it is with great pleasure that @nippur72 and I share with you our latest project (actually, more his effort than mine, I just soldered some component...).
As some of you had guessed: this is a microSD Storage Card add-on module that allows your Apple-1 to use one or more Commodore Disk Drives!
We told ourselves: almost everything is already on the Storage Card: RAM, ROM, VIA... let's add the few components that are missing and build a true serial IEC interface!
No sooner said than done!
The module is easy to build and connects to the Storage Card via the expansion connector already in place (just solder a 1x12pin stripline to it).
It is necessary to upgrade the Storage Card firmware to version 1.3 by rewriting the EEPROM, after having done all the checks described in Chapter 4 of the reference document.
Nippur72 ported most of the Disk Drive routines from the C64's Kernal to the Storage Card firmware, quite a miracle considering he managed to fit everything into just over 2kB of space.
Thus, all CBM DOS-specific commands (Initialize, Validate, Scratch, New, Error channel, etc.) plus Apple-1 dedicated commands are available.
Of course, the on-board microSD memory card remains accessible and functional.
Since this is an IEC interface it is possible to daisy chain and manage up to 4 Drives (or emulators: Pi1541 for example works great).
Tests were conducted with the 1541 and 1541/II but we are confident that any Drive offering "basic" serial IEC interface operation is compatible.
Any contributions to this Compatibility List will be posted on the project site.
The schematic for this Add-On, the related Gerber file, the SD OS source code, and all informations are available under a Creative Common CC BY 4.0 license.
Here's a short video. Enjoy! :-)
Claudio P-LAB / Nippur72