Constructing 485 bus communication network.
RS485 has two connection modes: two-wire system and four-wire system. Four-wire system can only realize point-to-point communication, and it is rarely used now. Two-wire system is used now. This connection mode is a bus topology, and up to 32 nodes can be connected on the same bus.
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Pay attention to several key points of RS485 hardware: interface chip, terminal resistance and grounding.
1, the interface chip depends on the data sheet of the chip. Generally, there is no wrong pin connection, the power supply is normal, the connection is normal, and there will be no problem. If the communication is abnormal, first check whether the line is connected incorrectly. a-& gt； +，B-& gt； -。
2. The terminal resistance is very important for RS485 communication. Generally, there are two resistors across two signal lines, with the resistance value of120Ω, one for the master device and the other for the slave device at the end. If there is a communication problem, you can check whether the terminal resistance is connected normally and whether the resistance between signal lines is normal (both parallel connections are 60Ω).
3. For grounding, RS485 detects the difference between two signal lines, so it can communicate without grounding. If you encounter communication abnormality, you can try grounding or ungrounded.