It's not clear from what I've been able to
find that the adapters
between the serial port and SD cards are passive (as opposed to having
active circuitry in them); I haven't found any description of what's in
them beyond "it Just Works with this software". Do you have reason to
think the adapter is a passive device?
Yep, I have. I built one.
http://www.tabajara-labs.blogspot.com.br/2012/03/sega-pride-and-joy-o-sauda…
This is the schematic I used:
http://www.dc-swat.ru/blog/hardware/14.html
As you see, just wires...