Zeroplus Arduino starter kit with Logic Analyzer Manual page 44

DESCRIPTION
The serial monitor of the Arduino communicates with the computer using
a serial protocol.
A protocol is a set of rules that a signal follows in order to transmit
data. If everyone agrees on the same set of rules (i.e protocol) it
becomes easier to communicate with other devices.
There are hundreds of protocols. Some were developed for specific
industries; others are ubiquitous, such as I2C, SPI or 1-wire. Serial
protocols use a single line to send data, one bit at a time, whereas
parallel protocols send data over several wires, in one clock cycle.
Serial protocols require fewer wires but are slower.
The UART is the hardware part that formats the signal; the interface
that converts the data bytes into a serial stream of bits. On the
receiver side the UART re-assembles this stream into the original data
bytes. That's how the Arduino receives the strings sent through the
serial monitor .
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