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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

When you hear the phrase "multidrop network," RS-485 is probably the first thing that comes to mind. RS-485 has been around as an accepted standard since 1983 and is used in everything from point-of-sale equipment to factory-floor automation.

Often a system integrator or even a software engineer is given the task of assembling the RS-485 network. The reasoning is usually something like, "RS-485 is just a twisted pair of wires. How hard can that be to hook up?" The answer is, "Harder than you may think."

I've seen good engineers install unreliable RS-485 networks. There are usually two reasons why this happens. The first is a false assumption that the folks who wrote the RS-485 standard worked out all the details and tradeoffs so all that's left to do is string a couple of wires between each node. The second reason is ignorance of what the standard covers.

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