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In practical applications of the RS485 bus, when the transmission distance exceeds a certain limit, the bus’s ability to resist interference decreases. In such cases, 120Ω termination resistors must be connected at both the beginning and end of the RS485 bus to ensure its stability. The correct way to connect the matching resistors for RS485 communication is to install a 120Ω terminal resistor at the output of the device at the beginning of the RS485 bus, and another 120Ω terminal resistor at the input of the device at the end of the bus. These terminal resistors are connected between the positive and negative wires of the RS485 bus. As shown below: ① When using an RS232 to RS485 converter alone, the installation location of the terminal matching resistor: http://yunrun.com.cn/upload/202005/02/202005021843541069.png ② When using an RS485 repeater, the installation location of the terminal matching resistor: http://yunrun.com.cn/upload/202005/02/202005021854571467.png ③ When using an RS485 hub, the installation location of the terminal matching resistor: http://yunrun.com.cn/upload/202005/02/202005021915531050.png I remember one user calling Changhui Instrument Technology on-site and saying that his PLC simply couldn’t read the data from the flow meter. After checking, it was found that there were no issues with the program or the hardware connections; testing with serial port debugging software on a computer also showed no problems. Yet, the data from the flow meter still couldn’t be retrieved. A technician from Changhui Instruments asked: What communication protocol is used between your PLC and the flow integrator? The user remained silent for a moment before saying, \"Weren’t PLCs and instruments all communicated using the RS485 protocol? Mine uses the RS485 protocol as well!\" This user’s answer likely reflects the thoughts of many people: the terms \"RS485 communication protocol\" may seem to go well together, but they actually shouldn’t be used together, as a protocol is one thing, while RS485 communication refers to the medium (the physical layer); these two concepts cannot be confused! I recommend that everyone read the article titled “Differences and Connections between RS485 and MODBUS”. For example, in phone calls, the phone line represents the physical layer, while the language spoken by the people talking to each other is the protocol. The same physical layer can transmit different protocols, just as people on either end of a phone call can communicate in various languages (protocols). RS485 communication is akin to the physical layer of telephony; it can transmit protocols such as PPI, Modbus, and Profibus, among others. After more than an hour of discussion, it was learned that there was an inverter near its flow meter, with the power cables and communication cables intertwined together. Therefore, users are required to route the power lines at a distance from the signal lines, and to add termination resistors at the exits and ends of the communication lines. The reason no terminal resistor was added was that he thought the flow meter was relatively close to the PLC (a few meters to a dozen meters), but this is a misconception; distance is not the issue here – the main problem is signal interference. Of course, if some communication ports already have isolation measures in place, then there is no need to add one! What was the result of that user’s communication attempt? Guess~ Some experience regarding the selection of the value for the terminal resistance: Changhui Instruments believes that the value of the terminal resistance is related to the device interfaces and cables, and there are specific calculation formulas available; in essence, it’s a range of values. My approach is to use a value ranging from 0 to 120Ω! The effect becomes more pronounced as the distance increases; therefore, to ensure the quality of RS485 communication for the instruments, it is best to include terminal matching resistors! Source: http://yunrun.com.cn/tech/3023.html