Six foot PC Engine controller extension cable already modified into a RetroSpy cable. The cable splice uses 12″ Dupont wires about 1 foot away from the female end of the cable. This will work with a TurboGrafx-16 with the proper adapter(s).
For a fully working RetroSpy setup you will also need a programmed microcontroller board. For instructions on setting up a board see https://retro-spy.com/wiki.