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.