Introduction

RS-232, RS-422, RS-485, and CAN bus are among the most common communication methods in IoT applications, used for data transmission between different devices. Each interface has its own transmission speed and reach—but how far can they actually go, and how can the distance be extended? This article compares these standards and explains the key factors that limit or extend real-world transmission distance.

The Standards at a Glance

RS-232

RS-232 (EIA RS-232) is one of the oldest serial communication interface standards. It is a full-duplex communication mode requiring three lines: ground, transmit, and receive. RS-232 is limited to point-to-point communication between a PC serial port and a device, with relatively poor noise immunity. It typically uses DB9 or DB25 connectors, and its standard signal levels range from −12 V to +12 V. In industrial control, only the RXD, TXD, and GND lines are usually used. RS-232 is suitable for local device communication—mice, printers, industrial instruments.

RS-485

RS-485 supports multi-drop and long-distance transmission in half-duplex mode. Using balanced transmission and differential reception, RS-485 delivers high data rates, strong interference resistance, and can carry signals over a kilometer—solving the short-distance limitation of RS-232.

RS-422

RS-422 (EIA RS-422-A) is a full-duplex interface with four signal lines, so receiving and transmitting can happen simultaneously. RS-422 uses differential signaling, giving it better noise immunity and longer range than RS-232—particularly well suited to industrial environments.

CAN Bus

CAN (Controller Area Network) was developed by Bosch in the early 1980s. It is a multi-master bus that can run over twisted pair, coaxial cable, or optical fiber, with a maximum rate of 1 Mbps. CAN-based distributed control offers stronger real-time data communication between network nodes.

Transmission Distance Comparison

• RS-232: max data rate 110 bps – 115.2 kbps, typical max distance ~15 m
• RS-485: max data rate 10 Mbps, typical max distance 1,200 m or more
• RS-422: max data rate 10 Mbps, typical max distance ~1,200 m
• CAN (high-speed): 125 kbps – 1 Mbps, bus length ≤ 40 m
• CAN (low-speed): 10 – 125 kbps, up to 1,000 m at 40 kbps

Under normal conditions, the communication distance ranks: RS-232 < CAN < RS-422 < RS-485.

Three Factors That Affect Real-World Distance

① Baud Rate — Higher baud rate means faster transmission but shorter distance and weaker noise immunity. At the common 9600 bps, RS-232 is recommended to stay within about 13 meters.

② Cabling — Twisted-pair cable reduces noise coupling; shielded twisted pair adds protection against strong EMI. RS-485 can only reach its maximum specified distance below 100 kbps; over a 100-meter run, the maximum rate is only about 1 Mbps.

③ External Interference — Separate high-voltage power lines from low-voltage signal lines and keep them apart; if unavoidable, cross them perpendicularly. RS-232’s short range is partly due to its single-ended signaling, which cannot reject common-mode interference.

Conclusion

To maximize real-world distance: choose an appropriate baud rate, add shielding, and route away from interference sources. For greater distances, a wireless transmission solution is recommended.

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