Serial UART Programming
You should already be fairly familiar with the serial interface. You use it when you program your Arduino board, and you also use it to communicate with the Serial Monitor to send data back and forth to the Arduino from your computer. As well as communicating via the USB interface, you can also communicate directly to the serial interface. The direct interface that skips the USB connection is often referred to as TTL Serial, or just Serial. TTL is a reference to Transistor-Transistor Logic, a now redundant technology that used 5V logic levels.
Serial communication, of this kind, is not a bus. It is point-to-point communication. Only two devices are involved—generally an Arduino and a peripheral.
Peripherals that use ...