I haven't tried this with VISA serial ports, but you might experiment with setting the timeout to either 0 or FFFFFFFF. Sometimes those are used for "infinite."
Hello Les I have tried 0, it does wait 0 ms. I do not think I can try FFFFFFFF, because given in ms will wait 1.5 month :-). I do not have the time to play with this :-).
Yes, 0xFFFFFFFF is the timeout for infinite. It is a special value and should not time out after 1.5 months. (As you had noticed, 0 does mean immediate.)