LabVIEW Real-Time 2018 on a PXI target with PharLap does recognize a NI USB-232 adapter as a USB-RAW device.That's bad. (e.g. you can't set Baud rate etc.)
It should be recognized as an additional RS232 port like VISA "ASRL3::INSTR".
(like same way, when I connect the USB-232 adapter to a Windows-PC)