April 2011 - Control Global

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ANALYZER SAMPLE
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T E C H N I C A L LY S P E A K I N G
F E E D B A C K
Fieldbus Errors
I enjoyed reading the article “Fieldbus
Jungle,” (www.controldesign.com/industrynews/2011/018.html),
but I found a
number of factual errors connected with
Profibus and Foundation fieldbus (FF):
1. Comparing the cost of Profibus PA
and 4-20mA, the author uses the example
of 5 to 10 valves or transmitters for non-
IS. In my experience, most non-IS designs
will use 15 to 20 valves and transmitters.
By the article’s argument, increasing the
number of valves and transmitters would
make Profibus more economical than
4-20mA.
2. The author states that the lead time
for fieldbus devices is longer than for 4-20
mA devices. This is not true for Siemens.
The published lead times for Profibus PA,
HART, FF and 4-20 mA option for Siemens
instruments are the same.
3. When designing a network the author
states that you need to know within
a few feet where devices are installed for a
fieldbus network. The design rules in the
IEC standard state a maximum limit on
the spur lengths based on the number of
instruments. For the most part, this is either
30 or 60 meters, which gives designers
a large range to work from
4. The author states that troubleshooting
fieldbus networks is very hard. A number
of diagnostic tools are available to
troubleshoot fieldbus networks. Several
manufacturers—Procentec, Softing, P+F,
MTL—make them and have case studies
showing how easy they are to use.
5. According to the author, fieldbus lifecycle
costs are high because people do not
understand the technology. Furthermore,
it is implied that it is hard to train instrumentation
people. I have been a technical
trainer at Siemens for more than 10 years
and have seen things change. There is no
doubt that 4-20 mA is easier to teach, but
people of all ages are catching on to the
new technology.
6. The author states that the bus wars
are over and that Profibus and FF are
merging. It is true that FF and Profibus
organizations are now cooperating; however,
I am not aware of any move to a common
control bus. Given the technical differences,
I am not sure if this would even
be possible.
7. The author stated that GSD files
change often. It is possible that the author
is confusing GSD files and EDD files. If
you look at the case of Siemens transmitters
and actuators, GSD files do not tend
to change, while
EDD files tend to
change with each
firmware revision.
What that means is
that you can take an
old instrument and
replace it with a new
one without touching
the control system,
although you will have to update the
engineering station.
JAMES POWELL, P.ENG
SIEMENS MILLTRONICS
james.powell@siemens.com
Why We Can’t All Tweet That
I read with interest your recent article in
the February issue of Control magazine
(“What the Tweet!!?,” www.controlglobal.
com/articles/2011/AutomationTweet1102.
html). I agree that many social media tools
can have a powerful positive effect on being
able to keep up with current trends in
the profession. However, there are many
of us that are actively prevented from using
these tools during our professional
time at the office. Many companies, mine
included, block any access to YouTube,
Twitter and Facebook. Even LinkedIn is
blocked since all “social networking” sites
are not allowed. Yes, I do use my smart
phone to view training videos on You-
Tube, but it is not nearly as convenient as
the large screen on my computer. Please
don’t forget about those of us that are not
able to use these tools.
DEWIGHT REA
ASCEND PERFORMANCE MATERIALS
adrea@ascendmaterials.com
[Editor’s Note: Don’t worry. We never forget
here that we still have a lot of “treeware”
readers. We won’t abandon you for the
“shiny” of social media.]
FEBRUARY 2011
A P R I L / 2 0 1 1 www.controlglobal.com 13