Not easy to find, you may well have to analys it yourself and design a circuit to emulate it.
I have analysed some protocol cheaply before by using the pins on the parallel port as an inputs. And use a software to sample the pins at a fixed rate and save them to a txt file as "1" and "0". or ASCII graphics for clarity.
If you want it you can play back the file to see if you have succesfully captured it and the changer reacts to it. Just be sure to have a consistent sample rates. I used turbo pascal for dos back then.
Its painful but you can atleast see whats going on in there. Do verify the signal with a scope first to see the shortest time its changes. So you can set the sample rate to 16 times or so.
It be alittle bit trick if the bus lines are bidirectional.
Unless you have acces to some equipment that can do that for you already.