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LED display signal unstable reason!

2023-11-21 23:38:35

The LED display in use suddenly appears garbled due to signal problems, if it is in a serious opening ceremony, it is irreparable. How to ensure the reliability and stability of signal transmission has become a subject that engineers have to face. The signal weakens with the increase of distance during transmission. The choice of transmission medium is particularly important.

Attenuation of signal

It is not difficult to understand that no matter what medium the signal is transmitted by, it will be attenuated during transmission. We can regard the RS-485 transmission cable as an equivalent circuit composed of several resistors, inductors and capacitors. The effect of the resistance of the wire on the signal is small and negligible. The distributed capacitance C of the cable is mainly produced by two parallel wires 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 baud rate, the greater the signal attenuation. Therefore, in the case that the amount of data transmitted is not very large and the transmission rate requirement is not very high, we generally choose a baud rate of 9 600 bps.

Signal reflection in communication circuits

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 the signal reflection of bus construction. Impedance mismatch, 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 entire communication line signal is messed up, and once such a reflected signal triggers the comparator at the input end of the 485 chip, the error signal will occur.

Our general solution is to add the bias resistance of the A and B lines of the bus with a certain resistance value, and separate the pull up and pull down, so that there will be no unpredictable chaotic signals. Second, impedance discontinuity, and light from one medium into another caused by reflection is similar. At the end of the transmission line, the signal occasionally encounters little or no cable impedance, and the signal will cause reflection in this place. The most common way to eliminate this reflection is to straddle a terminal resistor of the same size as the characteristic impedance of the cable at the end of the cable, making the impedance of the cable continuous. Since the transmission of the signal on the cable is bidirectional, a terminal resistor of the same size should be crossed at the other end of the communication cable.
Effect of distributed capacitance on bus transmission function

The transmission cable is generally twisted pair, and the capacitor will occur between two parallel wires of the twisted pair. Similarly small capacitors exist between the cable and the earth. Since the signal transmitted on the bus is composed of a lot of “1” and “0” bits, when special bytes such as 0x01 are encountered, the level “0” makes the distributed capacitor charge at the time it is satisfied, and when the level “1” occasionally calls, the charge gathered by the capacitor cannot be discharged in a short time, resulting in the deformation of the signal bit. Then it affects the quality of the entire data transmission.

Develop simple and reliable communication protocols

When the communication distance is short and the application environment is annoying, we sometimes only need simple one-way communication to complete the full function of the project, but most application environments are not so. The preliminary stage of the project is whether the wiring is professional (such as the signal line and the power line to adhere to a certain distance), the indecisive communication distance, the annoyance degree around the communication line, whether the communication line is selected as a twisted pair shielded line, etc., these elements have a great impact on the normal communication of the system. Therefore, it is particularly important to formulate a set of perfect communication protocols.


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