Electronoobs shows you more or less how i2c, UART and SPI serial communications work with a few examples.
Tony Gambacorta shows you how to explore your hardware with a serial port.
If you’re interested in hardware but haven’t had a chance to play with any yet, this one’s for you. In this “hello world”-level reversing project we’re checking out a UART (serial port) and using it to access a shell on a *very* soft target. If you decide to try it on your own you’ll find an equipment list, walkthrough references, and some troubleshooting ideas at the link below.