What Is the Difference Between RS232 and RS485? And How Can the Two Ports Be Connected?

RS232 and RS485 are two common serial communication standards used in industrial automation, device communications, and embedded systems. Although both are serial communication protocols, they differ significantly in their physical characteristics, communication methods, and application scenarios. The following article will detail their differences and explain how to connect devices with different interfaces.Basic Introduction to RS232 and RS485

RS232: The early mainstream serial communication method

RS232 is a standard established by the Electronic Industries Alliance (EIA) in the United States, with its original designation as EIA-RS-232, commonly abbreviated as RS232. It was originally designed for communication between computers and modems but later became widely used for point-to-point connections between various serial devices such as printers, PLCs, sensors, and instruments.

RS232 communication uses single-ended signal transmission, meaning data is transmitted via a single signal line and a ground line. It operates in full-duplex mode, allowing devices to send and receive data simultaneously. However, since the RS232 interface uses negative logic levels (e.g., logic “1” uses -3V to -15V, and logic “0” uses +3V to +15V), this high voltage difference design can easily damage chip circuits and is not directly compatible with TTL levels. Therefore, in microcontrollers and embedded devices, additional level conversion chips are often required to achieve connectivity.

Additionally, RS232 has relatively low communication speeds. In asynchronous transmission, the bit rate is 20 Kbps, and the transmission distance is also limited, typically not exceeding 15 meters in practical applications. These limitations make RS232 more suitable for point-to-point, short-distance, low-speed data communication scenarios.

RS485: An enhanced standard designed for industrial communication

To address the limitations of RS232 in terms of transmission distance, interference resistance, and networking capabilities, the RS485 standard was developed. RS485 was also established by the EIA but employs differential signal transmission, which transmits data through the voltage difference between two signal lines (typically labeled as A and B lines). Logic “1” is typically represented by a voltage difference between A and B greater than +2V, while logic “0” is represented by a voltage difference less than -2V. This differential method significantly enhances communication interference resistance, enabling stable transmission even in environments with strong electromagnetic interference.

Another key feature of RS485 is its support for a multi-point communication structure, meaning multiple devices can be connected to the same bus for communication simultaneously, with support for up to 32 or even 128 nodes in parallel. This structure is highly suitable for building distributed systems or data acquisition and control networks between multiple devices, finding widespread application in industrial automation, building control, security surveillance, smart meters, and other fields.

Additionally, RS485’s communication distance significantly outperforms RS232. At a transmission rate of 9600 bps, the transmission distance can reach 1200 meters, and in some low-speed scenarios, actual tests have shown distances of up to 3000 meters. Its maximum communication rate can also reach 10 Mbps (provided the distance is short).

RS485 has lower voltage standards and is more compatible with TTL voltage levels, making it more convenient and safer when interfacing with embedded systems, as well as more suitable for long-distance cabling and complex industrial environments.

Differences between RS232 and RS485

Different communication methods

RS232 uses a point-to-point communication method, meaning that only two devices — a master device and a slave device — can be connected on a single communication line. This is commonly used in direct connections between computers and individual devices.

RS485 supports multi-point communication, enabling multiple devices to be connected on a single bus, making it more suitable for multi-device networking in industrial environments.

Communication distance and speed

RS232 has a short communication distance, typically recommended for use within 15 meters, with speeds up to several dozen kbps; RS485 can achieve communication distances of up to 1,200 meters, with speeds of up to 10 Mbps (depending on distance), making it more suitable for long-distance transmission.

Interference resistance

RS232 uses single-ended signal transmission, where the logic state of the communication signal is referenced to ground. This method is susceptible to external interference; RS485 uses differential signal transmission, where the voltage difference between two signal lines represents logic 0 and 1. Additionally, the two lines are twisted together in a twisted-pair configuration. Even if interference occurs, both lines are affected simultaneously, so the voltage difference remains largely unchanged, ensuring signal stability is unaffected by interference. Therefore, in comparison, RS485 can maintain more stable and reliable communication performance in industrial environments with strong electromagnetic interference and long-distance transmission scenarios.

How do RS232 and RS485 interfaces connect?

If one device uses an RS232 interface and the other uses an RS485 interface, they cannot communicate directly because the electrical characteristics of the two interfaces are completely different. In order to achieve data communication between the two, an RS232 to RS485 converter, also known as an interface converter, is usually required.

Common methods for achieving connectivity include:

Using an RS232-to-RS485 converter

• This converter converts the single-ended signals of the RS232 interface into the differential signals of RS485. Typically, one port connects to the RS232 TX, RX, and GND pins, while the other port outputs the RS485 A and B lines. Come-Star’s RS232-to-RS485 converter ensures seamless conversion between RS-232 and RS-485 interfaces, enabling easy interconnection of devices with different serial protocols.

Use a serial server or gateway

For more complex industrial places, you can use a serial server or industrial gateway with RS232 and RS485 interfaces. These devices not only perform physical signal conversion but also support serial protocol management, data caching, Modbus RTU/ASCII protocol conversion, and other functions.

Software-level processing

Some devices achieve compatibility at the hardware level, such as supporting RS232/RS485 multiplexed ports, where the communication method can be switched via configuration software. In such cases, communication can be established by correctly connecting the lines and configuring the parameters, without the need for additional converters.

Conclusion

RS232 and RS485 are both classic and widely used serial communication protocols, each suited to different application scenarios. RS232 is suitable for short-distance, point-to-point communication, while RS485 is more suitable for long-distance, multi-point communication and has greater interference resistance. When devices with different interfaces need to communicate, reliable data transmission can be achieved simply by using an appropriate converter or gateway. In practical applications, the choice of standard should be determined by comprehensively considering factors such as communication distance, number of devices, and interference conditions in the on-site environment.