I'm not sure about this one, but I think that there were two separate adaptors for the DPF:-
The comms adaptor allowed the user to connect the DPF to their PC and transfer files.
The CD-ROM adaptor allowed the user to connect a CD-ROM (I've no idea what particular type, but I assume some sort of standard parallel one) to the DPF.
Both of these adaptors plugged into the top of the DPF (again, not sure which one).
Since I don't own one, I've no idea whether they worked when the DPF was in both Genesis & SNES modes, but I assume that the adaptors worked at least in SNES mode.
I've no idea where you can het either of these adaptors nowadays. There was talk a while back of getting someone to scan theirs and post them up, but nothing ever came of it.
Hope this helps
Agent X