The LED display in use suddenly appears garbled due to signal problems. If it is at a severe opening ceremony, it is irreparable to lose it. How to ensure the reliability and stability of signal transmission has become an issue that engineers have to face. During the transmission of the signal, the signal weakens with increasing distance. The choice of transmission media is particularly important.
Signal reflection in communication lines
In addition to signal attenuation, another factor affecting signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main causes of signal reflection from the bus. First, 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 messy. Once such a reflected signal triggers the comparator at the input end of the 485 chip, an error signal will occur. Our general solution is to add bias resistors with a certain resistance to the A and B lines of the bus, and pull them up and down, so that unpredictable clutter will not appear. Second, the impedance discontinuity is similar to the reflection caused by light entering from one medium to another. The signal at the end of the transmission line occasionally encounters a cable with a very small or no impedance, and the signal will cause reflections at this place. The most common method to eliminate this reflection is to connect a terminating resistor of the same size as the characteristic of LED sign parts impedance of the cable at the end of the cable to make the impedance of the cable continuous. Because the transmission of the signal on the cable is bidirectional, a termination resistor of the same size must be connected across the other end of the communication cable.
It is not difficult to understand that no matter what medium the signal is transmitted on, the signal will be attenuated and 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 can be ignored. The distributed capacitance C of the cable is mainly caused by the 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 communication baud rate, the greater the signal attenuation. Therefore, when the amount of transmitted data is not very large and the transmission rate is not very high, we generally choose a baud rate of 9 600 bps.
Draw up a simple and reliable communication protocol
When the communication distance is short and the operating environment is less disturbing, sometimes we only need a simple one-way communication to complete all the functions of the project, but most of the operating environment is not the case. In the early stage of the project, whether the wiring is professional (such as signal cables and power cables must adhere to a certain distance), the indeterminacy of communication distance, the degree of annoyance around communication lines, whether to use twisted shielded cables for communication lines, etc., all these factors are given to the system. Normal communication has a huge impact LED Video Processor. Therefore, it is particularly important to develop a complete communication protocol.
Influence of distributed capacitance on bus transmission function
Transmission cables are generally twisted pairs. Capacitance will occur between the two parallel wires of the twisted pair. A similarly small capacitance exists between the cable and the ground. Because the signal transmitted on the bus is composed of many “1” and “0” bits, when special bytes such as 0x01 are encountered, the level “0” allows the distributed capacitor to be charged at a time that meets the requirements. When the level “1” is accidentally called, the electric charge accumulated in the capacitor cannot be discharged within a short time, so the signal bit is deformed, and then the quality of the entire data transmission is affected.