[Discuss-gnuradio] how to specify the device of b210 that running uhd_fft if connect more than 1 b200

Hi ALL,

i remember fft for usrp1 before have an option -w to choose usrp device to run fft if connect more than 1 usrp.

now the uhd_fft have an option -a but it seems only for N series device. so if connect more than 1 b200 device how to choose which one to run uhd_fft?