uCHobby has a helpful primer on interfacing electronic components via the handy I2C protocol and Arduino - I?C uses only two bidirectional open-drain lines, Serial Data (SDA) and Serial Clock (SCL), pulled up with resistors. Typical voltages used are +5 V or +3.3 V although systems with other, higher or lower, voltages are permitted.
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What exactly does the I2C protocol and usage mean to the average robot builder? Well, it can mean a lot of different things,such as easily accessible devices like temperature sensors, accelerometers, and even setting up a small on board network on a robot. - Introduction to I2C
Bare Bones Arduino Board Kit (Unassembled)
http://blog.makezine.com/archive/2008/09/understanding_i2c.html