RS-485 typical distribution network structure RS485 bus advantages and disadvantages

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RS-485 typical distribution network structure RS485 bus advantages and disadvantages

Posted Date: 2024-02-03

Definition of rs485 bus

RS485 bus is a differential signal transmission technology, also known as RS-485 bus or EIA-485 bus. It is a standard approved by the Electronic Industry Association (EIA) in 1983. It is an electrical standard and defines the physical layer of the interface, such as voltage, impedance, etc., but does not involve software protocols, communication timing and communication data.

The RS485 bus uses balanced transmission and differential reception to achieve communication, and has strong resistance to common mode interference. The bus network topology generally adopts a terminal-matched bus structure, that is, a bus connects various nodes in series, and does not support ring or star networks. If you need to use a star structure, you must use a 485 repeater or 485 hub. The RS485 bus generally supports 32 nodes. If a special 485 chip can reach 128 or 256 nodes, it can support a maximum of 400 nodes.

The RS485 bus is a low-cost serial communication bus with the advantages of high sensitivity, long-distance transmission and multi-node support. In the field of industrial automation, the RS485 bus is widely used for serial communication between various devices, such as sensors, actuators, PLC, HMI, etc.

RS-485 bus networks generally use terminating resistors for matching, but this does not need to be considered in short distances and low speeds. In theory, when sampling at the midpoint of each received data signal, matching can be disregarded as long as the reflected signal attenuates low enough at the beginning of sampling. In other words, when the signal conversion time (rise or fall time) exceeds more than 3 times the time required for one-way transmission of electrical signals along the bus, no matching is required.

Figure RS-485 typical distribution network structure

RS-485 is a serial bus electrical characteristic specification. The specific distribution network structure also needs to consider the upper layer protocol requirements. Modbus is a common communication protocol based on the electrical characteristics of RS-485. Zeyao Technology digital radio supports RS-485 interface. The most typical use is to connect to PLC equipment to replace traditional RS-485 cables, which facilitates flexible deployment and reduces engineering costs.

The figure shows a schematic diagram of using DTU to replace RS-485 cable. In the figure, the DTU on the right inputs the R-S485 signal, and the DTU on the left outputs the corresponding RS-485 signal.

Since wireless devices have a certain delay, when using DTU to replace traditional cables, the user equipment timeout waiting time needs to be increased to a certain extent.

What are the advantages and disadvantages of RS485 bus

The RS485 bus has the following advantages:

Long transmission distance: RS485 bus has a long transmission distance and can achieve communication distances from tens of meters to several kilometers. This makes it suitable for applications requiring long-distance communication.

Strong anti-interference ability: The RS485 bus uses differential signal transmission, so it has strong anti-interference ability, can resist external interference and line noise, and maintain stable signal transmission.

Multi-node support: The RS485 bus can be mounted with multiple devices and can support up to 32 nodes. If a special 485 chip is used, it can reach 128 or 256 nodes, and the largest one can support 400 nodes.

High reliability: Because the RS485 bus uses differential signal transmission, it is not easily affected by external interference and line noise, and has high reliability.

However, the RS485 bus also has some disadvantages:

Low communication rate: The communication rate of the RS485 bus is generally low, with a maximum transmission rate of 10Mbps, which cannot meet the needs of high-speed data transmission.

Trade-off between communication distance and number of nodes: As the number of nodes increases, the communication distance decreases. Therefore, there is a trade-off between communication distance and number of nodes in practical applications.

High maintenance costs: If there are a large number of devices mounted on the bus, repeaters and other equipment need to be added, resulting in increased maintenance costs.

Poor networking flexibility: Since the network topology of the RS485 bus is relatively fixed, the networking flexibility is poor and it cannot be moved and accessed as easily as a wireless network.

Review Editor: Huang Fei

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