The I2C bus is a powerful feature of the NXT because it provides the NXT with practically unlimited expansion capability. Although it’s located on the input port connector, it’s actually used for expanding both inputs and outputs. Only two wires are necessary because the data is transmitted serially, or one bit at a time. One wire is sending and receiving data (SDA), and the other is providing a clock (SCL). On the input port connector, SDA is pin 6 and SCL is pin 5, or blue and yellow in the NXT cable. You need a third wire for ground, which is either pin 2 or 3, black or red in the NXT cable.
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