The running led display suddenly appeared garbled due to signal problems. If it was at an important opening ceremony, the loss would be irreparable. How to realize the reliability and stability of signal transmission has become a major issue for engineers to solve. In the transmission process, the signal will weaken as the distance increases, so the choice of transmission medium is particularly important. How about the LED display network cable transmission signal?

1. The attenuation of the LED display signal: It is not difficult to understand that the signal will attenuate during the transmission process regardless of the medium transmission. 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 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 requirement is not very high, we generally choose a baud rate of 9 600 bps.

  2. Signal reflection in the communication line of the led display screen: 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 the bus. Reason 1: 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 signal of the entire communication line is messed up. Once this kind of reflection signal triggers the comparator at the input of the 485 chip, an error signal will occur. Our general solution is to add bias resistors of a certain resistance to the A and B lines of the bus, and pull them high and low separately, so that there will be no unpredictable messy signals.

   The second reason is that the impedance is discontinuous, which is similar to the reflection caused by light entering another medium from one medium. At the end of the transmission line, the signal suddenly encounters a small or no cable impedance, and the signal will cause reflection at this place. The commonly used method to eliminate this reflection is to connect a terminal resistor of the same size as the characteristic impedance of the cable at 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 be connected across the other end of the communication cable.

  3. The influence of distributed capacitance on the bus transmission function: transmission cables are generally twisted-pair wires, and capacitance occurs between two parallel wires of the twisted-pair wires. There is also a similarly small capacitance between the cable and the ground. Since the signal transmitted on the bus is composed of a lot of "1" and "0" bits, when it encounters special bytes such as 0x01, the level "0" makes the distributed capacitance meet the time to charge, and when the power is When the level "1" comes suddenly, the charge accumulated by the capacitor cannot be discharged in a short time, which causes the deformation of the signal bit, and then affects the quality of the entire data transmission.

4. A simple and reliable communication protocol for the led display screen: When the communication distance is short and the application environment is less disturbing, we sometimes only need a simple one-way communication to complete all the functions of the project, but most of the application environment is not the case. ambition. In the early stage of the project, it is summarized whether the wiring is professional (such as keeping a certain distance between the signal line and the power line), the indeterminability of the communication distance, the degree of disturbance around the communication line, whether the communication line uses twisted-pair shielded wire, etc., these elements are for the system Normal communication has a great impact. Therefore, it is particularly important to draw up a complete communication protocol.