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My search will never end for an official cable, but atleast I can get to work on a home made one for a cabinet...maybe.
A poster named Quad posted this link and text in another forum a year ago;
Dreamcast Programming - PC Serial Adaptor
Marcus Comstedt Dreamcast PC-Serial Adapter Guide is still the best guide to building a dev cable from scratch. Since you're just connecting two Dreamcasts together, you don't need to worry about voltage conversion. So, you can skip the entire MAX3222 business all together and solder to a DB9 or DB25. Then, walk to any Radio Shack and purchase a null modem connector and use that to tie your two cables together.
The connector on the Dreamcast is a Japanese ISDN plug. Sorry, but it's been so long that I've lost the part # on Black Box.
Long and short story, the DC serial is normal serial except CMOS voltage rather than TTL. Additionally, Virtual-On uses an odd (very fast) baud rate so make sure the cables are short (under a foot for each side).
I'm having a hard time seeing what parts of the coders cable need to be ignored and where this "null modem connector" goes. Who is good enough at this magical stuff to see it?