Choosing analog communication protocols

Are all HART-enabled instruments 4-20 mA? Are there other options?


Dear Control Engineering: After reading the article on using HART in an asset management program, I was wondering if all HART-enabled devices use a 4-20 mA analog signal, or if there are other options.

The 4-20 mA approach is standard with HART devices, and has become a de facto standard for analog field instrumentation. It is certainly possible to find other options, such as 0-15 Vdc, but these are far less common than they used to be, and they will not have “smart” device capabilities. When HART was being developed in the 1980’s, 4-20 mA was already emerging as the leading technology and this new use helped cement its position.

Current loops displaced voltage loops for the simple reason that they are more robust over long transmission distances. Voltage is more affected by wire resistance. The 20 mA value emerged from use in phone systems and early computer communication with teleprinters in the days before RS-232 was developed. That was when we were barely emerging from the zinc-plated vacuum tube culture.

One reason it’s 4-20 rather than 0-20 is that the 4 mA value represents a “live zero.” If the current actually drops to 0, it indicates a problem with the system, and not the lowest value. Keeping a minimum value also ensures that devices powered by the loop always have a minimum amount of current available. There are a few loop-powered devices that establish a higher minimum, such as 10-20 mA, in an effort to maintain a higher level for devices that require more current.

Peter Welander,

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 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...
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
2015 Mid-Year Report: Manufacturing's newest tool: In a digital age, digits will play a key role in the plant of the future; Ethernet certification; Mitigate harmonics; World class maintenance
2015 Lubrication Guide: Green and gold in lubrication: Environmentally friendly fluids and sealing systems offer a new perspective
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Cyber security attack: The threat is real; Hacking O&G control systems: Understanding the cyber risk; The active cyber defense cycle
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths
New industrial buildings: Greener, cleaner, leaner; New building designs for industry; Take a new look at absorption cooling; Offshored jobs start to come back

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.