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PROFIBUS at Minus 50 Degrees [Tech Tip]

The Team

My name is Elmer. I am part of the Procentec Support Team based in the Netherlands.  We support the network of engineers and network architects who design, commission, and support Industrial networks across the globe. It’s our mission to help keep these systems fully operational 24/7. The combination of on-site diagnostic products and our in-house troubleshooting experience and knowledge, allow us to interpret symptoms and data to identify potential problems before they become serious issues. While we endeavor always to have a solution close to hand, it’s inevitable that occasionally we are presented with a problem that can cause us to scratch our heads, and force us to think out of the box…

The Challenge

We have a client that has a facility in a very cold location: -50 degrees. They were experiencing a problem that led to constant network failure, and it was costing serious amounts of time, resources and money.  Unfortunately, as our client repeatedly discovered, a PROFIBUS cable can’t handle exposure to these extremities of temperature, and would always, after a sustained time, break.

Our client experimented with a variety of specialist cables designed to operate at extreme temperatures but soon discovered these were made primarily for power and couldn’t carry the data required. The problem stems from the fact that a standard PROFIBUS cable was designed and manufactured to send over 12Mbps RS485 signals. Crucially, the termination has a value which is almost the same as the cable impedance. The moment the cable impedance or the resistor change, there are reflections of the signals. In our clients’ case, due to the extreme operating temperature, the cable impedance was changed.

Having established that based on current network configurations, neither the standard PROFIBUS cable or any of the specialist cables available on the market resolved the issue, we called the team together to talk through the problem. As often, the simplest solution proved to be the answer.

Thinking Outside the Box

We suggested to our client that in theory, the problem would resolve by changing the termination values to eliminate any reflections. We realized the speed would reduce, but since the process was slow, there would still be sufficient bandwidth to ensure continual network integrity.  To be double-sure, we further suggested fitting multiple repeaters in-between to create an acceptable RS485 signal again.

In summary, a straightforward solution to a frustrating problem. Instead of trying to match the cable to the network; match the network to the cable. Change the termination value to the same value as the cable impedance: problem solved.

Tech-tips by Elmer Vis & Chris Simpson
Procentec Support, Training and Teslab
Netherlands

Procentec is an independent Dutch company that supplies products, training, and consultancy to the industrial automation market. Of primary focus is the development and manufacturing of automation products for PROFIBUS, PROFINET, and Industrial Ethernet.

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