May 14, 2026 · Guide

Serial Communication Protocols: RS232 vs RS485 vs UART Explained

By Dzulfikar Ats Tsauri, founder of Voltrus / JoyoDigitama. He builds the protocol tools covered in these posts. Updated November 1, 2026.

Serial communication is how embedded devices, industrial equipment, and computers exchange data one bit at a time over a single communication channel. Despite being one of the oldest digital communication methods, serial protocols remain the dominant physical layer in industrial automation. Modbus RTU runs on RS-485. GPS receivers speak RS-232. Every microcontroller has a UART. For a broader view of how these fit alongside Ethernet-based protocols, see our industrial protocol comparison. Here is how these protocols differ and when to use each.

UART: The Foundation

UART (Universal Asynchronous Receiver-Transmitter) is not a protocol in the wiring sense: it is the digital logic that converts parallel data to serial and back. Almost every microcontroller has at least one UART peripheral. UART defines the frame format: start bit, 5-9 data bits, optional parity bit, and 1-2 stop bits.

UART operates at logic levels: 0V for low, 3.3V or 5V for high. These levels work on a PCB or between two chips on the same board. But raw UART signals degrade rapidly over cables. That is where RS-232 and RS-485 come in: they define the electrical characteristics for transmitting UART data over longer distances.

• Topology: Point-to-point (one transmitter, one receiver)
• Duplex: Full-duplex (separate TX and RX lines)
• Speed: Typically 9600 to 115200 baud
• Distance: Centimeters on a PCB, not suitable for cable runs
• Noise immunity: None: single-ended signals pick up noise easily

RS-232: The Legacy Standard

RS-232 (EIA-232) was introduced in 1960 to connect terminals to modems. It uses single-ended signaling with voltage swings of ±3V to ±15V (typically ±12V). A logic 1 (mark) is a negative voltage; a logic 0 (space) is positive. These large voltage swings give some noise margin, but the single-ended design means noise affects the signal directly.

Key Specs

• Topology: Point-to-point (1 device on each end)
• Duplex: Full-duplex (TX, RX, and hardware flow control lines)
• Speed: Up to 1 Mbps (typical: 9600–115200 baud)
• Distance: 15m (50 feet) at 9600 baud, less at higher speeds
• Wiring: DB9 or DB25 connector, minimum 3 wires (TX, RX, GND)
• Noise immunity: Moderate (large voltage swing), but single-ended

RS-232 is still found on configuration ports of industrial switches, GPS receivers, scientific instruments, and older PLCs. Its main limitation is point-to-point only: you cannot daisy-chain multiple devices on a single RS-232 bus.

RS-485: The Industrial Workhorse

RS-485 (EIA-485) uses differential signaling over twisted pair. Two wires (A and B) carry complementary signals. The receiver measures the voltage difference between A and B, not the voltage relative to ground. This makes RS-485 highly immune to common-mode noise (the noise that affects both wires equally cancels out).

Key Specs

• Topology: Multi-drop bus (up to 32 unit loads, up to 256 with low-load transceivers)
• Duplex: Half-duplex (2-wire) or full-duplex (4-wire RS-422 mode)
• Speed: Up to 10 Mbps at short distances
• Distance: 1,200m (4,000 feet) at up to 100 kbps
• Wiring: Twisted pair (A/B), optional shield, daisy-chain
• Noise immunity: Excellent: differential signaling rejects common-mode noise

RS-485 is the physical layer for Modbus RTU, BACnet MS/TP, PROFIBUS, and many other industrial protocols. It handles the electrically noisy environment of factory floors, motor rooms, and cable trays better than any other serial standard. For termination, biasing, and topology, see the RS-485 wiring section of the Modbus RTU over serial guide.

RS-422: The Full-Duplex Alternative

RS-422 is essentially RS-485 with dedicated pairs for transmit and receive (4 wires total), enabling full-duplex communication. One driver can transmit to up to 10 receivers. RS-422 is used in legacy industrial systems and some CNC machines, but has largely been replaced by RS-485 for new installations due to the higher wiring cost.

Comparison Table

USB-to-Serial Adapters

Modern computers rarely have built-in serial ports. USB-to-serial adapters bridge this gap. Common chipsets:

• FTDI FT232: The gold standard. Reliable drivers on all platforms. Works on macOS without kernel extensions. Slightly more expensive.
• CH340/CH341: Very cheap, widely available. Requires a driver on older macOS versions but works well. Most common on Amazon/AliExpress adapters.
• Silicon Labs CP2102: Good balance of reliability and cost. Solid macOS support.
• Prolific PL2303: Older chipset with known driver issues on newer macOS versions. Avoid for new projects.

Further Reading

• Modbus RTU vs TCP: Which Protocol Should You Use?
• Modbus RTU Over Serial: Frames, CRC, Timing, and RS-485 Wiring
• Serial Terminal for macOS: Alternatives to screen, minicom, and CoolTerm
• Modbus Register Types Explained: Coils, Discrete Inputs, Holding Registers, Input Registers