We have an Elev-8 and an SF11-C rangefinder, which both work just great...separately! Together, not so much. I found documentation on the github page that describes the process for accomplishing altitude hold with this rangefinder, but even after modifying the firmware as described and triple-checking the electrical connections, no joy.
It also appears that work done on this particular hardware was courtesy of several _former_ Parallax employees, which is slowing down the tech support considerably! If anyone out there has cracked this problem, we'd sure appreciate hearing about it.
Westhill High School