May 14, 2026 · Guide

Serial Communication Protocols: RS232 vs RS485 vs UART Explained

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 communicating with PLC programming environments. 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, extendable to 256 with repeaters)
• 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.

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?
• Serial Terminal for macOS: Alternatives to screen, minicom, and CoolTerm
• Modbus Register Types Explained: Coils, Holding Registers, Input Registers