This listing is for a bare PCB only. If you would like a assembled and cased unit, see my other listing at https://www.tindie.com/products/tersteeg/apple-iii-to-fujinet-or-floppy-emu-adapter/

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An IDC-26 to IDC-20 adapter allowing to to use yourApple II Fujinet, Floppy Emu, or Disk II drive on the external floppy port of an Apple III. Based on the work of ThorstenBr, but redesigned to plug directly into the external port and adds the option to route the drive 2 enable signal to pin 17, since Fujinet uses a different pin than the Floppy Emu which expects that signal on pin 5.

Schematic

BOM

Interactive BOM

Case

The case parts are designed to printed at a 0.2mm layer height and snap together by inserting the wider tab first and then pushing down on the shorter tab side near the 72LS32 chip. I found the tab on the lid broke off when using matte PLA with it's lower layer bond strength, but regular PLA or PLA+ (and probably PETG) worked fine and could be assembled and disassambled multiple times. Your mileage may vary.

Solder Jumpers

-12V - Routes -12V to pin 9

12V - Routes +12V to pins 13 and 15. Also pin 17 and 19 depending on their jumper positions.

PIN5 - Routes either the Drive2 signal or GND to Pin 5. For use with the Floppy Emu, do NOT also then route Drive2 to Pin17.

PIN17 - Routes either the Drive 2 signal or +12V to Pin 17. For use with the Fujinet, do NOT also then route Drive2 to Pin5.

Pin 19 - Routes either the Drive 3 signal or +12V to Pin 19. For use with the Fujinet, experimental, not currently supported in hardware or firmware.

Here's a table of the different ways to can build this and what works. If you route either Drive2 or Drive3 to pin 17 and 19, you must NEVER then attempt to plug an actual Disk II drive into this since they internally bridge +12V on all four pins and will backfeed that voltage into your Apple III drive enable lines.

*If you don't plan on using a Disk II, I would recommend NOT closing any of the 12V jumpers for safety's sake, since these voltages are not needed by any of these modern drive emulators.

Assembly

Assemble the SMD components and solder the jumpers as needed. Note that the 74LS32 chip mounts upside down with pin 1 on the lower right. Then solder on the two IDC connectors, make sure to pay attention to which side of the board they go on.

I like to then add a label printed on a P-touch (0.23mm black on transparent tape) to denote the mode the adapter was built for.