I don't know if many of you noticed, but the DC GD-ROM port is a plain standard ATA/IDE port.
The software protocol is likely to be proprietary, but the hardware signaling is industry standard. Someone with knowledge on DVD drive firmware hacking could even mod a existing DVD drive to work with the DC should one made an physical adapter for the connections.
Also, I don't know if any of you noticed that the GD drive controller boards contain industry standard ATAPI chips such the OTI found on the early models (2008) or the SEGA-rebranded TOSHIBA CD-ROM parts on the newer "Samsung" branded GD-ROM units...
They're standard CD-ROM drives with customized circuitry and firmware to be able to handle the GD-ROM format and the SEGA security barcode reading. :shrug:
I posted this a while ago and I have no clue why nobody stickied it up...
(Dreamcast VA0 schematics)
I also posted Saturn schematics which gotten stickied almost instantly. :shrug: