Understanding Serial Communication: RS232, RS422, RS485 Explained

1. Serial Communication vs Parallel Communication

In computer science, serial communication refers to protocols that transmit data bit by bit over a single line, such as RS232, RS422, RS485, USB, I2C, and SPI. Although slower than parallel communication, serial links require only two wires for transmission.

2. Serial Communication Modes

Serial communication can operate in simplex, half‑duplex, or full‑duplex modes. Simplex transmits data in one direction only, half‑duplex allows two‑way transmission but not simultaneously, while full‑duplex supports simultaneous two‑way data flow.

3. Serial Communication Protocols

Early interfaces used RS232 for point‑to‑point links, which later evolved to RS422 and RS485 to support multi‑device networks and improve noise immunity. RS422/RS485 use differential signaling, offering higher speeds (up to >10 Mb/s) and longer distances (over 1000 m), though speed decreases with distance.

4. RS232 Detailed Introduction

4.1 Basic Features

RS‑232 is a serial data interface standard defined by the Electronic Industries Alliance (EIA) to ensure compatibility across devices.

4.2 Physical Characteristics

RS232 typically uses a DB9 connector; the transmit (TXD) and receive (RXD) lines are crossed between devices. Pin assignments are defined in standard tables.

4.3 Communication Protocol

RS232 connections require proper wiring of DB9 pins; transmit and receive signals must be correctly crossed.

4.4 Baud Rate

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

4.5 Data Frame Structure

A serial data frame consists of a start bit (logic 0), data bits (typically 5‑8 bits, LSB first), optional parity bit, stop bit(s) (logic 1), and an idle period. The start bit synchronizes the receiver, while the stop bit marks the end of the character.

4.6 RS232 vs TTL Levels

RS‑232 voltage levels (±3 V to ±15 V) are not directly readable by microcontroller logic, so a level‑shifting chip converts them to TTL levels (0 V or 3.3 V/5 V) for proper interpretation.

5. RS422 Overview

RS‑422 defines a full‑duplex bus with four signal lines, suitable for point‑to‑point or star topologies but not for multi‑drop bus networks.

6. RS485 Overview

RS‑485 shares electrical characteristics with RS‑422 but uses a half‑duplex mode with two signal lines, making it ideal for multi‑drop bus networks.

7. Example: Aircraft Network Radio Serial Port

An aircraft network radio uses a composite interface (J30J‑25) that includes RS232, RS422, and RS485 pins. The pinout is documented in a table and wiring diagram.