This library allows you to communicate with I2C / TWI devices. On the Arduino boards with the R3 layout (1.0 pinout), the SDA (data line) and SCL (clock line) are on the pin headers close to the AREF pin. The Arduino Due has two I2C / TWI interfaces SDA1 andSCL1 are near to the AREF pin and the additional one is on pins 20 and 21.As a reference the table below shows where TWI pins are located on various Arduino boards.
|Board||I2C / TWI pins|
|Uno, Ethernet||A4 (SDA), A5 (SCL)|
|Mega2560||20 (SDA), 21 (SCL)|
|Leonardo||2 (SDA), 3 (SCL)|
|Due||20 (SDA), 21 (SCL), SDA1, SCL1|