I have very good experience with Prolific drivers, all devices with Prolific drivers always work.Also you must have a look at this link.
http://digital.ni.com/public.nsf/allkb/BF72229C53F7633386256EDD0069331B
That has not been my experience. I have had issues with Prolific drivers for USB-RS232. I try to avoid them and it's not just with Visa. I have had issues with other software as well. It just has not been as reliable as RS-232, in my experience.
-----------------------------------------------------------------------------------------
Reese, (former CLAD, future CLD)
Some people call me the Space Cowboy!
Some call me the gangster of love.
Some people call me MoReese!
...I'm right here baby, right here, right here, right here at home