USRP B210 gpio on J504

Hi, I want to control an antenna multiplxer via J504 gpio port. Actually
I
want to have logic 1 and logic 0 in order to switch the multiplxer. But
it
is difficult to find gpio example for B210. I have new version of b210
(green board) and 3.8.005 uhd version. I m very much thankful to if you
can
help me with gpio example.