Re: Daisychaining 5.25 " Drives

[Don Bruder <dakidd@sonic.net>]

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. 

Fixing that limitation isn't going to be easy, as logic on the circuit 
board of the first drive in the chain is responsible for allowing the 
signals on the control and data lines to "pass through" to the second 
drive connector. Any drive connected to the second connector on drive 1 
is *ALWAYS* "Drive 2" of the slot the card is in due to the way the 
second connector is wired. A third drive added to the chain would never 
get any signals because the second drive's circuitry would never see a 
"pass it along to the next drive" signal from the card. (The way the 
connector is wired makes the second drive think that it is the only 
drive on the card when the drive-select signal is high, thus there's no 
way for the controller card to tell the second drive "pass it along to a 
3rd, 4th, Nth drive") 

In a nutshell: If you want more than two floppies on the system, you 
must have X/2 (where X is the total number of drives), rounded up to the 
next whole number, cards in X/2 (also rounded up) slots in the machine.

This means that the maximum number of floppy drives that can be used on 
any one Apple //e computer is either 12 or 14, attached in pairs to 
either 6 or 7 controller cards. For a machine with an 80-column card 
(whether "extended" or "regular" flavor) installed, you can use 6 slots 
- 1, 2, 4, 5, 6, and 7 - each holding controller cards for up to two 
drives - Slot 3 cannot be used for a card containing ROM (which a drive 
controller card does) when an 80-column card is installed in the aux 
slot, because the aux slot circuitry and the 80-column card "takes over" 
the address space of slot 3 to operate when there's one installed, 
disabling the ROMs of the slot 3 card, and usualy rendering the card in 
slot 3 non-functional unless you do the work of talking to it directly 
through your own custom drivers residing somewhere in RAM. No 80-column 
card allows slot 3 to be used like any of the other slots, permitting 2 
more drives to be attached.

-- 
Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd> for more info