How to ensure the reliability of LED display signal transmission

The LED electronic display that is in use suddenly has garbled characters due to signal problems. If it is at a major opening ceremony, the loss is irreparable. Therefore, how to improve the reliability and stability of signal transmission has become a problem that engineers have to face.

The signal is weakened as the distance increases during transmission. Therefore, the choice of transmission medium is particularly important. The following should be noted when using RS-485 as a long-distance data transmission line.
1, signal attenuation
It is not difficult to understand that the signal will be attenuated during transmission regardless of the medium transmission. We can think of the RS-485 transmission cable as an equivalent circuit composed of several resistors, inductors and capacitors, as shown in Figure 1.

The resistance of the wire has little effect on the signal and can be ignored. 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 of the cable and the distributed inductance. The higher the communication baud rate, the greater the signal attenuation.

The attenuation coefficient of the conventional cable is shown in Table 1.

Therefore, in the case where the amount of data transmitted is not very large and the transmission rate requirement is not very high, we usually use a baud rate of 9 600 bps.
2. Signal reflection in the communication line
In addition to signal attenuation, another factor that affects signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main causes of signal reflection on the RS-485 bus.
1 Impedance mismatch, impedance mismatch is mainly the impedance mismatch between 485 chip and communication line. The reason for the reflection is that when the communication line is idle, the entire communication line signal is disordered. Once such a reflected signal triggers the comparator at the input of the 485 chip, an erroneous signal is generated. Our usual solution is to increase the pull-off resistors of the A and B lines of the RS-485 bus with a certain resistance value, so that there is no unpredictable messy signal.
2 Impedance discontinuity, as the name implies, is similar to the reflection caused when light enters another medium from one medium. The signal suddenly encounters a small or no cable impedance at the end of the transmission line, and the signal will cause reflection at this place. The most common way to eliminate this reflection is to connect a terminating resistor of the same magnitude as the cable's characteristic impedance at the end of the cable to make the impedance of the cable continuous. Since the transmission of the signal on the cable is bidirectional, a termination resistor of the same size is also connected across the other end of the communication cable, as shown in Figure 2.

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 the occurrence of signal reflection.
3. Influence of distributed capacitance on transmission performance of RS-485 bus
RS-485 transmission cables are usually twisted pairs, and capacitance is generated between two parallel conductors of a twisted pair. At the same time, there is also a similarly small capacitance between the cable and the earth. Since the signal transmitted on the RS-485 bus is composed of a myriad of "1" and "0" bits, when a special byte such as 001 is encountered, the level "0" causes the distributed capacitance to be charged for a sufficient amount of time. When the level "1" suddenly comes, the charge accumulated by the capacitor cannot be discharged in a short time, thus causing the deformation of the signal bit, thereby affecting the quality of the entire data transmission.
Therefore, on the one hand, 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 RS-485 communication protocol
When the communication distance is short and the application environment is small, we sometimes need only one-way communication to achieve all the functions of the project, but most of the application environment is not so ideal. Whether the integrated wiring in the early stage of the project is professional (such as the signal line and the power line must maintain a certain distance), the communication distance is undeterminable, the degree of interference around the communication line, whether the communication line uses twisted-pair shielded lines, etc., all of which are given to the system. Normal communication has a huge impact. Therefore, it is particularly important to develop a comprehensive communication protocol.
The specific method is to packetize the data transmission, and pack the data by adding the frame header and the frame tail to each packet of data, 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.
5 Conclusion
In the whole process of rectification of the project, we have adopted the following methods: Because the distance between the communication line and the power supply line is very close, and the communication line has no shielding layer, we replace all communication cables; both the upper computer and the lower computer Set the communication baud rate to be adjustable, so that you can select the appropriate value according to the specific conditions during the debugging process; optimize the communication protocol; add 120 Ω termination resistors at both ends of the communication cable.

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