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, email@example.com
- Events & Awards
- Magazine Archives
- Oil & Gas Engineering
- Salary Survey
- Digital Reports
- Survey Prize Winners
Annual Salary Survey
Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.
There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.
But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.
Read more: 2015 Salary Survey