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What's this I²C Thing?
I²C, prounounced I-Squared-C, is short for the “Inter-Integrated-Circuit bus”. It was invented in 1982 by Philips, although the patent is now expired.
More than you'll ever need to know is on the I²C Wikipedia page.
Sometimes it is also known as the “two-wire” bus, which is also descriptive, because it has two signal wires.
I²C in FRC
Since the introduction of the RoboRio, there has been an easy way to use I²C for FIRST robots. The connector on the Rio looks like this.
Around 2018-2019 we started to see packaged color and distance sensors from Rev Robotics and others, and for 2020 Rev introduced a cable to make using one of them easy with the RoboRio.
I²C in other maker/hobby wiring
QWIIC
Sparkfun has many sensor boards and controllers that use a tiny 4-pin connector they call QWIIC. It carries the SDA and SCL signals, ground, and +3.3v power.
(picture)