How to ensure the stability of the LED display signal and the emergence of garbled code

　　The LED display in use suddenly appears garbled due to signal problems. If it is on a major opening ceremony, the loss cannot be made up. Therefore, how to improve the reliability and stability of signal transmission has become a topic that engineers have to face. During the transmission process, the signal is weakened as the distance increases. Therefore, the choice of transmission medium is particularly important.

　　Below the RS-485 as a long-distance data transmission line.

　　1. Attenuation of the LED display signal

　　It is not difficult to understand that no matter what medium transmission of the signal, it will generate attenuation during the transmission process. We can treat the RS-485 transmission cable as an equivalent circuit composed of several resistors, inductors and capacitors. The resistance of the wires has a small impact on the signal and can be ignored. The distributed capacitance C of the cable is mainly generated by two parallel wires of the twisted wire. The loss of the signal is mainly due to the LC low -pass filter composed of the distributed capacitor and distributed inductance of the cable. The higher the communication Potter rate, the larger the signal attenuation. Therefore, when the amount of data is not large and the transmission rate requirements are not high, we usually use the Potter rate of 9 600 BPS.

　　2. Signal reflection in LED display communication lines

　　In addition to signal attenuation, another factor affecting signal transmission is signal reflection. The impedance does not match and the impedance incompleteness is the two main reasons that cause the RS-485 bus to form signal reflection.

　　First, the impedance does not match, and the impedance does not match the impedance between the 485 chip and the communication line. The reason for reflection is because when the communication line is free, the entire communication line signal is chaotic. Once such reflex signals trigger the comparator at the 485 chip input terminal, an error signal will be generated. Our usual solutions are to add the bias resistance of the RS-485 bus A and B to the bias resistance of a certain resistance value, which will be pulled and lower, so that there will be no unpredictable clutter signals.

　　Second, the impedance is discontinuous, as the name suggests, it is similar to the reflection caused by the light from one media to another media. The signal suddenly encountered the cable impedance at the end of the transmission line very small or even without, and the signal would cause reflection in this place. The most commonly used method to eliminate this reflection is to cross the end of the cable to cross a terminal resistance of the same size as the cable characteristic impedance, so that the impedance of the cable is continuous. Because the transmission of the signal on the cable is two -way, it is also necessary to cross a terminal resistor of the same size at the other end of the communication cable.

　　3. The effect of LED display distributed capacitance on RS-485 bus transmission performance

　　The RS-485 transmission cable is usually mostly twisted lines, and the capacitance will generate between the two parallel wires of the twisted wire. At the same time, there are similar capacitors between cables and earth. Because the signals transmitted on the RS-485 bus are composed of countless "1" and "0" bit, when encountering special bytes such as 0x01, the level "0" makes the distributed capacitor sufficient time to charge. When the level "1" suddenly came, the charge of the capacitor concentration could not be released in a short time, which caused the transformation of the signal bit, which affected the quality of the entire data transmission.

　　4. LED display sets 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 achieve all the functions of the project, but most of the application environment is not so ideal. Whether the comprehensive wiring of the project is professional (such as maintaining a certain distance between the signal line and the power line), the irreversible communication distance, the degree of interference around the communication line, whether the communication line adopts a twisted shield line, etc. These factors are given the system to the system Normal communication has a great impact. Therefore, formulating a complete communication agreement is particularly important.

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