> @hadess whats wrong with just using that markiii to megadrive converter with the retrode? The schematic looks pretty straight forward so it should work.
Well, because it didn't work

I should probably have added this information in my first post, but I had been putting off posting for so long, that I didn't even think to try and explain why I went down this path.
I have 2 cartridges that work out of the box with the adapter on my Megadrive II PAL:
- Enduro Racer (Mark III)
- Alex Kidd In Miracle World (Mark III)
Those 3 games will not boot without stack init. A trick that's mentioned on the Raphnet page is to swap a working cartridge quickly with a non-working one, while powered off. Another way, which I wouldn't recommend but works is to start the Megadrive with the adapter but not the cartridge, insert the cartridge and press Reset. Anyhoo, the 3 games:
- Alex Kidd BMX Trial (Mark III)
- Space Harrier 3D (Mark III/SMS)
- Opa Opa (Mark III/SMS)
All 5 of those are detected as 4 megs "GenericRomInSegaSlot.dat" dumps. Dumping the ROMs ends up in garbage.
There's a few active components on the Mark III/SMD adapter, where the Retrode SMS adapter has none.
Hope that clears any doubts about the reason why I'm asking

(and if you want to share your pinout for the card reader, probably in a separate thread, it'd be appreciated!)