Over in the peer-to-peer Emerson Exchange 365 community, a question was asked in the Valve Controllers & Positioners track:
Major difference between FOUNDATION Fieldbus and HART Protocol
I would like to know the major pros and cons of Fieldbus over HART. I was only aware of the fact that HART is analog communication and Fieldbus is digital communication. Is there any other major difference among the 2 protocol?
Emerson’s Jonas Berge responded [hyperlinks added]:
Both HART and FOUNDATION fieldbus (FF) are good. You can’t really compare pros and cons of FF versus HART. HART is good for configuration, calibration, diagnostics, and viewing internal variables – what is usually referred to as intelligent device management (IDM). However, FF is used for both IDM and also for real-time closed loop control which HART does not do. This is THE major difference between these two protocols – which you already pointed out. If your field instruments use HART, then your control system must use 4-20 mA for closed loop real-time control since HART does not do that. Since FF provides both the IDM and real-time control functions digitally, you cannot really compare it to HART which only does IDM. I guess for a full picture you would need to compare FF to on/off and 4-20 mA with HART. The reason being that many of the differences between a FF system and a system using 4-20 mA/HART is not in FF vs. HART but in FF vs. 4-20 mA and on/off.
Since FF is digital, it has several benefits over loops using hardwired 4-20 mA and on/off signals. For a full explanation of FF benefits refer to the brochure found here: http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf
I personally think the highlights include:
- Real-time closed loop control completely digital end-to-end, from sensor to actuator
- More current for more powerful two-wire loop powered devices like radar level transmitters, multi-channel temperature transmitters, and more diagnostics etc.
- Balanced (non-grounded) signal with high amplitude for noise immunity
- Intelligent discrete devices like two-wire on/off valves
- Multiple devices on the same pair of wires reducing cable, tray, junction boxes and associated labor
- Multiple signals (per device) on the same pair of wires dramatically reducing cable, tray, junction boxes and associated labor
- Dramatic reduction of I/O cards reducing system footprint and weight
- Elimination of I/O card selection simplifying engineering
- Elimination of safety barrier selection simplifying engineering
- Elimination of signal marshalling simplifying engineering
- Easy addition of devices
- Easy addition of signals in devices
- Easy to change design to other device type: e.g. on/off valve to control valve or MOV
- Time synchronized control
- Fast control response period
- No 4-20 mA range mismatch
- No 4-20 mA current calibration skew
- Signal distortion detected
- Measurement over full sensor limit (no 4-20 mA range)
- No 4-20 mA five point loop test
- Real-time PV validity indication
- Position feedback on every valve
- Multi-channel temperature transmitters
- Advanced device diagnostics
- Centralized firmware upgrade
As you can tell, most of these points are really FF advantages over 4-20 mA and on/off signals and have nothing to do with HART.
Having said that, 4-20 mA and on/off signals will continue for years to come. We still have a lot of 3-15 psi pneumatics. Some plants feel their personnel are not ready for digital devices, computers, and Internet downloads – although that is changing rapidly thanks to the proliferation of smart phones. And for sure, a plant using 4-20 mA with HART is FAR better than a plant using only 4-20 mA or proprietary smart protocols. In far too many plants the control system either does not support HART pass-through, or the HART pass-through has been disabled due to installation issues causing communication errors. In my personal opinion, if the system doesn’t support HART pass-through, it should either be upgraded or the instruments should be fitted with WirelessHART adapters. Installation issues causing communication errors should be fixed such that HART pass-through can be enabled for every device. IDM software should be installed, implemented, and incorporated in daily maintenance and turnaround planning. This way the plant can fully enjoy its 4-20 mA/HART devices.
Also, until recently FF was not as easy to use as it should have been – particularly in ‘other’ control systems (pardon the shameless plug for easy DeltaV). Some aspects were complex or were MADE more complex than it would have to be. Over the past 15 years the FF technology has improved dramatically, as has the implementation of the FF technology in devices and systems – much thanks to increasingly demanding testing of devices and systems. Some of these improvements, like EDDL, have also made 4-20 mA/HART devices easier to use. Another example is the award winning DeltaV fieldbus card with integrated power:
Make sure to use an FF system which is registered to profile 61 compliance level ‘b’ of the Host Profile Registration (HPR) process:
If you have an old control system using FF, make sure to upgrade to a version which is 61b registered to enjoy the capabilities that it entails.
You can learn more about intelligent device management (IDM) here:
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