Well that's an interesting response Hans! However I'm not too sure the blanket solution is to use a PCI 485 board - many of them require the RTS toggle to work also. I found this with some MOXA boards some time ago and that ADVANTECH have hardware to do the 485 turnaround. This may have changed. The ADAM 4520 not only opto isolates the 232 side but it does the 485 turnaround also - no magic software - if you tx data to it it puts the 485 driver to tx, when you stop tx, if puts the driver into receive.

BTW: I'm not allied to ADVANTECH in any way apart from using their products. Beware of their ethernet virtual serial ports - they have a few 'issues'.