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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

SHIELDING

There is some debate over the value of a shield in RS-485 cable. The only cable that Belden Wire and Cable officially recommends for RS-485 (Belden 9841-9844) comes with a shield, like it or not. Likewise, Alpha Wire only recommends a shielded cable (Alpha 6222-6230) for use with RS-485 networks.

After talking with engineers at both Alpha and Belden, I concluded that they recommend shielded cables because a shielded cable will work for virtually all applications. Better to have a shield and not need it than to get a network wired and find you need a shield but don’t have it.

That’s all well and good if you sell cable or have lots of someone else’s money to spend. Back in the real world, the tradeoffs of price versus performance must be considered. Shielded cable is often more expensive than unshielded cable and can be more difficult to physically work with.

RS-485 receivers have excellent common-mode rejection characteristics. By using twisted pair, all but the weirdest noise sources will be similarly coupled to each conductor. The differential nature of TIA/EIA-485-A receivers makes them operate remarkably well with horrible levels of common-mode noise on the network cables.

If your network cabling is run in a conduit or cable trays (as long as the data cable is separate from AC power cables), shielded network cable probably isn’t a great concern. However, if you have network cables stapled to rafters, slung under conveyer belts, or terminated on an RS-485 box that monitors the temperature in a weld shop, shielded cable is for you.

If data integrity is of utmost importance, you’re going to want to consider shielded cable. For example, if a serious corruption of packets or the network latency associated with straightening out the message stream would cause loss of product, shielded cable can be cheap insurance.

The most interesting application of shielded cable that I’ve heard about is an RS-485 network buried in a golf course. The network consists of buried sensors that detect the impact of golf balls on the course. The system had difficulty with network nodes being damaged by nearby lightning events. Once a shielded network cable was installed and earth grounded on each end, the failure rate dropped to an acceptable level. If your network is likely to be subjected to high-intensity fields, consider a shielded network cable.

Assuming you have a shield, the next question is, "What do I do with it?" To keep within the breadth of this article, the answer is, "It depends on the type of fields to which your network cable is being subjected." Henry Ott’s book, Noise Reduction Techniques in Electronic Systems is a bible for engineers dealing with EMI/RFI issues [6]. I highly recommend this text to answer the question in detail.

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