4 important issue of the reliable signal transmission for LED display

The running LED display suddenly has garbled characters due to signal problems. If it is at an important opening ceremony, the loss cannot be compensated. Therefore, how to improve the reliability and stability of signal transmission has become a problem that engineers have to solve

In the transmission process, the signal will weaken as the distance increases, so the choice of the transmission medium is particularly important

1. Attenuation of signals

It is not difficult to understand that no matter what medium is used for signal transmission, it will attenuate during 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 two parallel conductors of the twisted pair. The loss of 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 affecting signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main causes of bus reflection.

1st: Impedance mismatch. The 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 a certain resistance to the A and B lines of the bus, and pull them high and low, so that no unpredictable messy signals will appear.

2nd: The impedance is discontinuous, which is similar to the reflection caused when light enters another medium from one medium. When the signal at the end of the transmission line suddenly encounters a cable with little or no impedance, the signal will cause reflection at this place. A common method to eliminate this reflection is to connect a termination resistor of the same size as the characteristic impedance of the cable at the end of the cable to make the cable impedance continuous. Since the transmission of the signal on the cable is bidirectional, a terminating resistor of the same size must be connected across the other end of the communication cable.

In this way, the influence of signal reflection can be somewhat reduced. However, in practical applications, since the characteristic impedance of the transmission cable is related to the application environment such as the communication baud rate, the characteristic impedance cannot be exactly equal to the termination resistance, so we cannot completely avoid signal reflection.

3. Influence of distributed capacitance on the transmission function of the bus

The transmission cable is usually a twisted pair, and the capacitance will occur between the two parallel conductors of the twisted pair. A similar 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 a special byte such as 0x01 is encountered, the level "0" makes the distributed capacitor meet the charging time, and when the power When the "1" came suddenly, the charge accumulated in the capacitor could not be discharged in a short time, so it caused the distortion of the signal bit, and then affected the quality of the entire data transmission.

Therefore, we should try to use a cable with a small distributed capacitance or better quality as the communication line. On the other hand, we should try to reduce the baud rate of communication and give the capacitor enough time to discharge.

4. Develop a simple and reliable communication protocol

when the communication distance is short and the application environment is less troublesome, 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 so ambitious. The early stage of the project summarizes whether the wiring is professional (such as the signal line and the power line must adhere to a certain distance), the indeterminacy 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 all given to the system Normal communication has a great impact. Therefore, it is particularly important to develop a complete communication protocol.

The specific method is to packetize and transmit data, and pack the data by adding each frame of data to the frame header and the end of the frame, wherein one byte is reserved as a check byte at the end of the frame. The lower computer compares the check byte calculated by itself with the check byte transmitted by the host computer, thereby issuing an instruction to the host computer, whether to resend the data of the packet or send the next packet of data. After the package is sent, send another packet until it is finished. Through such a verification retransmission mechanism, we can discard the probability of error and make the communication system operate normally.

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