1. Signal attenuation
It is not difficult to understand that no matter what medium is used to transmit a signal, attenuation will occur during the transmission process. We can regard the RS-485 transmission cable as an equivalent circuit composed of several resistors, inductors and capacitors. The resistance of the wire has little effect on the signal and can be ignored. The distributed capacitance C of the cable is mainly produced by two parallel wires of the twisted pair. The signal loss is mainly due to the LC low-pass filter composed of the distributed capacitance and distributed inductance of the cable. The higher the communication baud rate, the greater the signal attenuation. Therefore, when the amount of transmitted data is not very large and the transmission rate requirement is not very high, we usually use a baud rate of 9600 bps.
2. Signal reflection in communication lines
In addition to signal attenuation, another factor that affects signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main reasons that cause signal reflections on the RS-485 bus.
①Impedance mismatch, the impedance mismatch is mainly the impedance mismatch between the 485 chip and the communication line. The reason for the reflection is that when the communication line is idle, the signal of the entire communication line is messy. Once this kind of reflection signal triggers the comparator at the input of the 485 chip, an error signal will be generated. Our usual solution is to add bias resistors with a certain resistance to the A and B lines of the RS-485 bus, and pull them up and down respectively, so that there will be no unpredictable messy signals.
② Impedance is discontinuous, as the name implies, it is similar to the reflection caused by light entering another medium from one medium. When the signal suddenly encounters the cable impedance at the end of the transmission line is very small or not, the signal will cause reflection in this place. The most common method to eliminate this kind of reflection is to connect a terminal resistor of the same size as the characteristic impedance of the cable across the end of the cable to make the impedance of the cable continuous. Since the signal transmission on the cable is bidirectional, a terminal resistor of the same size should also be connected across the other end of the communication cable.
In this way, the influence of signal reflection can be reduced to a certain extent. However, in practical applications, because the characteristic impedance of the transmission cable is related to the application environment such as the communication baud rate, the characteristic impedance cannot be exactly equal to the terminal resistance, so we cannot completely avoid the occurrence of signal reflection.
3. The influence of distributed capacitance on RS-485 bus transmission performance
RS-485 transmission cables are usually twisted pairs, and capacitance is generated between two parallel wires of the twisted pair. At the same time, a similarly small capacitance also exists between the cable and the ground. Since the signal transmitted on the RS-485 bus is composed of countless "1" and "0" bits, when encountering special bytes such as 0x01, the level "0" makes the distributed capacitors have sufficient time to charge. And when the level "1" suddenly comes, the charge accumulated by the capacitor cannot be discharged in a short time, which causes the deformation of the signal bit, thereby affecting the quality of the entire data transmission. Therefore, on the one hand, we should try our best to use cables with smaller distributed capacitance, that is, with better quality, as the communication line; on the other hand, try to reduce the baud rate of the communication as much as possible to allow sufficient time for the capacitor to discharge.
4. Develop a simple and reliable RS-485 communication protocol
When the communication distance is short and the application environment interference is small, we sometimes only need simple one-way communication to realize all the functions of the project, but most application environments are not so ideal. Whether the integrated wiring in the early stage of the project is professional (such as keeping a certain distance between the signal line and the power line), the indecisability of the communication distance, the degree of interference around the communication line, whether the communication line uses twisted-pair shielded wire, etc., these factors are all for the system Normal communication has a great impact. Therefore, it is particularly important to develop a complete communication protocol.
The specific method is to transmit the data in packets, and pack the data by adding a frame header and a frame end to each packet of data, and one byte at the end of the frame is reserved as a check byte. The lower computer compares the check byte calculated by itself with the check byte transmitted from the upper computer, and then issues an instruction to the upper computer whether to resend the data just now or to send the next packet of data, that’s it After one package is sent, send the next one until the end is finished. Through such a check and retransmission mechanism, we can discard the probability of error and make the communication system operate normally.
