This simple port-powered CI-V interface circuit can be built inside a DE-9 shell. I built mine with an 1/8" jack mounted in the shell's cable hole, and bought a R/S 1/8" mono patch cable to connect the equipment. If you need to connect more than one piece of gear, the signal and ground wires from each item should be paralleled. You could buy a long cable, cut it in half, and tie it directly to the circuit in the shell (nixing the jack in the process).
(full size version)
WARNING: D2 on this schematic is depicted backwards. I'll fix that one of these days. ;-)
If I had to do it over again, I'd change R1, R3, R4, and R5 to 22k -- the threshold on Q1 is excessively low, and keeping the values the same makes purchasing easier. I'd also tie DE-9 pins 6 and 8 to pin 4 -- this provides a better control signal picture to whatever program is driving the port.
As with all port-powered items, you need a reasonable amount of current out the port for this to work properly ("mouse compatible"). For what it's worth, this circuit works on my notebook computer.
I also wrote a quick little DOS program to monitor the CI-V protocol: MON_CIV.EXE. It is hard-coded for 1200bps on COM2: -- might be useful for troubleshooting, if you have nothing else.