Error loading usrp firmware

Hi there.
I am trying to make work my usrp.

after running anything using usrp (for example usrp_oscope.py) i get
this error mesasge:
RuntimeError: Unable to find USRP #0

Probably i don’t have firmware installed.
after connecting power, right (closer to usb) LED is blinking at 2 Hz.

So i tried ‘usrper load_firmware /usr/share/usrp/rev4/std.ihx’
i got message: ‘usrper: found unconfigured usrp; needs firmware.’

LED blinging changed to 1 Hz.

usrper always return message ‘failed to find usrp’ :

root@ubuntu:/home/mato# usrper load_firmware
/usr/share/usrp/rev4/std.ihx
usrper: failed to find usrp[0]
root@ubuntu:/home/mato# usrper load_fpga
/usr/share/usrp/rev4/std_2rxhb_2tx.rbf
usrper: failed to find usrp[0]
root@ubuntu:/home/mato# usrper led0 on
usrper: failed to find usrp[0]

after unplugging and plugging again power, LED always start to blinking
at 2 hz.

am i doing something wrong?
during test, no boards were connected to the board.

thanks for help.

Sitec.