This error occurs if the COM port that was last used by PC-TDR cannot be opened because it is already opened or no longer available. The error is most often seen when either of these two scenarios occurs:
To resolve the error, open the TDR100.ini file found in C:\Campbellsci\pctdr and change the CommPtEx setting to a valid COM port number (for example, CommPtEx=1). Alternately, the TDR100.ini file can be deleted, although this will reset any previously entered wave-form values, as well as any user-defined coefficients.