First, the attenuation of the signal: It is not difficult to understand that the signal will be attenuated during the transmission process no matter what medium is used for transmission. We can regard the RS-485 transmission cable as an equivalent circuit composed of several resistors, inductors and capacitors. The resistance of the wires has little effect on the signal and can be ignored. The distributed capacitance C of the cable is mainly generated by the two parallel conductors of the twisted pair. The loss of the signal 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 generally choose a baud rate of 9 600 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 causes of signal reflections that make up the bus. Reason 1. Impedance mismatch. Impedance mismatch is mainly due to 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 such a reflection signal triggers the comparator at the input end of the 485 chip, an erroneous signal will occur. Our general solution is to add a bias resistor with a certain resistance value to the A and B lines of the bus, and pull them up and down respectively, so that there will be no unpredictable and cluttered signals. The second reason is that the impedance is discontinuous, which is similar to the reflection caused when light enters another medium from one medium. At the end of the transmission line, the signal suddenly encounters little or no cable impedance, and the signal will cause reflection at this place. A common method to eliminate this reflection is to connect a terminal resistor with the same size as the characteristic impedance of the cable at the end of the cable, so that the impedance of the cable is continuous. Since the transmission of the signal on the cable is bidirectional, a terminating resistor of the same size should be connected across the other end of the communication cable.

3. The influence of distributed capacitance on bus transmission performance: The transmission cable is generally twisted pair, and capacitance will occur between the two parallel wires of the twisted pair. There is also a similarly small capacitance between the cable and the ground. Since the signal transmitted on the bus is composed of many "1" and "0" bits, when encountering special bytes such as 0x01, the level "0" allows the distributed capacitance to be charged for a sufficient time, and when the power When the level "1" suddenly comes, the electric charge accumulated by the capacitor cannot be discharged in a short time, which leads to the deformation of the signal bit, and then affects the quality of the entire data transmission.

Fourth, formulate a simple and reliable communication protocol: when the communication distance is short and the application environment is less disturbing, we sometimes only need simple one-way communication to complete the full function of the project, but most application environments are not so ideal. In the early stage of the project, whether the wiring is professional (for example, the signal line and the power line must maintain a certain distance), the indeterminacy of the communication distance, the level of disturbance around the communication line, whether the communication line uses twisted-pair shielded wire, etc., these elements are all for the system. Normal communication has a great impact. Therefore, it is particularly important to formulate a complete set of communication protocols.