Just like RS232, RS422 & RS485 specify: voltage
levels, driver type (single ended,differential)
connector type and pinout, and signal conventions.
They do _not_ specify protocol. You can run standard
Async. on any of these. The sender and receiver just
need to talk/listen in the same language (protocol).
The overhead for multidrop can be done with hardware
or it could be done in a PIC application just using
the PIC. I have worked on a project where we used
RS485 & Std Async. It was 80C186 based, not PIC.

Brian Read