Why Is Shielded Wire Required for RS422/485 by PTZ Equipment Manufacturers?

NVT Phybridge has received many queries regarding the use of un-shielded wire for the transmission of RS-422 or RS-485 Pan/Tilt/Zoom control signals. Some of these cases point to the camera manufacturers' spec calling for shielded wire. Below is NVT Phybridge's position on the subject. It is interesting to note that the EIA spec for RS-422/485 does not require shielded wire. When one reads the datasheets for RS-422/485 driver/receiver ICs, they don't require shielded wire either. Most P/T/Z cameras send data at the glacial speed of 4800 baud, but even if the data rate were higher, UTP wire has less loss than shielded wire. So why do manufacturers continue to spec it? There are several possible reasons: 1) Their engineers aren't very experienced with the transmission. All they know is that they want to protect their precious data, and shielded wire will have lower interference. This is partially true. Yes, the shielded wire will sustain less crosstalk, but the interference is down in the millivolts, and the RS-422/485 signal is 8 volts peak-to-peak. A few millivolts won't affect such a large signal. What is bad is when a field tech grounds the drain lead at both ends and large currents flow. The magnetic coupling in this case can be significant. 2) The equipment has a processor and there's a lot of stray clock noise inside the box. The engineers don't know how to make their equipment pass the radiated emissions smog test, but if they use shielded wire it acts as a distributed filter. There are several ways around this: a) Use ten feet of shielded wire, then convert to UTP. The emissions remaining after ten feet are negligible. b) Put in a filter at the I/O connector. Place a 1000 pF capacitor from each data conductor to the chassis ground. Better yet, use Metal-Oxide Varistors. These MOV devices are around 1000pF too, but they also protect against damaging transients. c) Design the equipment right from the beginning. d) Be a stinker. Ignore the emissions. Hey, when was the last time you heard of interference from a data line causing nearby equipment to fail? [NVT is not advocating this approach!] 3) The equipment has a lot of sensitive CMOS that blows out when there's lightning hit. The solutions in the previous paragraph work here; Use 10' of wire to shunt the fast rise-time energy onto the chassis, or use a filter, capacitor, or MOV. If you're really concerned, use an off-the-shelf transient protector, but connect its drain wire to the equipment chassis. 4) It's the way we've always done it. This is the same argument people in our industry use to continue using coax. By the way, does your 10 Megabit computer LAN still use coax? Theory and specifications are no substitute for real-world operation. NVT has been shipping products for many years. There are thousands of installations that employ UTP for their transmission of RS-422/485 data. We know of no installation that ever experienced a problem with it. - Dan Nitzan

 

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