USRP2+RFX2400, TX only and RX only configuration

HI,
I would like to configure two USRP2+RFX2400 boards one as the TX only
and the other one as RX only, so on the TX board I set:
io_tx_05=1 => the enable(pin 8)of the transmitter mixer (U101) is
always high, so the mixer is always active
io_tx_06=0 => the TX/RX (U202) works always as TX

and on the RX board I have:
io_rx_05=1 => the enable(pin 15)of the receiver mixer (U2) is always
high, so the mixer is always active
io_rx_06=1 => the RX (U209) works always as RX

but now when I look at the output of the U2(pin 8 and 22), there is no
signal there (why?) and GRC is giving me this message: “Channel 0 is not
receiving”. what does it mean?
Thanks,
Malihe