Are you using the cable that came with it?
What kind of USB port is on the unit?
A? B? Mini? Micro?
I had a meter once, not that brand, but it had a USB mini. So you would assume USB right..wrong.
I guess they just got a good deal on that when they made them.
They sent a unique cable with it. One end had a USB plug the other was RS-232.
So it really was RS-232 and not USB, it was all TTL, so a modern computer would not give you that "sound", when plugged into a RS-232.