That would depend upon the particular piece of test equipment. How are you talking to the instrument (IVI, Plug&Play, SCPI commands through LabVIEW VISA calls)?
Look in whatever instrument documentation you have for status register information. You should be able to set it up so that a status bit gets set whenever there is data available. You can either SPOLL the status register and look at that bit, or if you feel lucky you can set it up for an interrupt. [EMAIL PROTECTED]
