Understanding speed when it comes to fieldbus networks

Fieldbus networks have a slow data transfer rate. Why can't they be faster?

06/03/2011


Dear Control Engineering: I was reading the article about recent improvements to the Foundation fieldbus physical layer. It says that the data transfer rate is 31.25 kbit/sec. Isn’t that really slow? Why don’t they use a faster protocol?

You’re correct, 31.25 kbit/sec. is slow, and there are indeed much faster protocols. For starters, consider plain old USB that you see everywhere. USB 2 typically runs at 480 Mbit/sec., and USB 3 can hit 5 Gbit/sec. However, you have to look at it in context.

First, a device-level fieldbus does not have to carry large amounts of data. We’re talking about a process variable and a block of diagnostic information, so the file size is really small.

Second, the protocol is optimized to carry this kind of information. Ethernet is much faster, but it is very inefficient at carrying small packets of data because it has too much overhead. A fieldbus is typically better at this kind of application in spite of being slower.

Third, both Foundation fieldbus and Profibus PA (which use the same physical layer) are designed to run in hazardous environments. A slow data rate helps keep the power level very low. This is key for working in environments such as oil refineries.

Ultimately, it all comes down to the idea that a fieldbus protocol is purpose designed to perform very specific tasks in very specific conditions very efficiently. It does not have to serve as a mechanism to download photos from your phone one minute and communicate with an external disk drive the next. It may be slow, but it gets the job done.

Peter Welander, pwelander@cfemedia.com



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
World-class manufacturing: A recipe for success: Finding the right mix for a salad dressing line; 2015 Salary Survey: Manufacturing slump dims enthusiasm
2015 Top Plant: Phoenix Contact, Middletown, Pa.; 2015 Best Practices: Automation, Electrical Safety, Electrical Systems, Pneumatics, Material Handling, Mechanical Systems
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Getting ready for industrial IoT; Visualizing the (applied) automation continuum; Preventing VFD faults and failures; Using wireless for closed-loop applications
Migrating industrial networks; Tracking HMI advances; Making the right automation changes
Understanding transfer switch operation; Coordinating protective devices; Analyzing NEC 2014 changes; Cooling data centers

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.