just use the negation of drive 1 for drive 2, because the
"normal" state is *neither* drive selected.  Since encoding these
three states would also require two wires (plus decoding logic in
each drive), the two drive select lines are provided fully decoded.

-michael

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