You used the loopback VI from here [link removed; example no longer available]? If that's the case, which port number did you select? If it was 0, in the Basic Serial Write and Read, which VISA Resource Name did you use? Did you just leave it as the default or did you click on the arrow on the right side of the control to select a serial port? Assuming that you have a recent version of LabVIEW, the loopback example uses the actual VISA Resource Name of ASRL1::INSTR for port 0. The default of Basic Serial Write and Read has a default of Com1 which is actually an alias for ASRL1::INSTR. You should make sure that Com1 actually exists as an alias by opening MAX and under Devices and Interfaces, look to see what is listed under Ports. You can also try communicating here by first selecting a port, then right click and select Open VISA Session. The Basic I/O tab has Write and Read selections.
