Understanding Serial Communication: RS232, RS422, and RS485 Explained

Serial vs. Parallel Communication

In computer science, serial communication refers to protocols that transmit data bit by bit over a single pair of wires, such as RS232, RS422, RS485, USB, I2C, and SPI. Although slower than parallel communication, serial links require fewer physical connections.

Serial Communication Modes

Serial ports can operate in simplex (one‑way), half‑duplex (alternating two‑way), or full‑duplex (simultaneous two‑way) modes. Simplex transmits data only in one direction, half‑duplex allows both directions but not at the same time, and full‑duplex uses separate transmit and receive lines.

Serial Communication Protocols

Early data acquisition used analog signals; the introduction of RS232 enabled point‑to‑point communication but lacked networking capability, leading to the development of RS422 and RS485 for differential signaling, multi‑device support, and improved noise immunity.

RS232 Detailed Introduction

Basic Features

RS‑232 is an EIA‑standard serial interface ensuring compatibility across manufacturers. It defines voltage levels, connector types, and signaling conventions.

Physical Characteristics

RS232 typically uses a DB9 connector; pins carry transmit (TXD), receive (RXD), ground, and control signals. The wiring follows the RS‑232 voltage standard.

Communication Protocol

RS232 uses crossed TX/RX lines between two devices. The standard defines pin assignments and signal polarity.

Baud Rate

Common baud rates for RS232 include 300, 1200, 2400, 9600, 19200, 38400, 115200, and 230400 baud. Higher rates reduce transmission distance.

Data Frame Structure

A typical RS232 frame consists of a start bit (logic 0), data bits (LSB first), optional parity bit, stop bit(s) (logic 1), and idle bits. The frame ensures synchronization and error detection.

RS232 vs. TTL Levels

RS‑232 voltage levels (±3 V to ±15 V) are not directly readable by microcontroller logic, so a level‑shifter converts them to TTL (0 V–5 V or 0 V–3.3 V) levels.

RS422 Overview

RS‑422 uses differential signaling, supports full‑duplex communication with four wires, and is suitable for point‑to‑point links such as star or ring topologies.

RS485 Overview

RS‑485 shares the same electrical characteristics as RS‑422 but operates in half‑duplex mode with two wires, making it ideal for multi‑drop bus networks.

Network Radio Serial Interface Example

An in‑vehicle network radio (model J30J‑25) integrates RS232, RS422, and RS485 connectors. The table below shows the pin assignments for each protocol.