Understanding UART, RS-232, RS-422, and RS-485 Serial Communication Standards

UART (Universal Asynchronous Receiver/Transmitter) is a key module for asynchronous serial communication, converting between parallel and serial data streams and defining frame formats; with matching baud rates and frame formats, two devices can communicate over just two signal lines (Rx and Tx) without a shared clock.

Common serial interface standards include RS-232, RS-422, and RS-485, each with distinct electrical characteristics and usage scenarios. RS-232 uses a 25‑pin (or DB‑9) connector with higher voltage levels, suitable for point‑to‑point links but limited in speed and distance. RS-422 provides four signal lines (full‑duplex) and better noise immunity, while RS-485 uses a differential two‑wire bus (half‑duplex) that supports up to 128 nodes, longer distances (up to 1.2 km or more), and higher data rates (up to 10 Mbps).

The article details the frame structure of UART communication, including start bit, data bits, parity, stop bit, and idle level, and explains how baud rate relates to bits per second. It also outlines the electrical specifications of each standard, such as voltage levels, signal polarity, and termination requirements.

Comparisons highlight RS-485’s superior anti‑interference capability, longer transmission distance, multi‑node support, and higher speed versus RS-232’s limited distance, lower speed, and single‑pair communication. RS-422 differs mainly in having separate transmit and receive pairs, enabling full‑duplex operation, whereas RS-485 shares the pair for half‑duplex communication.

Practical design considerations are discussed, including the need for termination resistors to match cable characteristic impedance, bias resistors to set a defined idle voltage, and the impact of cable capacitance, impedance discontinuities, and signal reflections on communication reliability. Formulas for reflection attenuation factor (RAF) and load calculations are provided.

Finally, the article emphasizes that serial ports are widely used in embedded devices, industrial control, and PCs, and that understanding these standards is essential for reliable hardware design and troubleshooting.