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The connection method of RS485 communication

Date:2022-07-06

The connection method of RS485 communication

RS485 is widely used and can support ModBus protocol. Because it is simple, cheap, reliable and mature, it is widely used in industrial control, smart instrumentation and power communication.

In theory, the transmission distance of an RS485 bus can reach 1200 meters. However, in practical applications, it is affected by the communication chip, baud rate, wire, interference, etc., which is actually far from this value.

Theoretically, one RS485 bus can be connected to 254 devices, but in reality it is far from reach. In normal applications, it is about 30 devices.

Regarding the selection of wires:

The communication cable should choose two-core shielded twisted pair, and the wire diameter should not be less than 0.5 square. Twisted pair is because 485 communication adopts the principle of differential mode communication, and the anti-interference of twisted pair is the best.

There are several ways to ensure the quality of communication:

1. Common ground method: Connect the GND ground of all 485 devices with one line or shielded line, so as to avoid the potential difference between all devices that affects communication.

2. Terminal resistance method: connect 120 ohm terminal resistance in parallel to the 485+ and 485- of the last 485 device to improve the communication quality.

3. If the communication distance is too long, it is recommended to use a repeater or 485HUB to solve the problem if it exceeds 500 meters. If there are too many loads, it is recommended to use 485HUB to solve the problem if there are more than 30 units on a bus.

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