Could you provide some details on what exactly you mean by "cannot recognize"? Do you get a specific error when you try to use the driver or does it just not respond to a command you issue? I've only used the driver with GPIB and have no problems there but I'm not sure the driver is written for serial. I know NI's web page says it is, but I don't see anything in there that sets the serial port. I have not tried to redownload so it's possible that I have a downlevel version but there's usually some code in the initialize function to set com paramaters and termination character. Cehck the manual and see if the scope requires a LF or CR at the end of the command string when using the serial port. The driver that I have does not add that. Try a simple VISA Write of *IDN? with either or both characters and see if you get a response. If you would like to look at a driver that I know supports both serial and GPIB, download the driver for the HP34970A. In the Initialize function of that driver, you can see how to set com port and term character. Adding this to the scope driver is simple.