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.