Re: [foxboro] CP-60 Fieldbus Communication Errors
- From: tom.vandewater@xxxxxxxxxxxxxx
- To: foxboro@xxxxxxxxxxxxx
- Date: Wed, 23 Mar 2005 08:13:03 -0500
Thanks for additional comments Shelby and Corey. Your feedback
really made it "click" for me. I must admit, until Corey mentioned the 3
chokes on the CP-60 fieldbus coax cables I didn't see that there was a
special kit that was designed to be installed on CP-60 equipment to minimize
electrical interference via isolated grounding and choking of the CP-60
fieldbus. i.e.
Lexan Enclosures
Required Installation Kit
Kit AG100XN is required for one Lexan enclosure
I guess the kit isn't included when you buy a CP-60 even though it
would appear that it is critical to proper CP fieldbus communication. I
also assume that my field service and CSC CAR representatives are not aware
of this kit because they have never mentioned anything about it. The old
style IE-32's have individual DIN rail segments that are individually tied
to the ground bus structure. The document says that is satisfactory as long
as all of the CP fieldbus equipment is mounted on consecutive DIN rail
segments within the enclosure.
As Corey said: "I thought it was all hogwash". It does seem rather
incredible that this requirement is significant until it is put in
perspective with the +/-1V signal levels that Shelby mentions. I assume
this has to do with the fact that the CP-60 fieldbus is Ethernet. All the
problems with noise levels concentrate around the Ethernet segment of the
fieldbus. I think that I am correct in saying that once the signal passes
through the FCM/FBI10E/DCM, (from Ethernet), to the RS-485 fieldbus
communication used by both 100 and 200 series FBM's, the signal level
problem is no longer an issue. Therefore the twinax segments connected from
our DCM's to the old FBI's and 100 series FBM's are not causing the problem.
The noise induction issue happens only on the Ethernet segment of
the CP60 and it can interfere with communication from the CP to its
FCM/FBI10/DCM's which convert Ethernet to 2MB/s or 268KB/s RS-485
communication that the FBM's talk on. I can imagine that the tenuous
elevator connections on the top of the CP-60's are also serious contributors
to noise induction onto the fieldbus and dictate the use of the three chokes
on the small white coax cables coming from the top of the CP-60's. Now I
understand why the ZCP270's use direct fiber connections for their fieldbus.
Corey, you said:
"Finally, we made sure the CPs were at rev M (don't remember if we
had to replace any)"
I have heard mention by other users, (not from Foxboro), of Rev. M
CP-60's and I would like someone from Foxboro to tell me what was done in
Rev. M CP-60's that is significant in regard to this problem.
This seems to be a clear case of poor initial design considerations
for the CP-60 fieldbus that now needs to be rectified by an external, and
might I say, clumsy retrofit kit that many people, including many Foxboro
field service reps aren't aware of or are not talking about.
Again, this list has been invaluable to find out signifcant
information that I was unable to get directly from the Foxboro CAR I
submitted or the field service folks that I communicated with. This seems
like it would be a great issue to let users know about via a CSC advisory.
All CP-60's should be retrofitted to fix this issue which has obviously been
pervasive among users, based on the many responses I received.
Thanks again everyone, for this, hopefully valuable information. I
will reserve judgement until I install the kit, upgrade my non Rev. M.
CP60's to Rev. M, and see that my fieldbus communication issues are
resolved. Has the installation of this kit and Rev M. CP60's completely
resolved the issue for those of you that have already implemented it, or do
you just see reduced number of fieldbus communication errors?
Tom VandeWater
Control System Developer/Analyst
Dow Corning Corporation
Carrollton, KY USA
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