Hi Thomas
I do not recommend using a PIC for an IP-Bus Emulator. The reason is, its low speed. I had a working emulator with an 18F452 @40Mhz, but the PIC barely managed it to respond to Messages on the bus. Maby it is possible to do it with speed optimized assembler code, but i wouldt use the circuit with the ATMEGA, that vitat built. It will save your time and keep you from getting angry.

I don't know if i still have the PIC software somewhere, but if i find it, i will post it here.
Cya
Mike