Serial communication between PIC and Atmega controileer trough RS485

[Dave]

Pullup and Pulldown resistors are mearly safeguards for disconnect conditions. I do not hear of any bus termination resistors in place? There should be a termination resistor on each END of the bus between bus+ and bus-. I would a quick verification of the IDLE state of the bus by allowing it to go IDLE (with no traffic) and look at the RX output pin of the MAX487 to make sure it is in the correct IDLE state. This will infact tell you if the bus connections are reversed. SERIN statements do not act very well when they are in the reversed states. The program tends to hang. I would also place a pullup resistor on the RX pin of the PIC to ensure it's state during the time the TX enable of the MAX487 as the RX output line will go tristate.