Just notes about different embedded hardware protocols in the wild. Touching on both physical and theoretical properties.
PWM - Pulse With Modulation. Things which look like square waves. Used to control things like ESCs etc.
I2C - or I^2C serial protocol, with 2 wires. Synchronous. one wire for data, another for clock. up to 5mbps, depending on mode
SPI - 4 wires, synchronous similar to the i2c. Up to 60mbps
UART - up to 5 mbps, needs synchronized clocks and predefined speed. 2 wires, tx/rx.
DMX512 - protocol used for stage lightning.
CAN Bus - mainly developed for automotive, but useful in other automation. Two wire interface, with twisted wire pair which allows daisy chaining and addressing all the nodes. Also power supply and ground are needed, making it total 4. Given it’s automotive heritage, it’s rather robust. One of the good things about it, is it’s ability to save on copper wiring. Though, it’s limited speed is about 1mbps. One of the reasons beeing the lengh required for cables etc.
LIN Bus - cheaper alternative to can bus. less resilient?
MOD bus - bus protocal, designed for PLCs specifically.