Now the most used is a two-wire connection, this connection is a bus topology, can be connected to a maximum of 32 nodes on the same bus.
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The ideal line is twisted pair: the two lines of half duplex are best to use a pair of twisted pairs, so that the two lines are twisted, and the interference level on the two lines is offset to achieve anti-interference effect. One pair is used for receiving two lines and the other pair is used for sending two lines. RS485 has no power transmission requirements, so the line diameter requirements are not high.
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As far as possible, the bus architecture is adopted, the star connection is reduced, the branch line is as short as possible, and the Daisy chain connection method is adopted as far as possible, that is, the bus is connected to the first node and then jumps to the next node
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Extension and expansion:
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rs-485 adopts balanced transmission and differential reception to achieve communication: the sending end converts the ttl level signal of the serial port into differential signal a and b two outputs, and the differential signal is restored to ttl level signal at the receiving end after cable transmission. Because the transmission line usually uses twisted pair and differential transmission, it has a strong ability to resist common mode interference, and the bus transceiver is very sensitive and can detect voltage as low as 200mv. Therefore, the transmission signal can be recovered from kilometers away. The maximum communication distance of rs-485 is about 1219m, the maximum transmission rate is 10mb\/s, the transmission rate is inversely proportional to the transmission distance, under the transmission rate of 100kb\/s, the maximum communication distance can be reached, if you need to transmit a longer distance, you need to add 485 repeater. rs-485 adopts half-duplex mode and supports multi-point data communication. rs-485 bus network topology generally adopts the terminal matching bus structure. That is, a bus is used to connect each node in series, and ring or star networks are not supported. If you need to use a star structure, you must use a 485 repeater or 485 hub. rs-485 bus generally supports a maximum of 32 nodes, if the use of special 485 chip, can reach 128 or 256 nodes, the maximum can support up to 400 nodes.