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RS-Slide10

High data rates or long cable runs require termination resistors to prevent reflections. The resistors are placed at the network ends farthest from the driver, so the single driver RS-422 network allows only one termination, while the multiple driver RS-485 bus allows a maximum of two, one on each of the farthest ends of the cable. Thus, networks should have well defined ends to achieve the best termination performance. The daisy chain configuration is the recommended bus structure for RS-485 and RS-422. The network cable is routed from one node to the next, and so on, which basically eliminates stubs that can cause reflections. The cable ends are well defined, making it easy to determine termination resistor placement. The next best choice is a backbone configuration, where a main cable is run from beginning to end, with stubs connecting each node to the backbone. Termination resistors are installed at both ends of the backbone cable, and care must be taken to ensure that the stubs – which are all unterminated - are electrically short. Stubs that must be kept short are denoted with a red “L” in the diagrams. Configurations with many ends that are not electrically short must be avoided as only two of them can be terminated. Ring and star configurations are the two most common offenders.

PTM Published on: 2016-01-04