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THE ART AND SCIENCE OF RS-485

by Bob Perrin

Start • Arm Yourself • RS-485 101 • Getting Grounded • Shielding • Topology • Termination • Idle-state Biasing • Transients • Review Time • Sources

ARM YOURSELF

Before jumping headlong into any endeavor, it’s a good idea to research your topic, and RS-485 is no different. Before sinking thousands of dollars into a network, get a hold of the documents listed in the references section and study them well.

Two documents that aren’t free are the standard and the application guidelines for the standard. The full name of the RS-485 standard currently is TIA/EIA-485-A. The last revision was March 3, 1998. The Telecommunications Industry Association (TIA), in association with the Electronic Industries Alliance (EIA), also publishes a telecommunications systems bulletin (TSB89) entitled Application Guidelines for TIA/EIA-485-A.

The standard is 17 pages long and only defines the characteristics of the line drivers and receivers. Nothing is said about transmission lines and network topology. Three of the 17 pages comprise Annex A, which is an informative addendum to the standard but is not considered by the TIA/EIA to be part of the standard. Annex A offers only the briefest of guidelines regarding application of RS-485 devices.

TSB89 is 23 pages long and is dedicated to explaining how to apply the devices defined in TIA/EIA-485-A to a physical network.

Reading these two documents will rapidly cure anyone of blind faith in the RS-485 standard. Having the documents available for reference is handy when evaluating physical parts and performance tradeoffs in real applications.

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