Arduino Wire.h Library -

void requestEvent() Wire.write("Hello Master");

void setup() Serial.begin(9600); Wire.begin(8); // Join I2C bus with address 8 Wire.onReceive(receiveEvent); Wire.onRequest(requestEvent); arduino wire.h library

You can also make one Arduino act as a slave. This is useful for multi-processor projects. Master Code (Sends a command): #include <Wire.h> void setup() Wire.begin(); Serial.begin(9600); void requestEvent() Wire