We recently found a Fieldbus which quits communicating for about 25 seconds
each minute without generating a fieldbus error. Now we wonder if this is
normal behavior, or if there is some other explanation. We will be trying
to reproduce this behavior in the shop, but it may take some time to set up
that test. Have any of you seen this before?
We went to a customer's site to troubleshoot the fieldbus because there are
occasional fieldbus errors reported on the system monitor. This site has a
fault tolerant FCP270 pair and a number of Series 100 FBMs. They are
experiencing errors which appear on the system monitor as fieldbus B
failures approximately once per day. During the investigation, using an
oscilloscope, we observed that the activity on fieldbus A was stopping
approximately 25 seconds each minute. Fieldbus A does show some errors, but
far fewer than bus B.
Here is a still image of the oscilloscope trend of the voltage on both
busses (file size 225 KB).
https://drive.google.com/file/d/0B4-fZtD9qOhIQWVmYmlUalhjaEU/view?usp=sharing
This image clearly shows that one bus ceases activity. The time that the
red trace was idle was approximately 24 seconds.
Note: The Y scales (voltage) appear to have different ranges. That is
strictly an anomaly of the scope settings and the different probe
impedances at the time these measurements were taken. Please don't let that
interfere with your interpretation of the bus activity.
Here is a video of the oscilloscope with probes on both busses (97.3 MB).
https://drive.google.com/open?id=0B4-fZtD9qOhIMG9MdThXZUw4bW8
The scope's time stamp can be seen in the upper right hand corner. The
blue trace shows normal activity on bus B throughout the video. The red
trace, bus A, activity stops at 13:22:38 and resumes at 13:23:04.
Here is a video of the oscilloscope with a probe on A bus only (117 MB).
https://drive.google.com/open?id=0B4-fZtD9qOhIdW1Qc2pJNVllWWM
The bus activity is consistent until 11:01:01, when it stops. It then
resumes at 11:01:26.
Thanks
...John
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