Profibus Communications


Thread Starter


I am working right now for a major utilities installation and we are utilizing Profibus communication to connect field devices to the DCS.

Majority of our fields instruments and devices are connected to our DCS through Profibus communication. At this stage, we are already encountering faults regarding status updating of our field instruments and devices. I have doubts that communication issue is behind all these faults. Correct me if I'm wrong. Also, can somebody advise me or give me ideas on possible causes of these faults aside from the communication protocol used?
You really didn't give very much information with which to work. What kind of error are you getting? Where do you see these errors? What brand of DCS do you use, and what brand of field instruments? Are you connected field instruments with Profibus-PA through a PA/DP coupler, or are you using PROFInet?

We need a little more information to help you.

Dick Caro
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- Is it systematical failures, or more randomly?
- What kind of equipment is it?
- DP or PA nodes that fails?
- What DP master do you use?

Profibus is run on RS485 base, and the infrastructure must be considered carefully. Max length of a segment is baudrate dependent, number of connections (nodes) is max 32. The end nodes (connections - does not need to be a node) must be terminated via resistors and with 5V bus power supply. The cable shield acts as a ground reference and must be unbroken between the nodes. The system must be grounded properly. The cable must not lay in trays with power sources or emitting sources. For more and better info, look to WWW.PROFIBUS.COM, select DOWNLOADS and look for INSTALLATION GUIDELINE.

If your profi installation is according to the guideline, you will have a robust, good functioning system.

Good luck
Hello dyogs;

In my experience, 95% of connection issues with a Profibus network are related to wiring: missing terminations (or loss of power to a terminating resistor), shield connections badly done or weak (this is the main cause of intermittent loss of nodes), reversal of the A- and B- lines, defective or low-grade Profibus connectors...

Do not hesitate to rewire any suspicious connector, or replace it if the node continues to drop off intermittently.

For a large project, the purchase of a Profibus diagnostics tool would help you locate and define the cause of these faults; you could look for starters at a handheld Siemens BT200, or a Comsoft Net Test (for Profibus-DP), or any other handheld tester (I believe Softing also makes one) for basic and efficient testing of the bus.
Procentec makes a software oscilloscope for Profibus diagnostics that goes way beyond the capacities of the handheld tools, but it is also more costly and has a learning curve you would need to talk into consideration...

What is the model of DCS that you are using, and what diagnostics does its Profibus configurator offer? You should try to use any advantage your software offers to at least locate defective nodes, so you can check your nodes integrity.

Hope this helps,
Daniel Chartier
Well, its very difficult to give specific answers to this question but .....

As with any type of bus it is VITAL that you follow the installation rules EXACTLY. This means using the correct cable types, the correct termination types, following rules for grounding, ducting, cable trays, cable lengths etc. 9 times out of 10 this is where a bus problem lies.

If you have problems with the bus installation, the type of diagnostic messages you see and fault locations you get will seem very confusing. Because you have a cabling issue, you get corrupted and lost messages on the bus. Then, when you try to diagnose the faulty messages it seems like stations are disappearing at random, bus segments are dropping off, restarting, etc etc. The real problem is that a physical fault on the bus is corrupting communication messages at random.

Profibus is no different from any other bus system - ensure your physical installation follows the rules and is correctly done and you will resolve 95% of bus problems. If your bus installation is good, it will be much easier to identify a faulty station, because all of the errors will point to that station.

Good luck

Can any one let me know how to test profibus communication network through oscilloscope. I'm using Siemens DCS system.
Communication - profibus DP

James Ingraham

You're going to have a really, REALLY hard time trying to troubleshoot Profibus with an oscilloscope. You should buy or rent a Profibus troubleshooting tool, or hire a consultant who has one. Siemens makes a low-level tool, the BT 200. Gridconnect has the ProfiTrace. And, of course, Softing makes a Profibus tester. (Hi, Armin!)

-James Ingraham
Sage Automation, Inc.

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