I can check out the wiring and protocol (if not propietary) later.
At
http://www.cs.cmu.edu/afs/andrew.cmu.edu/usr/shadow/www/ibmrt/faq/hardware.…
(H.11) I found some info. The mouse (or locator in IBM RT speak) is more
a co-processor on a wire than a humble mouse :-).
Fred Jan