RS485 has two-wire system and four-wire system. Four-wire system can only realize point-to-point communication and is rarely used at present. At present, the two-wire system is widely used. This is a bus topology, and the same straight bus can connect up to 32 nodes.
When the host sends data, it must have the address of the slave source machine, and all slaves will receive the data sent by the host machine. However, to compare the address with the local address, do the corresponding operation and then return the data to the host. Slaves can't actively send data to the host, and slaves can't send data to each other.
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principle of operation
Use differential signals. When discussing a/D, we will discuss the concept of differential signal input and introduce the advantages of differential input. The biggest advantage is that it can suppress common-mode interference ... especially in complex industrial environment, the difference method can effectively improve the reliability of communication.
RS485 adopts two communication lines, usually represented by A and B or D+ and D- .. Logic "1" means that the voltage difference between the two lines is (0.2 ~ 6) V, and logic "0" means that the voltage difference between the two lines is -(0.2 ~ 6) V, which is a typical differential communication.
The communication speed of RS485 is fast, and the highest transmission speed can reach more than10mb/s. The internal physical structure is RS485, and the anti-interference ability is greatly improved by combining balanced drive and differential reception.