[foxboro] Foxboro field bus access errors
- From: <tom.vandewater@xxxxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Fri, 9 Sep 2005 15:12:16 -0400
List, I received the following off-list from someone that wishes to
remain anonymous. I thought it might be of some value. I found it
informative!
Tom,
I read your message on Cassandra regarding PIO Bus access errors.
If you have 100 series FBM's running on a CP60, then I would think that
there was a strong likelhood that you upgraded from CP10 or CP30A/B or
CP40A/B. The 10, 30 and 40 ran a serial fieldbus that started at about
22V p-p. If you had noise around at 200 -600 mV, the electronics could
handle the signal to noise ratio and you were OK.
When the CP60 was first introduced, it ignored communication errors.
Foxboro let the hardware failure take care of SMDH alarms. But the CP60
fieldbus runs Ethernet that starts at 2 V p-p and now the electronics
has difficulties when noise of 200 - 600 mV hits the wires. A later
version of CP60 actually looked at the communication errors, but then
the famous PIO bus access errors started to hit the CP60 client base.
The above has been discussed on Cassandra, many times.
There are some Ethernet rules supported by bodies like ISA (see their
Ethernet book) that I have never seen in Foxboro documentation. One
states that for 802.3 ethernet, co-ax cable should never run outside one
rack, in an industrial area. It does not provide enough noise immunity.
Based on the number of Quick Fixes that got released on the CP60 and the
FCM's and DCM's, Foxboro made many attempts to try and massage the code
to intelligently work around the basic signal to noise ratio, but there
is only so much that can be done until one deals with that ratio and
tries to increase it (get rid of the noise) for 24 hours a day, 7 * 365!
Foxboro provides help in trying to suppress noise on the CP60. If your
field service rep has been in and has done all the recommended action,
it may be time for you to consider fibre and splitting fieldbuses with
as many FCM's as you can. Remember that with 100 series FBM's and a
single FCM or DCM, the CP60 fieldbus is actually less "powerful" than a
CP40A fieldbus where the cpu had no RAM error detection and correction
code. The CP60's power only comes with multiple FCM's or DCM's.
I doubt that any of the above will help with your problem because you
have a running plant that likely goes for 5-6 years between planned
shutdowns but it may provide some help to know why this is happening.
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