How to connect the RS485 bus with 120 ohm matching resistor?

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

How to connect the RS485 bus with 120 ohm matching resistor?

Posted Date: 2024-02-03

RS-485 is a differential (balanced) transmission standard approved by the American Electronics Industry Association in 1983. RS-485 is widely accepted and used in industrial, medical and consumer applications and is now a common interface in the industry.

RS-485 is built on RS-232 and RS-422, solving the shortcomings of RS-232 and RS-422. It has stronger anti-interference ability, relatively low interface level, and longer transmission distance. , more nodes, more flexible usage.

RS-485 is an electrical characteristic specification and does not include protocol content. The simplest RS-485 transmission includes at least one pair of differential (balanced) lines, half-duplex communication, and a maximum communication distance of 1200 meters.

The RS485 chips of different devices are usually different, and there are different load types of chips, which are usually not directly visible to the engineering companies. So that means the maximum number of connected devices for different devices on the bus is uncertain. For the number of connected devices for the same device, just refer to the device description requirements. The following is a daisy chain connection diagram. Please pay attention to the location of the 120 ohm resistor.

In addition, if you want to make RS485 communication more stable, the following two points are also very important.

1. Shielded wire

485 signal lines can use shielded wires as wiring, or unshielded wires as wiring. Since the 485 signal is transmitted in differential mode, the voltage difference between 485+ and 485- is used as the signal.

If there is an external interference source interfering with it, when using twisted pair for 485 signal transmission, due to its twisted pair, the interference effect will be the same for 485+ and 485-, and the voltage difference will still remain unchanged. , the interference to the 485 signal is reduced to a minimum. In the same way, if there is a shielded wire that acts as a shield, the interference from external interference sources can be minimized as much as possible.

2. RS485 grounding

The 485 bus must be grounded. Strictly speaking, the 485 bus must be reliably grounded at a single point. Single point means that there can only be one point grounded on the entire 485 bus, and multiple points cannot be grounded, because it is grounded to keep the voltage on the ground wire (usually a shielded wire used as a ground wire) consistent to prevent common mode interference. , if multiple points of grounding are counterproductive.

When grounding reliably, the ground wires of the entire 485 line must have good contact to ensure consistent voltage, because in actual construction, for the convenience of wiring, the wires are cut into multiple sections and then connected, but the shielded wires are not well connected, thus The ground wire is divided into multiple sections, and the voltage cannot be kept consistent, resulting in common mode interference.

Analysis of the working principle and characteristics of rs485 bus

The RS-485 bus is a commonly used serial communication standard. Its working principle and characteristics are as follows:

working principle:

1. Signal transmission method: RS-485 uses differential signal transmission, which uses two lines for data transmission, namely line A (positive line) and line B (negative line).

2. Differential signal transmission: The logic 1 and logic 0 of the data are represented by the level difference between the A line and the B line. The larger the difference, the logic 1, and the smaller the difference, the logic 0.

3. Asynchronous communication: RS-485 bus supports asynchronous communication, and the data transmission rate can be set as needed.


1. Multi-point connection: RS-485 bus supports multi-point connection and can connect multiple devices to form a bus network.

2. Long-distance transmission: RS-485 bus can transmit data over a long distance, usually up to several hundred meters or even further.

3. Strong anti-interference ability: The RS-485 bus uses differential signal transmission and has good anti-interference ability, and can deal with noise and electromagnetic interference in industrial environments.

4. Half-duplex communication: RS-485 is a half-duplex communication protocol, which can only carry out one-way data transmission at the same time.

5. Flexibility: The RS-485 bus can perform data communication by setting the master-slave mode to achieve flexible device control and data exchange.

The RS-485 bus can be used in various fields, and is especially suitable for long-distance data transmission and scenarios with harsh noise environments, such as industrial automation, building automation, smart agriculture, etc. At the same time, the reliability and anti-interference ability of the RS-485 bus also make it a popular communication standard.

#connect #RS485 #bus #ohm #matching #resistor