I am using a USB<->DB9 converter to accomplish USART communication with PIC micro.It works fine if I select the VISA resource name as "ASRL6::INSTR" but fails to communicate if I select COM6 which is the alias assigned to the same port and throws an error :-1073807194..But once I work with "ASRL6::INSTR",I am able to work with "COM6" VISA name for subsequent runs..
.Also,if I connect it to some other USB port present on the system,the problem persists.However, it works fine with COM1(default COM port available on system).
Though,I have tested it with hyperterminal and it works well for all USB ports.
How does the port show up in Measurement and Automation Explorer (MAX)? MAX is where the name of the port (Com6) is associated with its VISA address. Usually serial ports show up correctly and Com6 would work correctly right away. But perhaps there is some corruption in the MAX database that is interfering with that.
What brand of USB/RS-232 converter are you using? Some work better than others.