Write serial port

I have got USB to Serial device realized on PIC18 microcontroller.
I need to write to and read from it. How to do it?
I don't care about latency. Driver would work in servo thread.
I know there is rtapi_outb function but I don't really understand its usage.

Probably the easiest way is to use a userspace python routine/component:

linuxcnc.org/docs/html/hal/halmodule.html

It will not be real time so expect 10s of ms delays