Hello,
Is the USB-Serial converter an NI product? What is the model? Are you sure that the hardware is working correctly? I suggest that you perform a loopback test to confirm that it is working up to that point. See the following article:
http://zone.ni.com/devzone/conceptd.nsf/webmain/D48BA43C59B60277862569EE000501FD?opendocument
Also, you can try the LabVIEW <-> Serial.vi example that ships with LabVIEW by going to Help...Find Examples. This will allow you to test simple basic reads and writes from LabVIEW through VISA, the communications driver that LabVIEW uses.
If that works, then you can focus on what could possibly be going wrong in your applications. If not, then you can look more into your hardware (OS drivers, hardware, cabling).
Le
t me know if I can be of further assistance by posting here.
Scott B.
Applications Engineer
National Instruments