This is like my 5th post or something, but I wanted to give back. I didn't see a "guides" section and this doesn't really meet many standards in way of a guide, but I thought it could really help someone get started, especially REALLY new people.
Youtube video showing physical hookup:https://www.youtube.com/watch?v=P2DYBKIAHQA
Excellent post with schematic:http://pskillenrules.blogspot.com/2009/08/arduino-and-rs485.html
Modbus Master Library:http://www.arduino.cc/playground/Code/ModbusMaster
// instantiate ModbusMaster object as slave ID 2
// This is how to set the serial port, 3 is serial # and 2 is the SLAVE ID of the node
const int txRxPin = 4;
int txState = LOW;
// initialize Modbus communication baud rate
//creating a counter that transmits "i"
static uint32_t i;
uint8_t j, result;
txState = HIGH;
// slave: write TX buffer to (2) 16-bit registers starting at register 0
result = node.writeMultipleRegisters(0, 2);
Essentially this code shows you how to transmit over modbus a 32bit int