A few easy to ensure that the LED display signal stability
Being used in the led display screen suddenly due to signal problems appear garbled, if it is in a major opening ceremony, the loss is irreparable. Therefore, how to improve the reliability and stability of signal transmission has become a topic that engineers have to face. Signal in the transmission process, as the distance increases and weakens. Therefore, the choice of transmission medium is particularly important.
The following RS-485 as a long-distance data transmission line to pay attention to the following matters
1, the attenuation of the signal
It is not difficult to understand, no matter what kind of medium the signal is transmitted, will produce attenuation in the transmission process. We can view the RS-485 transmission cable as a number of resistance, inductance and capacitance of the joint composition of the equivalent circuit. The resistance of the wire has a small, negligible effect on the signal. The distributed capacitance C of the cable is mainly generated by the 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 will be. Therefore, in the case that the amount of transmitted data is not very large and the transmission rate requirement is not very high, usually we use a baud rate of 9 600 bps.
2、Signal reflection in the communication line
In addition to signal attenuation, another factor affecting signal transmission is signal reflection. Impedance mismatch and impedance discontinuity are the two main reasons that lead to the formation of RS-485 bus signal reflection. First, impedance mismatch, impedance mismatch is mainly between the 485 chip and the communication line impedance mismatch. The reason why the reflection is caused by the communication line is idle, the entire communication line signal clutter, once such reflected signals triggered by the 485 chip input comparator, it will produce the wrong signal. Our usual solution is the RS-485 bus A, B line with a certain resistance value of the bias resistor, respectively, pull high and pull low, so that there will be no unpredictable clutter signal. Second, impedance discontinuity, as the name suggests, and light from one medium into another medium caused by the reflection is similar. Signal at the end of the transmission line suddenly encountered cable impedance is small or even no, the signal in this place will cause reflection. Eliminate this reflection of the most commonly used method, is at the end of the cable across a cable with the same size of the cable's characteristic impedance of the terminating resistor, so that the cable's impedance continuity. As the signal transmission on the cable is bidirectional, therefore, at the other end of the communication cable should also be connected across a terminating resistor of the same size.
