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):
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- 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
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 YouTube, 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.
Ascend Performance Materials
[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.]