I was looking at the schematics of Apple II and Apple III floppy disk drive analog boards on Bitsavers late last night, and made a surprising discovery: the DuoDisk 5.25 dual floppy drive sold for use on the Apple II is actually directly usable on the Apple III+, with the full functionality the Apple III SOS operating system expects.
The Apple III had one internal floppy drive, and could support up to three external drives. The internal and external drives used the same drive mechanism as the Disk II used on the Apple II, and the same motor control board, but a different analog board. Whereas the Disk II used a 20-position ribbon cable to connect to the interface card, the floppy drives for the Apple III use a 26-position ribbon cable, and there are two connectors and three drive select signals allowing for three external drives to be daisy-chained. Each drive shifts the drive select signals on the daisy chain down, so that the drives don’t need to be configured for their position in the chain. The analog card also has a flip-flop circuit which latches a disk changed signal, not present in the Disk II. The Apple III SOS operating system uses the disk changed indication to ensure that it does not write to the wrong disk even if the user swaps disks at an inopportune time. The disk change signal is derived from detecting an edge on the write-protect sensor.
The usual external floppy drive was known as the Disk III, model number A3M0004, and had a molded plastic housing, unlike the sheet metal housing of the Disk II. However, the earliest external floppy drive for the Apple III was the Disk II for Apple III, model number A3M0003, and did use the same (or nearly the same) sheet metal housing as the Apple II.
Bitsavers has schematics for the Disk II analog card, the Disk III analog card, the UniDisk 5.25 analog card, and the DuoDisk 5.25 analog card. The UniDisk 5.25 was the replacement for the Disk II, and like the Disk III, was in a molded plastic housing. The DuoDisk 5.25 contained two floppy drives in a single housing. The UniDisk and DuoDisk were most commonly used with the Apple IIe.
The suprise discovery was that the UniDisk analog card schematic shows that it has the same disk changed circuit as the Disk III, and that like the Disk III, it has an “Apple II Mode” pin on the connector to disable the disk changed circuit when used on an Apple II, or on an Apple III when running in Apple II emulation mode. When I discovered this, I was excited because I actually have a number of UniDisk 5.25 drives, and would like to use them on the Apple III. I hastily tweeted about my discovery.
Alas, I was almost certainly mistaken. Upon further study, it became apparent that the UniDisk 5.25 analog card schematic on Bitsavers is for a version of the drive that did not make it into production. The schematic and the PCB layout show a 25-pin D-subminiature connector, as used on the Apple III+, but the product as manufactured has a non-standard 19-pin D-subminiature connector, as used on all later Apple floppy drives. The 19-pin connector has only two drive select lines, and does not have the Apple II mode line. I haven’t yet cracked open a production UniDisk 5.25 drive to check, but I’m now fairly confident that will not contain the disk changed circuit needed for use on the Apple III.
However, I then realized that the production DuoDisk 5.25 drive actually _does_ have the 25-pin D-subminiature connector, and a quick check of the pinouts revealed that it appears to match the Apple III+ disk connnector pinout. This causes no end of aggravation to people wanting to use the DuoDisk on an Apple II because it needs a special 19-pin to 25-pin cable, which is very hard to find now.
After tweeting about jumping the gun regarding the UniDisk 5.25, I also tweeted that it was possible that the DuoDisk 5.25 might have the disk changed circuit, and might work directly connected to an Apple III+. I don’t have a DuoDisk 5.25 to test, and I have an Apple III with 26-pin header for the disk connector rather than an Apple III+, so I wasn’t able to test it myself. To my amazement, within a few hours @jrobj_ confirmed that the DuoDisk works on the Apple III+ and does have the disk changed circuit.
It is interesting to note that the DuoDisk schematic shows only a single disk changed detection circuit for both drives, rather than separate circuits for each drive. As a result, if the user changes either diskette, SOS will think that both have been changed.