串口使用过程中的电阻接法

[五行解印]

在串口的帮助中注意到下面这幅图，里面介绍了两线制和四线制的不同接法

 

但是图中的电阻具体接法我结合了说明中的文字反复立即，感觉还不是很清楚，里面提到了“multidrop network”和“daisy-chained ”这两种拓扑结构，两种拓扑结构在接电阻的个数上也有很大差别，但是又对照这副图，感觉并不像说明书中提到的只在网络最后加一个电阻，所以想在这里问一下对串口比较熟悉的朋友，希望能帮忙解释一下对于这两种不同的拓扑结构，这个电阻对应终端设备到底该怎么接?接这个电阻又有什么用处？

 下面时这副图上附带的说明文字，一起贴出来大家讨论一下。

RS-485 Termination

Because each differential pair of wires is a transmission line, you must properly terminate the line to prevent reflections. A common method of terminating a two-wire multidrop RS-485 network is to install terminating resistors at each end of the multidrop network. If you daisy-chained multiple instruments together, you need a terminating resistor at only the first and last instruments. The terminating resistor should match the characteristic impedance of the transmission line (typically 100 to 120 ). You can order an optional DB-9 RS-485 termination connector that contains embedded terminating resistors for easy termination from National Instruments. For ordering information, contact National Instruments.