so, on the bench it works fine, in the airplane it doesn't work? Run a new wire paying attention to the pin out and the serial ground. Don't use the shield for the serial ground, use 3 conductor for rx,tx gnd. 

If you're using the serial from the sl 30 for anything else , (grt. afs efis?) this might also be causing a problem.  Disconnect and try the MX 20.