How to ensure the stability of LED display signal
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The LED display that is in use suddenly has garbled characters due to signal problems. If it is at a severe opening ceremony, it will not be able to make up for it. How to ensure 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. The choice of transmission medium is particularly important.
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 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.
Signal reflection in communication lines
In addition to signal attenuation, another element that affects signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main causes of the bus's signal reflection. First, 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. Second, 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 encounters a small or no cable impedance, 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 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.
Influence of distributed capacitance on bus transmission function
Transmission cables are generally twisted pairs, and capacitance occurs between two parallel conductors of a twisted pair. 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 a flat "1" accidentally calls, 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.
Develop a simple and reliable communication protocol
When the communication distance is short and the application environment is small, we sometimes need only a simple one-way communication to complete the full function of the project, but most of the operating environment is not the case. 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.