VISA Write error when writing to Virtual COM port (through USB)

[Matthew Bailey]

I am having problems with VISA write to a virtual COM port through a
USB connection.

I am able to talk to the virtual COM port through any other terminal
program on my PC, but I can not get it to work through VISA.

I can communicate through COM1 successfully with VISA, but not the
virtual COM port.

The VISA Init goes ok, but I get an error (-1073807339) when the VISA
write is done.

I have updated to the latest VISA 3.31 and I have Labview 6.0.2.

I have a port monitor program (portmon) that allows me to see the low
level serial communication. With this I am able to see the problem.

The VISA driver seems to send a command IRP_MJ_FLUSH_BUFFERS after the
write (IRP_MJ_WRITE). On the virtual COM port the IRP_MJ_FLUSH_BUFFERS
command comes back with INVALID DEVICE REQUEST.

When I look at the same thing on a real com port (ie COM1), the
IRP_MJ_FLUSH_BUFFERS command comes back with SUCCESS.

I have checked a number of other terminal programs and they do not send
the command IRP_MJ_FLUSH_BUFFERS. How can I stop the VISA session
sending this command?

Thanks,

Matthew

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