First of all, what is the error code that you're getting now? You didn't attach the error display. Second, have you used Hyperterminal to see if you can communicate with your cable and port settings? Third, in your VISA write, you're not sending a termination character with the command. Most instruments require this. If everything works in Hyperterminal and doesn't in your LabVIEW program, this is a likely cause for an error. Lastly, have you tried one of the shipping examples for serial com. Both Advanced and Basic Serial Write and Read add a termination character to the write string (though in different ways). Have you tried one of those?
