PIC18f452 has both built in RS485 and SPI communication functions, if your receiving end has SPI and the transmission distance is not long then you should use SPI, otherwise if you have to communicate with an external device then use RS485.
Using RS485 is fairly simple its specifications available....google it.....a software protocol called Modbus based on rs485 is a good choice too but this will add some extra bytes before and after your datastream, it has built in CRC and an optional parity checking mechanism to assist in error checking