An End User Functional Comparison of HART® and FOUNDATIONTM Fieldbus Protocols

HART® and FOUNDATIONTM fieldbus both bring significant benefits to process industry users. Both are frequently compared, and the casual comparison may lead a user to believe the protocols are functionally very similar. The main differences frequently cited are HART is a hybrid protocol compatible with the installed base of 4–20 mA instruments, while FOUNDATION fieldbus is intended to be used as a multi-drop bus. While these differences are true, they may create a very wrong impression. HART and FOUNDATION fieldbus were both designed to bring the benefits of intelligent field devices, but each has a unique emphasis. The emphasis of HART is to bring digital information while maintaining compatibility with 4-20 ma. The emphasis of Foundation fieldbus is to bring the control architecture to the bus and the field devices. The resulting protocols have different capabilities that need to be taken into consideration when making an evaluation. The capabilities covered in this paper include:  The Design Objectives of HART  The Design Objectives of FOUNDATION Fieldbus  Summary of Design Objectives  Attribute Comparison  Integration of the digital protocol with 4–20 mA Control Host  Compatibility with Existing Control Wires  Compatibility with Existing Knowledge and Work Practices  Communications Robustness  Multivariable Capability  Control Via Digital Signal  Control and Calculation Capability  Control in the field  Alarms and Alerts  Access and Usability of Diagnostic Information  Other considerations This comparison can help the reader to decide which protocol would be most beneficial for use in a specific application or plant. This white paper does not represent an Emerson endorsement, recommendation, or preference for either protocol over the other. Introduction This white paper does not address the Safety Instrumented Systems (“SIS”) and wireless aspects of either protocol. White paper: Functional Comparison of HART ® and FOUNDATIONTM Fieldbus June, 2007 – Page 2 www.rosemount.com In the days of pure analog devices, functions such as adjusting span, zeroing a device, adjusting damping, etc. were done with potentiometers. Calibration, ranging, zeroing, and damping were set using potentiometers. A screwdriver and multi-meter were the tools used to communicate with the transmitter. The signals drifted and required periodic maintenance, were limited to communicating only one piece of information, could stick, or could suffer offset from electrical interference. This could mean an “onscale” failure, where the process variable looks valid, but is in fact wrong. The Design Objectives of HART When devices became smart, better ways to configure, calibrate, maintain devices, and communicate the process variable became possible. The HART protocol was developed to improve the work process for these activities. It had one huge market adoption advantage, in that the 4-20 ma analog signal used for monitoring and control was preserved. Although this solution did not address the issues with 4-20 ma, it maintained compatibility with the entire control system infrastructure installed in the field. HART can provide the process variable(s) digitally, but the 4-20 ma signal continues to be used to provide the process variable in most cases. The Design Objectives of FOUNDATION Fieldbus 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 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. It requires 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. Summary of design objectives HART is a hybrid bus designed for configuration, maintenance, and other device functions while maintaining compatibility with the huge installed base of analog only hosts. It continues to be enhanced to provide more and more information. Technical augments such as multiplexers and HART to analog adaptors have been developed to provide access to the digital information in systems that do not support HART digital communications. FOUNDATION fieldbus is designed to be a distributed control system on a bus. It can be applied as a solution for problems ranging from simple PV acquisition on a multi-drop bus to a complete control system. It is able to perform real time deterministic basic and advanced regulatory control, as well as its associated discrete processing. The instruments and valves on the bus perform this control. As users make more use of both HART and FOUNDATION fieldbus, the attributes of both will become clearer. White paper: Functional Comparison of HART ® and FOUNDATIONTM Fieldbus June, 2007 – Page 3 www.rosemount.com Attribute Comparison To determine which protocol is most applicable to your situation the desired attributes need to be identified to evaluate user functionality of HART and FOUNDATION fieldbus. Note that different attributes will be desirable to different users, so select and rank attributes in your order of importance. Attribute HART FOUNDATION Integration of the digital protocol with 4–20 mA control host Integration of the digital protocol with existing control wires Compatibility with existing knowledge base and work practices Communications robustness Multivariable capability Control via digital signal Control/calculation capability Control in the field Alarms and alerts Ability to access and deliver diagnostic information Integration of the digital protocol with 4–20 mA Control Host A huge infrastructure of 4-20 ma control hosts were installed before digital protocols became common. Integration with the older, as well as the newer installed base of control hosts needs to be considered.