Well this is for me a quite old project. If I recall correctly I built this around 1997-98. Why I built it was to both check that I correctly wired a straight cable (I had not), and to not require having two computers over null modem or similar while developing some serial based programs in qbasic.
It was made up via taking a look at how a standard null-modem cable works, and documentation on what signalling is used by qbasic.
How to wire one up
Images of my version