The first thing to check is the diagnostic LED on the M1XSP.
From the XSP manual:
Slow blink (1/2 sec.) = Normal communication with M1.
Fast flicker = Communicating with other equipment (Thermostat, Lighting Controller, PC, etc.)
No blink = No communication with M1. Unit might be unplugged or powered off.
Is the LED showing the slow blink condition?
If not, check that the XSP has good 12V power at the data bus connector using a multimeter.
After you've verified that the XSP has proper power, check the connections to the data bus, both on the XSP and also at the other end.
Disconnect the data bus cable at both ends, and use a multimeter on the ohms scale to check for shorts or opens on any of the wires.
If you have the data bus cable connected through a M1DBH data bus hub, you can try reterminating the RJ45 at the DBH end of the cable.
