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Views: 300 Author: Site Editor Publish Time: 2023-11-04 Origin: Site
RS485 does not support arbitrary topology, because arbitrary topology will produce a large number of standing waves and reflections at the branch. This is due to impedance discontinuity.
Impedance discontinuity:
At the end of the transmission line, the signal suddenly encounters little or no cable impedance, and the signal will cause reflection at this place. The principle of this signal reflection is similar to the reflection caused by light from one medium to another. The method of eliminating this reflection must be connected at the end of the cable with a terminal resistor of the same size as the characteristic impedance of the cable, so that the impedance of the cable is continuous. Since the transmission of the signal on the cable is bi-directional, a terminal resistor of the same size can be connected at the other end of the communication cable.
Impedance mismatch:
Another cause of signal reflection is the impedance mismatch between the array transceiver and the transmission cable. In high-frequency circuits, when the frequency of the signal is very high, the wavelength of the signal will be very short, and when the wavelength is short, it can be compared with the length of the transmission line, the reflected signal superimposed on the original signal will change the shape of the original signal. If the characteristic impedance of the transmission line does not match the load impedance, reflection will occur at the load end. The reflection caused by this reason is mainly manifested in the chaos of the entire network data when the communication circuit is in the idle mode.
When the signal is transmitted to the end of the bus, the instantaneous impedance changes (taking RSM485ECHT as an example, the impedance changes from 120kΩ to 96kΩ), resulting in signal reflection and affecting the quality of the signal.
If the power consumption is required and the communication distance is long, it is recommended to use the bus that supports the topology itself, such as POWERBUS,MBUS, etc.
