If the PV300 is intended to be connected to a SLC, then you can use passthrough to download it if you can successfully connect to the SLC. Since we have upgraded to Windows XP SP3, the PIC driver no longer works, so we download our PV300s using passthrough.
As long as the connection to the SLC connected to the PV300 is direct (not through a Controllogix gateway), then you should be able to see the PV in RSLinx. Just set the DH485 node number with the PV keypad first.