1 The RS-232 protocol
Ben Eater's explanation of how RS-232 serial works, often suggested to people wanting to understand a basic communication protocol, with one poster rating it very highly.
Topic
protocols
3 videos. Ranked by independent recommendations, not by views.
2 How I2C Communication Works and How To Use It with Arduino
A commonly suggested starting point for understanding the I2C bus, particularly for people linking microcontrollers together or adding sensors over two wires.
3 TSP #31 - Tutorial on Programming the NeoPixel (WS2812) RGB LEDs and Equipment Giveaway!
Explains the WS2812 NeoPixel data protocol and its timing in detail, useful for understanding the signal even though it uses a non-Arduino microcontroller.
