USRP B210 TX/RX switching between transmitting and receiving

Hi,

I want to use the single port (TX/RX) from USRP B210 to switch between transmitting and receiving. I have selected "Ch0: Antenna: TX/RX" for both the USRP sink and source blocks as shown in the figure. I expect the TX/RX port will show orange light. But the USRP hardware still transmits through TX/RX port (Red light on), and receives through RX2 port (Green light on). 

What needs to be set if only using a single TX/RX port for switching between transmitting and receiving?

Thanks

Karen