How about the signal transmission of the led display cable?

How about the signal transmission of the led display cable?

Date: August 19, 2019 Source: Tuosheng Optoelectronics

The running led display suddenly appears garbled due to signal problems. If it is at an important opening ceremony, the loss is irreparable. How to achieve the reliability and stability of signal transmission has become a major issue for engineers to solve. During the transmission process, the signal will decrease as the distance increases, so the choice of transmission medium is particularly important. How about the signal transmission of the led display cable?

First, the attenuation of the LED display signal: It is not difficult to understand, the signal will be attenuated during the transmission process regardless of the medium transmission. We can think of the RS-485 transmission cable as an equivalent circuit consisting 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 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. Therefore, in the case that the amount of data transmitted is not very large and the transmission rate requirement is not very high, we generally use a baud rate of 9 600 bps.

Second, the signal reflection in the LED display 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 the bus's signal reflection. The reason is that the impedance is not matched, and 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 of the 485 chip, an error signal will occur. Our general solution is to add the bias resistance of the A and B lines of the bus to the pull-down and pull-down, so that there will be no unpredictable clutter.

Cause two, the impedance is discontinuous, similar to the reflection caused by light entering from one medium into another. At the end of the transmission line, the signal is so low that the cable impedance is small or even, and the signal will cause reflection at this place. A common method of eliminating such reflections is to connect a terminating resistor of the same magnitude as the characteristic impedance of the cable at the end of the cable so that the impedance of the cable is continuous. Since the transmission of the signal on the cable is bidirectional, the same size of the terminating resistor must be bridged at the other end of the communication cable.

Third, the effect of distributed capacitance on the bus transmission function: transmission cables are generally twisted pairs, and the capacitance between the two parallel wires of the twisted pair will occur. A similarly small capacitor exists between the cable and the ground. Since 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" causes the distributed capacitance to be charged at the time of charging, and when the power is charged. When the flat "1" is coming, the charge accumulated by the capacitor cannot be released in a short time, thus causing the deformation of the signal bit, and then affecting the quality of the entire data transmission.

Fourth, led display to develop a simple and reliable communication protocol: When the communication distance is short, the use of environmental annoyance is small, we sometimes only need a simple one-way communication to complete the full function of the project, but most of the operating environment is not the case ambition. Whether the main line of the project is professional in the early stage (such as the signal line and the power line must maintain a certain distance), the undeterminable communication distance, the annoyance around the communication line, whether the communication line uses twisted-pair shielded lines, etc., these elements are given to the system. Normal communication has a huge impact. Therefore, it is particularly important to draw up a complete communication protocol.

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