UART
Universal Asynchronous Receiver-Transmitter.

UART predates most modern bus specifications. There is no single governing document; the framing conventions (start bit, data bits, optional parity, one or more stop bits) and the family of common baud rates are inherited from the RS-232 and teletype era. Modern ANSI/EIA-232 describes the electrical and timing characteristics of the most common variant.

The bus uses two unidirectional signals: TX (transmit) on one end is wired to RX (receive) on the other, and vice-versa. There is no shared clock line — both ends must be configured to the same baud rate, data-bit width (typically 8), parity (none, odd, or even), and stop-bit count (1 or 2).

Optional hardware flow control via RTS/CTS adds two more signals and lets the receiver throttle the transmitter when its buffer is full. UART is the default fallback for debug consoles, MCU-to-MCU links, GPS modules, Bluetooth/Wi-Fi modules, and any peripheral that needs a simple bidirectional pipe.