Check out the detailed tutorial on RS-485 MODBUS Serial Communication with Arduino as Master:
For Arduino as Slave tutorial, visit this link:
A Brief About RS-485 Serial Communication:
RS-485 is an asynchronous serial communication protocol which doesn’t not require clock. It uses a technique called differential signal to transfer binary data from one device to another. It provides a Half-Duplex communication when using two wires and Full-Duplex requires 4 fours wires.
Connecting RS-485 with Arduino:
RS-485 Module can be connected to any microcontroller having serial port. For using RS-485 module with microcontrollers, a module called 5V MAX485 TTL to RS485 which is based on Maxim MAX485 IC is needed as it allows serial communication over long distance of 1200 meters. It is bidirectional and half duplex and has data transfer rate of 2.5 Mbps. This module requires a voltage of 5V.