I was having a bit of fun with my Apple III connecting it to a FloppyEmu (and to a normal Disk II drive). My Apple III only had the single internal drive so far. It really needed an extension...
Since I wanted a convenient and safe solution for the connection, I made a simple adapter. It's basically the Apple III equivalent of the D-SUB to 20pin IDC adapter for Apple II, which BMOW sells with every FloppyEmu. Sure, Apple III with FloppyEmu is certainly a niche application, but hey, so is any retrocomputing anyway. Maybe someone else is interested after all. :)
Some background (what you may or may not know...)
The Apple III has two 26pin box headers as ports for internal and external drives. Their lower 20pins are identical to the Apple II's Disk II connector. The upper 6pins are Apple III specific. The Apple III needed more pins, since it supports up to four 5,25" drives.
And it added a signal for a hardware feature to detect disk swaps: they changed the logic of the write-protect signal, so it became sticky. Removing a disk from a drive inevitably toggles the write protect switch (no matter whether the disks were protected or not). The Apple III drives used this to keep any new disk write protected (even if the disk itself wasn't), until the OS recognized and acknowledged the disk swap. A nice safety feature to prevent accidentally overwriting a disk, when the user swapped disks while the OS wasn't expecting it.
But then the Apple engineers needed to add yet another signal to the connector: to disable the new disk swap protection feature. The Apple III had to be able to emulate an Apple II. And since Apple II software doesn't know how to handle the "sticky write-protect" feature, the drives had to revert back to the original Disk II behaviour in Apple II emulation mode.
I guess, that's a well-known story. But I always found this detail about adding logic for a corner-case, and then requiring even more logic, just to disable the extra feature again, was quite telling about how the Apple III project worked as a whole (and it reminds me of so many futile feature discussions I had with product managers in my own day job...).
Anyway, back to FloppyEmu
Steve (BMOW) has a blog post on connecting his FloppyEmu to an Apple III. He doesn't sell Apple III adapters though, and doesn't officially support it - since he doesn't have an Apple III - and there are too few machines out there, so it's not worth it.
However, it will still work and all you really need is a 26pin ribbon cable, a 26pin IDC connector at one end, at a 20pin IDC connector at the other (stripping 6 wires). Just connecting the 20 identical signals is enough (only if you do it right, that is! :) ). That will work, since the extra pins are not strictly needed by the Apple III to work with a drive. The disk electronics, mechanics and low-level sector format are otherwise unchanged.
However, swapping cables was not a convenient solution for me. It would require me to disassemble my FloppyEmu enclosure every time I wanted it to switch between Apple II and Apple III. Same issue with the Disk II: I don't want to disassemble a drive, just to swap cables, so I can more conveniently copy disks on the Apple III. Also, the simple cable wouldn't be able to support FloppyEmu's "dual drive" feature - where it is emulating two drives...
Well, and making a proper adapter is more fun anyway. :) Long story short, I made this simple PCB:
It can be connected to the Apple III's external disk port, using a simple 26pin ribbon cable. And it provides a 20pin connector for the original disk II or FloppyEmu ribbon cable. No disassembly or swapping cables required. It can also be used to connect to the Apple III's internal disk connector. That, however, requires to open the Apple III machine, of course...
Well, but does it really work?
Of course it does.
I have tested the combinations - using the FloppyEmu as internal or dual external drives. And using a Disk II as an external Apple III drive. When FloppyEmu is connected externally, the Apple III now even has 3 disk drives (Wohoo!).
Configuring a three-drive Apple III: SOS, SOS, ...
Two-drive configuration (one internal + one external drive) worked out of the box. But I was briefly struggling to make the third drive work (FloppyEmu as an external drive in dual-drive emulation mode). Until I found out, that the Apple III will not autodetect the number of its drives, like the Apple II does: no, you have to reconfigure the "Sophisticated Operating System (SOS)": load the "system utilties" disk, enter the "system configuration program (SCP)", select "read driver file", load your ".D1/sos.driver" configuration, select "change system parameters", "change the number of drives" to 3 (it was set to 2 on mine), then select "generate new system", write the configuration to ".D1/sos.driver". Finally reboot. Done! Now all three drives do work...
Oh, and as recommended, don't forget to make a backup of your boot disk before reconfiguring anything. Since, if that goes wrong - you're otherwise stuck with no boot disk. For example, if you forget to "read the driver file" before "generating the new system", then the new configuration will not contain a character set. The Apple III has no video ROM, but a character memory, which is loaded by software. So writing a new configuration with an empty character set really sucks (ask me how I know! :) )
Anyway. It all works now. And admittedly all this hassle with "SCP" was only necessary to make a third drive work. In any case, having the FloppyEmu option makes testing new stuff with the Apple III much more convenient...
And, btw, I also made a simple 3D-printed box which completely covers the PCB (just to be safe):
If anyone else was interested in connecting an Apple III to FloppyEmu - the project and Gerber files, and some even more detailed descriptions are in my github project:
Also, I also have 2 or 3 PCBs left, if anyone else had an Apple III + FloppyEmu and was interested. I do live in Germany though, so mailing these anywhere is really economical for EU destinations only (nowadays, the minimum postage of sending anything overseas is more than the total cost of ordering a complete custom batch of PCBs directly from China. Crazy, but true...).
Apple (I)II forever!