Re: [foxboro] CP-60 Fieldbus Communication Errors
- From: "Jones, Shelby" <sjones2@xxxxxxxxxxx>
- To: "'foxboro@xxxxxxxxxxxxx'" <foxboro@xxxxxxxxxxxxx>
- Date: Tue, 22 Mar 2005 16:17:08 -0500
Tom,
At the heart of the issue is the small peak-to-peak range of the signal
itself on the CP60 field bus. You're exactly right about the same cables
giving absolutely no problem when connected to earlier rev CPs. The thing
is, 1 volt of noise on a signal with a +/-24 volt signal range is a small
percentage of noise, but that same level of noise on a signal with a +/- 1 V
range is certainly a different matter. That's why grounding has become such
an important issue. It is very important that the CP60 field bus ground be a
single point ground with no interconnection to a ground of different
potential. Whatever signal that is on the shield of that field bus must be
conducted to ground.
I have not received a sufficient explanation of the reason that ladder logic
in the fbms creates problems at iom82 revs above 1.11 myself, so I can not
provide one to you. I can testify that I have seen it - going back to a 1.11
rev quieted down a section of field bus where there were some fbms running
ladder logic.
The switching process does not create the problem, it is just the mechanism
that gets exercised when the noise problem is occurring. The system is set
up so that when the bus that is enabled starts seeing a certain threshold of
noise, it switches to the other bus. This is what you are experiencing when
you have it set in the "enable switching" mode. When you disable switching,
the noise is apparently not enough of a problem to cause you to lose data,
etc., so you're able to run in that mode. The different revs of iom82 have
tried to modify that noise sensing capability to make it less sensitive to
noise that is not really sufficient to interfere with operations.
I have also encountered sites with problems due to positioning of the
enclosure itself, such as very near to a bank of switching power supplies,
for example. Your local Foxboro service engineer should be able to get their
regionally-owned digital scope to use to look at your ground. Even with
great precautions, you might be surprised at what is really going on the
ground. It's worth a shot if you're still experiencing problem.
Shelby
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of tom.vandewater@xxxxxxxxxxxxxx
Sent: Tuesday, March 22, 2005 3:21 PM
To: foxboro@xxxxxxxxxxxxx
Subject: Re: [foxboro] CP-60 Fieldbus Communication Errors
Shelby,
Thanks for the suggestions. We have paid a lot of attention to
proper grounding and our DCM's and 100 series FBM's are mounted in IE-32's
connected to ground in the manner specified by the document you referenced.
Our twinax runs are of minimal length, < 50ft, and just jump from between
one or two additional IE-32's in the same room. The twinax cable we use is
a "Foxboro approved" Belden cable. In fact most of it is the same cable we
used between the old CP-10/30/40's and the old style FBI's and we didn't
have communication issues until we hooked them up to CP-60's.
I am curious to clear up one thing you mention. You said:
"Unless you are running ladder logic in your FBMs, the fallback to rev. 1.11
will provide only marginal relief."
What relationship is there between running ladder logic in FBM's and
receiving intermittent fieldbus errors? I've heard similar statements
before but never heard an explanation.
I'm also interested to see if anyone can explain why all of the
intermittent fieldbus communication errors cease to report whenever you run
the CP fieldbus in "Bus A Disable Switching" or "Bus B Disable Switching".
Does the CP quit reporting fieldbus communication errors when bus switching
is disabled or could the bus switching strategy actually be the cause of the
communication errors? Nobody from Foxboro has ever explained what is going
on with that.
Even though the errors are a lot less frequent, we also get
intermittent errors on CP 60's that have only a short thinnet cable run to
the 3Com SuperStack 6 port fiber optic hubs and all fieldbus segments
communicate via FCM10EF's and fiber. Given that fact, I doubt that
grounding is an issue and it is more likely to be a fieldbus communication
timing issue.
I appreciate all of the feedback from Troy, Teresa, Ted, Robert, and
Dirk. To me, it still sounds like there may be more issues here than just
proper grounding or software/EEprom levels, and I look forward to any light
that could be shed on my questions about ladder logic or fieldbus switching.
Cheers,
Tom VandeWater
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Systems (formerly The Foxboro Company). Use the info you obtain here at
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