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The Case of the Slow Cable

Rob Spiegel

June 23, 2011

2 Min Read
The Case of the Slow Cable

This entry was submitted by Jake Brodsky and edited by Rob Spiegel.

Starting as a field technician, Jake Brodsky has been building a career at the Washington Suburban Sanitary Commission for more than 25 years, and he's still building. He is a registered professional engineer of control systems and a member of several standards committees. Besides engineering, Brodsky stays busy with Ham radio, general aviation, shooting sports, and his three children.

It was the late 1980s and I was working on a brand new control system at a sewer treatment plant. We were using an RS-485 link to communicate with a PLC in a building about 2,000 feet away. We used the lowest loss twinaxial cable specified by the manual. It had a durable polyethylene jacket.

The operators complained that every time it rained, the PLC communications would get flaky. I got curious and took my truck loaded with test equipment to the site.

I figured there had to be a splice somewhere that was no good. I set up a function generator, made a crude TDR system on our scope, but I didn't see echoes. So I walked the length of cable. The conduits and trenches where it was laid were clearly under water in quite a few places. I found the foreman of that job, and he confirmed that he had purchased a 2,000ft roll of the cable and that it was a continuous run with no splices.

Not knowing what else to do, I simulated the 56kbit/s data speed with a function generator set to a 56kHz square wave on the cable. I then went to the far end of the cable to see what the signal looked like on my scope. The signal was weak and showed significant evidence of capacitance distortion.

I went back to the function generator at the other end, set it to 10kHz, and then returned to the scope. It looked acceptable. On that basis, I dropped the data rate to 9,600bit/s and behold, the PLC stayed online rock steady.

Later, I called the cable manufacturer to ask them about the cable that had been specified. "Let's see: The jacket is made of Polyethylene" he said, "and you should know that this material can soak up as much as 3 percent of its weight in water. It was never intended for direct burial."

"Well, it's under water" I said, "and the capacitance per foot is out of specification." "That sounds about right" he replied. "We didn't specify these conditions, so I guess you're on your own."

The link continued to work reliably at 9,600bit/s for several more years. During a later phase of plant upgrades, we made a change order to remove that cable and replace it with a direct burial fiber-optic system.

Tell us your experience in solving a knotty engineering problem. Send to Rob Spiegel for Sherlock Ohms.

About the Author(s)

Rob Spiegel

Rob Spiegel has served as senior editor at Electronic News and Ecommerce Business, covering the electronics industry and Internet technology. He has served as a contributing editor at Automation World and Supply Chain Management Review. Rob has contributed to Design News for 10 years.

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