Well this is an really old thing, (not that old but anyhow). I think I built it around 1997-98 sometime (I was then 13 or 14years old). The reason I built it was to test if a straigt cable was working, wich i had built earlier (it wasnt ;)). How i came forth with the idea of how to build it was trou studying some shematics and how the standard null-modem cable works.
well here is the schem for this little fellow:
Images of my version (yea the oldie ;) )