RS485, RS means the abbreviation of Recommended Standard, which is the recommended standard. It is a commonly used half-duplex-asynchronous-serial communication bus. Half-duplex means that when two parties communicate, at the same time, only one of them can send, and the other can only receive, not at the same time.
RS485 protocol features
1.Support multiple nodes
Generally, a maximum of 32 nodes are supported, and data exchange and device control can be carried out between nodes through Modbus or other industrial control protocols.
2. Long transmission distance and strong anti-interference ability
This relies on a special differential communication method. This method only requires two signal lines to ensure communication. The voltage difference between the two data lines A/B represents the signal, which represents logic 0 and logic 1. , where the logic 1 signal is represented by the voltage difference between the two lines (A+ and B-) between +2V and +6V, and the logic 0 signal is represented by the voltage difference between the two lines (A+ and B-) between -2V and -6V. expressed between. Usually the limit of the voltage difference between the two is considered to be around 200mv. Signals with a voltage difference lower than 200mv are difficult to detect.
The reason why this method has strong anti-interference performance is also due to the advantages of differential signals, because electromagnetic interference noise will cause some burrs on the level waveform, resulting in the entire waveform after the transmitter transmits it to the receiver. There is some distortion, which has less impact in low-speed communication. However, if this kind of electromagnetic interference is superimposed during high-speed communication, it can easily cause errors in the received information. This is why industrial communication protocols basically have a verification method ( For example, the CRC check of the modbus protocol), but the RS485 differential signal is different. The glitches caused by electromagnetic interference will theoretically affect both communication lines A/B, but the voltage difference signal between the two does not occur. Changes. For example, electromagnetic interference causes the voltage of the A/B line to be 150mv higher than the signal source at a certain moment. Subtracting the two will find that the voltage difference between them has not changed, which is a good way to avoid interference. Influence.
3.RS485 wiring is simple and the bus cost is low
Because you only need to connect two data lines and one ground wire, the wiring is very simple, and the circuit of the 485 bus is also relatively simple. Here is a typical 485 circuit:
