Hi Rami,
I have the following questions/suggestions,
1) Is the behaviour with the DO occurring when using test panels?
2) Is the device attached to an external USB hub? If so, I recommend attaching directly to the PC. Often times an external hub does not provide the correct power nor the controller requirements the USB devices require.
3) Is the behaviour repeatable on another USB Port, PC?
4) What version of DAQmx do you have installed?
Regards,
Glenn