Hello!
For some reason, I can't seem leave the PCjr cartridge slots alone! I don't have a jrIDE, and somehow, I decided that I wanted an MMC/SD card reader cartridge to use as a hard disk replacement and as a way to move files easily from another system... so I made one! Just as I suspected, it was a very satisfying experience. Not always easy, especially getting the boot ROM to work correctly without the machine randomly hanging... (I spent a whole week looking for this bug and it was caused by a mismatched register push/pop.. duh!). After that, making the DOS device driver was almost easy!
The performance is not very impressive (only 33 kB/s reading) but at least it is faster than floppies which I timed (using the same tool) at 10kB/S. Being faster than floppies was all I was aiming for, but there are still a few optimizations I plan to try which may yet improve performance.
I posted pictures and schematics, as well as a bit of design information on my website:
https://www.raphnet.net/electronique/sd ... dex_en.php
And the source code is available here:
https://github.com/raphnet/sdcartJR