Warning 1073676424 from VISA Set I/O Buffer Size.vi on a serial port

[Optimelt]

I am porting an application from LabVIEW 6.1 on Windows to LabVIEW 7 on OS X (Mac). It was very painless except some GUI modifications.

The application involves 30 KB data from a instrument through a RS232 serial port. I found the application misses data whenever the computer is busy. The problem came down to the unchanged buffer size.

Attempt to change the buffer size of a serial port with "VISA Set I/O Buffer Size.vi" fails with a warning of 1073676424 (The specified I/O buffer is not supported). Even the example vi from NI web site "Advanced _Serial_Write_and_Read.vi" has the same warning.
I wonder what I am missing.

[Noel]

Under the hood VISA is using the POSIX serial interface for Mac OS X (same as for Linux and Solaris). This interface does not support changing the buffer size. Hence, the buffer size is fixed to the internal OS buffer size. The only thing that changing the buffer size will do (for the out buffer) is to have VISA not flush the data after every write. This is a limitation in the serial API for Mac OS X. Therefore, VISA reports a warning.