Basically, it is the UPB version of the TW513. So it talks to a hardware interface (liek a security panel), not a PC RS-232 serial port (without a lot of magic). It maps the X-10 addresses to UPB addresses like A1 = UPB 1, A2 = UPB 2, B1 = UPB 17, etc. It is one-way, so it sends UPB only.
I hear a two way PW523 is in the works.