I am running the latest git build of OpenBTS UHD on a USRP1 with RFX1800 cards reflashed to 900.  The USRP tests fine and I am able to run various gnuradio python scripts without issues.  I installed GNU Radio and UHD from http://www.sbrac.org/files/build-gnuradio.

I do not have a clock module, and i have the configuration file set to run the normal non-52MHz transceiver.  I am operating in the 900MHz band using responsible RF practices.   

However, after running OpenBTS, I get the following errors: 

Device open failed.  Exiting...

1314317403.3018 ALARM 3066751856 TRXManager.cpp:90:clockHandler: TRX clock interface timed out, assuming TRX is dead.

Could someone point me in the right direction?  What causes this error?