Advanced a bit more.
The commands are 7 bits wide + a clock from cartridge and an ack from uPD779
here is a schematic of SD208 8049 based cartridge:
basic config from 8049 is
P1 : bit 0-6 : command bit 7 : clock
P2: 4 line of IO + ack from uPD779
bus: 4 lines IO
IO lines are direction + fire + game select + game start + unknown.