The value 0x00 is the default error replacement character. The serial driver uses that to replace characters with errors (parity, framing, etc.). The existence of an error should also be returned by the appropriate VISA function, usually VISA Read.
There was a bug in older NI-VISA drivers that didn't properly report all transmission errors. We have fixed some of those errors in NI-VISA 3.0, at least on Win32 and LabVIEW RT.
Dan Mondrik
National Instruments