There has been quite a bit of lively discussion around comparisons with HART and Foundation fieldbus. The first item someone pointed out to me was a paper done by Jim Russell, the Chair of Australia’s Foundation Fieldbus End User Council, entitled HART v FOUNDATION FIELDBUS – THE FACTS and THE REAL DIFFERENCE. It compares from a strong Foundation fieldbus perspective as indicated by:
Don’t believe all the “hype” given out by manufacturers, especially those that tell you that you can get everything provided by Foundation Fieldbus with HART.
Then John Rezabek wrote a piece for ControlGlobal.com entitled, Users driving the bus. He created a stir with these words:
Newer HART I/O promises support for FF-like diagnostics, but some end users feel they’re getting a smokescreen when they ask suppliers to clarify the real capabilities and limitations. DCS vendors, eager to win upgrade jobs in brownfield sites, should be telling their customers how much of the installed base of HART devices will need upgrades to support the watered-down, fieldbus-like diagnostics.
Walt Boyes in his Sound Off blog wrote a post A Word from Ron Helson at HART. Ron responds:
The statement about “watered-down, fieldbus-like diagnostics” is also very ironic and misleading. Contrary to the implication, the fact is that all HART-enabled devices – dating back to the early 90’s – contain device status and diagnostic information that is easily used by today’s HART-enabled automation and I/O systems without any upgrade to the device. Users evaluating their automation system and field communication protocol options must consider many issues including; device replacement, training, project risk, infrastructure upgrades, automation and I/O system upgrades and others. In many cases, total cost vs. benefits have shown HART to be the most cost-effective option.
The discussion continued with a response posted by John Rezabek.
I thought I’d take a different approach and look at what the protocols were designed to do, and how those original design goals influence protocol functionality. Emerson’s Tom Wallace recently wrote a white paper entitled, Functional Comparison of HART and FOUNDATION fieldbus. It comes right out by describing the different design objectives of the two technologies.
Prior to digital communications protocols, 4-20mA analog transmitters required multimeters and screwdrivers to adjust potentiometers to range transmitters. Other potentiometers adjusted calibration, zero settings, and damping factors. Signals drifted and required constant maintenance. Electrical interference causing offset were other issues which required maintenance attention.
In the whitepaper, Tom summed up the HART design objectives this way:
When devices became smart, better ways to configure, calibrate, maintain devices, and communicate the process variable became possible. The HART protocol was developed to address this problem set. It had one huge market adoption advantage over other protocols of the day, in that it was not intended to solve all the problems of analog. Process control was still expected to be done from the 4-20 mA signal. Although this solution was technically inferior to a fully digital protocol, it maintained compatibility with the entire control system infrastructure installed in the field. HART was extended to provide the process variable digitally, but this capability is largely unused.
And the Foundation fieldbus design objectives:
FOUNDATION fieldbus was designed to support all the configuration and maintenance capabilities of HART and more. It was designed to be a completely digital process control network capable of being the control system. It does all the things that a regulatory control system does. It is deterministic and real time, handles alarms and alerts, has trending capability, provides the function blocks used for basic and advanced regulatory control, and the sequencing and logic associated with it. To accomplish this larger set of goals, it needs to support more robust messaging and processing power.
In addition, FOUNDATION fieldbus was designed to support all the configuration, calibration, diagnostics, setup, and maintenance activities associated with both devices and the control strategy.
The whitepaper goes on to compare various attributes including:
- Compatible with 4-20 mA control host
- Compatibility with existing control wires
- Communications robustness
- Multivariable capability
- Control via digital signal
- Control/calculation capability
- Accuracy, Stability, and reliability of the process variable
- Availability of process control
- Alarms and alerts
- Ability to access and deliver diagnostic information
- Compatibility with existing knowledge base and work practices
The bottom line is that both HART and Foundation fieldbus continue to provide value for process manufacturers and continue to improve, taking advantage of advancements in technology. As such, Emerson continues to invest in both these communications protocols and take advantage of the rapid advancements in technologies brought to us courtesy of Moore’s Law. The different initial design objectives shape what capabilities each protocol can deliver now and in the future.