85 (ASCII "U") to the host on the serial line

I have an XBee Pro S1 unit on a new board that seems to be sending 85 (ASCII “U”) to the host on the serial line. Is “U” a sign of a specific condition? I haven’t seen this from other systems before.I am diagnosing a problem that could involve a communications overrun that may be locking up the serial port on the host. I need to know if receiving multiple “U” bytes on the host com port means anything important.