When do I Need to Add a Terminating Resistor?
1. When the communication speed is low or the communication distance is short, it is recommended not to add a terminal resistor
When the communication speed is low or the communication distance is short, the signal reflection has little effect on the communication signal, and the power consumption can be greatly reduced without adding a terminal resistor. It has a higher differential voltage amplitude, which improves the reliability of communication.
2. The communication distance is long and the communication speed is fast, and the signal quality is required to be high.
At this time, the terminal resistance can be increased to prevent the signal reflection problem caused by the sudden change of impedance and improve the signal quality, but it should be ensured that the differential voltage of the bus is not within the threshold level range when the bus is idle.
3. Requirements for power consumption and long communication distance
Generally, the signal is sampled at the middle time of a bit. Due to the low communication speed, the time of each bit is longer, so the reflected signal has been consumed when it reaches the sampling point, and has no effect on communication. Figure 8 and Figure 9 show the waveforms of the RSM485ECHT at 1200m 9600bps without terminating resistors at the beginning and end. It can be seen that the reflected signal has been consumed before reaching the middle of each bit.
Therefore, for applications that have higher requirements on the power consumption of the RS-485 transceiver and a longer communication distance, the communication speed should be appropriately reduced.
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