RS232, RS422, RS485: Not Just Your Average “Serial Port Trio”

In the realm of industrial automation and equipment connectivity, the RS232, RS422, and RS485 standards are arguably the most familiar yet easily confused technical terms among engineers.

All three belong to the “family” of serial communication protocols. While they may share similar appearances—often utilizing DB9 connectors or terminal blocks—their signal transmission methods and application scenarios differ vastly. Choosing the wrong one can lead to anything from unstable communication to a complete system shutdown.

Today, we’ll explore the differences between these “Serial Port Trio” and how we help customers make the best choice across IOTRouter’s various gateways and applications.

How to Distinguish Them?

Simply put, here’s a quick breakdown: RS232 is like a “personal walkie-talkie,” RS422 functions as a “one-way loudspeaker,” and RS485 operates as a “multi-party conference call.” To grasp their essence, we must examine their core signal transmission principles.

RS232

First, let’s discuss the oldest sibling, RS232. It employs single-ended signal transmission: one wire represents logic “1” (negative voltage, -3V to -15V), another wire represents logic “0” (positive voltage, +3V to +15V), plus a ground wire as reference. This design resembles two people shouting across a distance, making it highly susceptible to external electromagnetic interference. Moreover, the signal voltage itself rapidly degrades with increasing wire resistance and length.

Consequently, its transmission distance is strictly limited to under 15 meters. Data rates remain low, peaking at around 20kbps, and communication is strictly point-to-point. Despite these limitations, its simplicity (requiring only three wires for communication) and extensive legacy device base make it irreplaceable for short-range PC-to-device configuration and debugging interfaces. Many IOTRouter gateway products still feature an RS232 configuration port for local debugging.

RS422

To address the distance and interference limitations of RS232, RS422 introduced the revolutionary concept of differential signaling.

It no longer relies on a common ground reference. Instead, it transmits a signal—data “1” or “0”—using a pair of twisted wires (Line A and Line B), where the signal is determined by the voltage difference between these two wires.

External common-mode noise affects both wires simultaneously and equally, while the receiver only cares about their “difference,” thus perfectly canceling out noise.

This design effortlessly extends RS422’s transmission distance to 1200 meters (at low speeds), achieves data rates up to 10Mbps, and provides robust one-to-many drive capability—a master device can broadcast to up to 10 slave devices simultaneously.

However, it remains unidirectional, meaning data flows in only one direction at a time. Bidirectional communication requires two pairs of wires.

RS485

Building upon the robust foundation of RS422, RS485 further enables “multiparty communication.”

It retains all the advantages of differential transmission while allowing multiple devices (up to 32 standard loads) to connect their transceivers to the same bus pair. Through enable signal control, any device can become the “speaker” when needed, with others acting as “listeners,” enabling true multipoint, bidirectional, half-duplex network communication.

This bus-based topology enables a single bus to connect dozens of sensors, instruments, or controllers, significantly simplifying wiring and reducing costs. This makes it the undisputed leader in industrial fieldbus, building automation, and data acquisition systems.

In IOTRouter’s industrial IoT gateways, the RS485 interface is nearly standard equipment, connecting various Modbus RTU protocol sensors such as temperature/humidity, power, and water meters.

Consequently, many IOTRouter industrial gateways, computers, and routers prioritize RS485 as one of their most critical underlying data acquisition interfaces.

To help you visualize their differences more clearly, here’s a simple comparison:

Frequently Asked Questions (FAQ)

Q1: My actual RS485 communication distance far exceeds 1200 meters. Why is that?

A1: 1200 meters is the theoretical guaranteed distance under standard conditions at lower baud rates (e.g., 9.6kbps). In practice, using high-quality low-attenuation twisted-pair cable, appropriate baud rates, and correctly installing 120Ω terminating resistors at both ends to eliminate signal reflections makes stable communication exceeding 1500 meters entirely feasible at lower speeds.

Q2: Why are terminating resistors required on RS485 buses? Can they be omitted?

A2: Signal reflections occur at cable ends, superimposing on the original signal to cause distortion and bit errors. Installing resistors matched to the cable’s characteristic impedance (typically 120Ω) at both ends (only both ends) absorbs signal energy and eliminates reflections. In low-speed, short-distance (e.g., under 50 meters) static environments, issues may not be “noticeable,” but for system reliability, it is strongly recommended to follow specifications and add them.

Q3: What is the relationship between RS232/422/485 and Ethernet? Will Ethernet replace them?

A3: These represent different layers of communication technology. The RS series defines electrical standards at the physical and link layers, specifying “how to represent 0 and 1 with voltage levels” and fundamental transmission/reception rules. Ethernet, conversely, is a more complex, high-speed family of network protocols.

In industrial settings, they coexist rather than replace each other. Many of our gateways act as “protocol converters,” efficiently and reliably converting device data from front-end RS485 buses and transmitting it to back-end Ethernet or wireless networks (4G/5G/Wi-Fi) for cloud integration.

In the realm of industrial communication, there are no obsolete technologies—only misapplied solutions. The three “classic veterans”—RS232, RS422, and RS485—have weathered decades of change. Thanks to their unmatched reliability, real-time performance, and cost advantages, they continue to play irreplaceable roles in their respective domains.

Understanding their inner workings unlocks the key to equipping diverse application scenarios with the most suitable “transportation tools.” For IOTRouter, our mission extends beyond providing premium hardware supporting these interfaces—we empower clients to build stable, efficient data pathways toward intelligent solutions amidst complex industrial environments.