How to improve the stability of long-distance communication of LED display screens

As we all know, data transmission is a very important link in the field of industrial control, and the stability of data transmission directly affects the reliability of the product. Therefore, how to improve the reliability and stability of data transmission has become a topic that engineers have to face. The following uses the problems that occurred during the implementation of our company's LED production billboard display project as an example to explain the matters that should be paid attention to when using RS-485 as a means of long-distance data communication.

The LED display screen in this project is installed in the production workshop to display the actual output and target output. Six display screens with the same specifications are distributed in six workshops. The length of the cable between adjacent workshops is about 150~200 meters. The long distance and the interference of the motors running in the workshop make the LED display screen communication abnormal, resulting in frequent garbled characters.

After careful analysis and multiple tests, we started from many aspects: improving the hardware circuit, improving the communication protocol between the host and slave computers, and finally solved the problem successfully. The key points of the final solution to the problem are analyzed and explained in detail below.

1. Signal attenuation during transmission

It is not difficult to understand that no matter what medium the signal is transmitted through, it will be attenuated during the transmission process. We can regard 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 generated by the two parallel wires of the twisted pair. The signal loss 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. The attenuation coefficient of conventional cables is shown in Table 1.

Therefore, when the amount of data to be 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 communication lines

In addition to signal attenuation, another factor that affects signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main reasons for signal reflection in the RS-485 bus.

① Impedance mismatch. Impedance mismatch mainly refers to the impedance mismatch between the 485 chip and the communication line. The reflection is caused because when the communication line is idle, the signal of the entire communication line is chaotic. Once such a reflected signal triggers the comparator at the input end of the 485 chip, an erroneous signal will be generated. Our usual solution is to add a bias resistor of a certain resistance to the A and B lines of the RS-485 bus, pull them up and down respectively, so that there will be no unpredictable chaotic signals.

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 even no cable impedance at the end of the transmission line, and the signal will cause reflection at this place. The most common 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 transmission of the signal on the cable is bidirectional, a terminal resistor of the same size should also be connected at the other end of the communication cable, as shown in Figure 2.

This method can reduce the impact of signal reflection to a certain extent. 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 completely equal to the terminal resistance, so we cannot completely avoid the occurrence of signal reflection.

3. Impact of distributed capacitance on RS-485 bus transmission performance

RS-485 transmission cables are usually twisted pairs, and capacitance is generated between the two parallel conductors of the twisted pair. At the same time, there is also a similar small capacitance between the cable and the ground. Since the signal transmitted on the RS-485 bus is composed of countless "1" and "0" bits, when encountering special bytes such as 0x01, the level "0" allows the distributed capacitor to get enough time to charge, and when the level "1" suddenly comes, the charge accumulated in the capacitor cannot be discharged in a short time, thus causing the signal bit to be deformed, thereby affecting the quality of the entire data transmission.

Therefore, on the one hand, we should try to use cables with smaller distributed capacitance, that is, better quality, as communication lines. On the other hand, we should try to reduce the baud rate of communication to 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 has little interference, we sometimes only need simple one-way communication to realize all the functions of the project, but most application environments are not so ideal. Whether the early stage of the project's integrated wiring is professional (for example, the signal line and the power line must maintain a certain distance), the uncertainty of the communication distance, the degree of interference around the communication line, whether the communication line uses a twisted pair shielded line, etc., these factors have a great impact on the normal communication of the system. Therefore, it is particularly important to formulate a complete communication protocol.

The specific method is to transmit data in packets, and pack the data by adding a frame header and a frame tail to each packet of data, with a byte left at the end of the frame as a check byte. The lower computer compares the check byte calculated by itself with the check byte transmitted by the upper computer, and then issues an instruction to the upper computer, whether to resend the previous packet of data or continue to send the next packet of data, and then send the next packet after sending one packet until all are sent. Through such a check and retransmission mechanism, we can eliminate the probability of error and make the communication system operate normally.

5. Conclusion

During the entire project rectification process, we successively adopted the following measures: Since the distance between the communication line and the power supply line is very close and the communication line has no shielding layer, we replaced all communication cables; the communication baud rate of both the upper and lower computers was set to adjustable, so that the appropriate value can be selected according to the specific situation during the debugging process; the communication protocol was optimized; and 120 Ω terminal resistors were added to both ends of the communication cable.