Firstly, have you got bias resistors on the RS485 connections?
These are normally 560R. One goes from D- (pin 7 on 75176) to 0volts and another from D+ (pin 6 on 75176) to 5volts. These resistors bias the 485 bus to idle so the 75176 will output a '1' to the rxd pin on the micro. Only one device on the 485 bus should have this. If you use a MAX485 instead of the 75176 will will avoid this issue but nevertheless it is good practice to include the bias resistors along with 120R termination resistors. See the references to RS485 in 'LINKS' for more info.

The reset problem is probably caused by the RXD pin being '0' due to the issues stated above. Your code may not be able to handle the 'break' condition and thus 'crashes'. I normally have circuitry to monitors the TXD and RXD levels and displays them on leds - red to transmit, green for receive. Normally these leds are off but blink when there is data received or sent. If the green is always on, this indicates a problem with the wiring of the RS485 bus.