chunyang 发表于 2016-10-26 00:18
不能这么认识。485终端电阻的主要作用是抑制信号反射，而信号反射影响信号质量的前提是信号波长与总线的 ...

“总线长度低于信号波长时可以不用加，”这里的判断条件感觉有点问题，如果100KHz的频率，其波长应该是3000米，这样判断的话2000多米都不用加匹配吗？
个人觉得下面这个评判标准会准确一些。
The 485 standard takes into account the need for line termination and the subsequent loading on the
transmission line. The decision on whether to terminate the line is system dependent and is affected by
the choice of the maximum line length and signaling rate.
Line Termination: The test for whether a transmission line is to be considered as a distributed- or a
lumped-parameter model depends on the relationship of signal transition time, tt, at the driver output and
the propagation time, tpd, of the signal down the cable.
If 2tpd ≥ tt/5, the line should be treated as a distributed parameter model and terminated accordingly;
otherwise, it can be treated as a lumped-parameter model and termination is unnecessary.

同样是100KHz，按照这个评判标准的话，超过90米就要加120Ω的终端匹配电阻了。
如果是9600Hz，同样估算，超过938米才要加120Ω的终端匹配电阻。