What is RS485 Bus

In industrial sites, it is often necessary to collect multi-point data, such as analog signals or switching signals, and the RS485 bus is generally used. RS-485 adopts a half-duplex working mode and supports multi-point data communication. The network topology of RS-485 bus generally adopts a bus structure with terminal matching, that is, a single bus is used to connect each node in series, and it does not support ring or star networks.

RS485 has no specific physical form; it is an interface adopted according to the actual situation of the project. RS485 uses differential signal negative logic: +2V～+6V represents "0", and -6V～-2V represents "1".

RS485 Protocol and Cable Applications

RS485 has two wiring modes: two-wire and four-wire. The four-wire mode can only realize point-to-point communication and is rarely used now. The two-wire mode is mostly adopted currently, which features a bus topology structure that can connect up to 32 nodes on the same bus.

RS-485 only requires twisted pair cables; in environments with higher requirements, coaxial cables with a shielding layer can be used. For a specific transmission line, the maximum allowable cable length for data signal transmission from the RS485 interface to the load is inversely proportional to the baud rate of signal transmission. This length is mainly affected by factors such as signal distortion and noise.

According to the theoretical structure of the 485 bus, under ideal environmental conditions, the transmission distance of the 485 bus can reach 1200 meters. However, in practical applications, the transmission distance is shorter than 1200 meters, and the specific distance depends on the surrounding environment.

During transmission, repeaters can be added to amplify signals, with a maximum of eight repeaters. That is to say, theoretically, the maximum transmission distance of RS485 can reach 9.6 kilometers. For long-distance transmission, optical fiber can be used as the transmission medium, with photoelectric converters added at both the transmitting and receiving ends. The transmission distance of multimode fiber is 5-10 kilometers, while that of single-mode fiber can reach 50 kilometers.

What are the commonly used standard protocols that use RS-485 as the physical layer? For example: industrial HART bus, Modbus protocol, and Profibus DP.

Precautions for RS485 Protocol Application

1.RS485 cables must use special shielded twisted pair to communicate with 128 connected devices; ordinary twisted pair network cables cannot be used as substitutes.

2.The shielding layer of the 485 communication cable is used as a ground wire. Connect the device grounds in the network (such as machines and computers) together and reliably connect them to the earth to eliminate common-mode interference.

3.Separation of strong and weak electricity: Communication lines should be kept away from interference sources such as high-voltage wires and fluorescent lamps. If communication lines cannot be separated from interference sources like power lines, they should be placed perpendicular to power lines (not parallel, let alone bundled together), and high-quality twisted pair routing should be adopted.

4.The RS485 bus must adopt a daisy-chain structure instead of a star structure. A star structure will generate reflected signals, which affect 485 communication. The branch line from the bus to each terminal device should generally not exceed 5 meters. It is advisable to connect 120Ω terminal resistors at both ends of RS485 devices.

Some RS-485 Products: