[Discuss-gnuradio] USRP2 reference clock

Dear Sir or Madam,

At present I work with 2 different USRPs.
One of this is the USRP2 and the another one a newer device is the X300.
This devices communicate each about QPSK and a test algorithm.
For the USRP2 I use the reference input and use reference clock output from
the x300 device to compensate frequency shift.
If I use the command
self.uhd_usrp_source.set_clock_source("external", uhd.ALL_MBOARDS)
, then data communication didn't work.
But If I set the master clock of the USRP2 to 200Mhz with
self.uhd_usrp_source.set_clock_rate(200e6, uhd.ALL_MBOARDS)
, then the communication works.
Is it possible to set the master clock at the USRP2 or what
could been the reason?

 

best regards