The LED display screen that is currently in use suddenly appears garbled due to signal issues. If it was lost during a severe opening ceremony, it would be irreparable. How to ensure the reliability and stability of signal transmission has become a topic that engineers have to face. The signal weakens as the distance increases during transmission. The choice of transmission medium is particularly important.
01 Signal attenuation
It is not difficult to understand that signals, regardless of the medium they rely on for transmission, will experience attenuation during the transmission process. We can consider the RS-485 transmission cable as an equivalent circuit composed of several resistors, inductors, and capacitors combined. 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 loss of signals is mainly due to the LC low-pass filter composed of distributed capacitance and distributed inductance of the cable. The higher the communication baud rate, the greater the signal attenuation. Therefore, when the amount of data transmitted is not very large and the transmission rate requirement is not very high, we generally choose a baud rate of 9600 bps.
02 Signal reflection in communication circuits
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 bus composition. 1、 Impedance mismatch, mainly refers to the impedance mismatch between the 485 chip and the communication line. The reason for reflection is that when the communication line is idle, the entire communication line signal is chaotic. Once this type of reflection signal triggers the comparator at the input end of the 485 chip, an erroneous signal will occur. Our general solution is to convert bus A Add a bias resistor with a certain resistance value to the B line, separate from pulling it up and down, so that unpredictable and chaotic signals will not occur. 2、 Impedance discontinuity is similar to the reflection caused by light entering another medium from one medium. When a signal encounters a cable with very low or even no impedance at the end of the transmission line, it will cause reflection at this point. The most commonly used method to eliminate this reflection is to connect a terminal resistor at the end of the cable that is the same size as the characteristic impedance of the cable, making the impedance of the cable continuous. Due to the bidirectional transmission of signals on cables, a terminal resistor of the same size needs to be connected across the other end of the communication cable.
03 The influence of distributed capacitance on bus transmission function
Transmission cables are generally twisted pair cables, and capacitance occurs between the two parallel wires of twisted pair cables. There is also a similar small capacitance between the cable and the earth. Due to the fact that the signal transmitted on the bus is composed of many "1" and "0" bits, when encountering special bytes such as 0x01, the level "0" allows the distributed capacitance to be charged within a certain time. However, when the level "1" accidentally calls, the accumulated charge of the capacitor cannot be discharged in a short period of time, resulting in the deformation of the signal bits and affecting the quality of the entire data transmission.
04 Develop a simple and reliable communication protocol
When the communication distance is short and the application environment is less troublesome, sometimes we only need simple one-way communication to complete the full function of the project, but most application environments are not like this. In the early stage of the project, factors such as whether the wiring is professional (such as maintaining a certain distance between signal and power lines), the uncertainty of communication distance, the degree of disturbance around communication lines, and whether twisted pair shielded wires are used for communication lines all have a great impact on the normal communication of the system. Therefore, it is particularly important to develop a comprehensive communication protocol.
Post time: Aug-26-2024
