UART (serial port)

A UART sends bytes one bit at a time without a shared clock; both ends agree on a baud rate in advance. Transmit, receive and ground lines connect two devices. It is the classic interface for debug consoles, GPS receivers, cellular and Wi-Fi modules, and anything driven by text "AT" commands.

The UART defines the framing of the bits; the electrical layer varies. Logic-level ("TTL") serial is for on-board links, RS-232 uses higher voltages for short cables, and RS-485 uses differential signaling for long, noisy, multi-device industrial links.

Where it bites: baud-rate mismatches and clock tolerance, dropped bytes when flow control is missing, and wiring a logic-level UART directly to RS-232 voltages, which can damage parts.