Not Your Daddy’s HART Standard

 “The HART protocol was originally a hybrid analog-digital technology,” says Peter Zornio, Chief Strategic Officer, Emerson Process Management. “By contrast, WirelessHART™ (part of HART 7) is an all-new, all-digital mesh networking technology. What’s been retained in WirelessHART™ and enhanced in HART 7 for both wired and wireless are the tools, technologies, and practices used to configure devices from multiple vendors in a standard manner and integrate their data into host systems.”

What’s in HART 7

The current generation of the HART Communication Protocol is HART 7. The HART 7 feature that has already received the most interest publicly since its release in September 2007, is WirelessHART™, an evolutionary extension to the HART Protocol. According to Sean Ireland, wireless product manager for Siemens Energy and Automation, “This would be rated extremely high in importance, both to us and our customers. Wired and wireless products must work together in a common network.”

But HART 7 is significantly more than WirelessHART™. There are significant differences and improvements between HART 5 and HART 6, the most commonly installed and used versions of HART, and HART 7—and not just wireless, either.

“There will be increased diagnostic capability built into the instrument,” says ABB’s Gareth Johnston, “and the ability to report by exception, based on time or event.” But in keeping with the HART Communication Foundation’s long-standing commitment to the users of the HART protocol, HART 7 is fully backward-compatible with all the HART versions that came before it.

The fact that WirelessHART™ is simply part of the HART 7 standard shows off an important characteristic. Wired HART and WirelessHART™ are intended to be used together seamlessly, as integral parts of a digital field sensor and control network. They operate the same way, program the same way, and are useable interchangeably, wired and wireless.

ABB’s Johnston says, “WirelessHART™ is the logical migration of the current HART technology to a wireless environment.” He continues, “Customers now have the capability to maintain their current legacy 4-20 mA loops and unlock the power of HART instrument data via HART wirelessly.”

What do the HART users get? Johnston continues, “This will enable customers to obtain real-time asset monitoring data. This data will permit them to better manage the health of their instruments, thus insuring a reliable measure of their process.”

HART is a global standard and works anywhere

The HART protocol is enshrined in several IEC global standards, and the WirelessHART™ portion of HART 7 has been submitted for ballot as an additional IEC standard. HART has become both an IEC and de facto global standard because it is easy to use, simple to understand, flexible, and adaptable, and it can be used anywhere in the world. WirelessHART™, because it uses the 2.4 GHz band, can also be used almost everywhere in the world without licensing challenges.

New Features in HART 7 for wired devices

In Table 1, you can see what’s been added to HART 7: new capabilities for increased intelligence and communication throughput efficiency, with new features such as enhanced data publishing for process variables, configuration change and device diagnostic status alerts, with support for “Publish by Exception” and “Automatic Event Notification.” Another exceptionally valuable new feature: “Time-Stamped Data,” applies to device process variables, diagnostics status, and configuration change events.  This new feature supports broader use of multivariable device capabilities in control systems and permits users to employ better forensics in the event of problems. Measurements or actions can be triggered at a specific time, and HART 7 allows synchronized operation across multiple devices.

In HART 7, Process Variable Trends can now be sent as 12 sequential time-stamped data points of the same process variable in a single packet, effectively multiplying communication throughput for the selected process variable. “This makes it easy to setup a plant-wide asset management system,” says Endress + Hauser vice president Victor Wolowec.

Faster configuration uploads are now possible in HART 7 with multiple read commands in one transaction.
Plus, HART 7 adds a number of additional standardized device diagnostic and status parameters such as “device memory,” “power,” and maintenance and environmental variables.

Wireless HART™ fundamentals

WirelessHART™ is an evolutionary extension to the wired HART Communication protocol that enables wireless communication with field devices. It is built on established standards and proven technologies, while preserving fundamental HART principles of low-cost digital connectivity, interoperability, and backward compatibility. This preserves the investments of both users and suppliers in wired HART technology, while providing a path forward to the wireless future. There are four WirelessHART™ specifications built on the 13 core HART specifications.
“WirelessHART™ is an important milestone,” says Yokogawa’s Kaoru Sonoda, communications and diagnosis for field instrumentation group manager.

WirelessHART™ is real, scalable, useable in all the ways HART wired devices are, capable of talking to any control system or asset management system over any backbone or protocol, and at least 17 companies are in the process of producing WirelessHART™ devices for sale. And every single one of them interoperates with each other, and with all the wired HART devices that have ever been made by any vendor.

Using the new features of HART 7 for wired HART devices

The first versions of HART were designed to bring smart field devices into the world of control and automation, and they succeeded wildly. Even today, many HART devices are operated in strictly analog mode, and the HART smarts are used strictly for setup and calibration.

Yet the push toward added value from intelligent field devices has also led to the use of HART devices in many control systems for enhanced system integration—taking the digital signal through HART multiplexers directly into the control system, for additional process variables, online diagnostics, and better alarm management. 

Now, with HART 7, the HART protocol integrates wired and wireless devices into a common control network and adds additional functionality to improve alarm management and maintenance diagnosis.

“The unsolicited messaging capability is probably one of the most immediate benefits,” says Scott Saunders, vice president of sales and marketing for Moore Industries-International. “All plants are doing more with less personnel. Having HART slave devices that are able to perform predictive maintenance on their own is a real plus.”

About Interoperability, Interchangeability and Coexistence

A major reason for the success of the HART protocol is backward-compatibility and Electrical Interchangeability. HCF’s definition of these terms is rigorous:

• Freedom to use any manufacturer of HART devices and mix manufacturers and products in the same network
• Controllers use the same commands with all devices, and there are standardized diagnostics in every device
• Same toolset works with all manufacturers, and one EDD works on all EDD-enabled hosts
• Freedom to use any revision of the HART protocol or of the device without mandatory upgrades to tools or applications.

“There is no necessity for existing HART 5 users to move to HART 7 because they are satisfied with existing features,” Yokogawa’s Sonoda points out. “However some users intend to move to HART 7 from HART 5 so they can use the new functions of HART 7.”

Protocol Versatility

The watchword of the modern HART 7 standard is protocol versatility—a key to interoperability.
You can use “traditional” HART, which is a 4-20 mADC current loop, with a digitally encoded signal: a 1200 bps Frequency Shift Keyed (FSK) sine wave modulated on the current signal that enables bi-directional communications of configuration, process variables, and diagnostics information to and from the intelligent device. Or you can use the High-Speed HART PSK specification. This specification provides for a digitally encoded signal on a 4-20 mADC current loop, but this time, it is Phase Shift Keyed at 9600 bps; eight times faster than “traditional” HART FSK signalling. And, of course, PSK HART is completely backward-compatible with FSK HART.
In addition, HART Communication can be run over RS485 (Modbus), Profibus and Profinet, Ethernet, OPC, and WirelessHART™.

Defined Command Set

Interoperability also requires a defined command set that every device must do. Every HART device must accommodate the Universal Command set, and be able to perform Identity Verification (Unique ID, Tag, Descriptor, etc.), as well as read device dynamic variables, loop-current values, diagnostics, and other variables in the device.

Every device should also use the Common Practice Commands, such as device management commands for ranging, loop-current setup, sensor trim, and other standard capabilities. Most HART devices support the Common Practice Commands.

In addition, every WirelessHART™ device must support the defined set of Wireless Commands required for setup, operation, performance mointoring, and diagnosis of the WirelessHART™ network. The Wireless Command set specifically defines all requirements for joining, security, message routing, diagnostic reports, bandwidth allocation, and other necessary capabilities to ensure interoperability, proper performance, and network coexistence. Key required diagnostics include remaining battery life (in days), average communication latency, and packet loss counter (normally “0”).

Universal Diagnostics

All HART devices have an interoperable universal set of diagnostics. These universal diagnostic alerts are designed for continuous monitoring by HART-enabled control and intelligent I/O systems, for immediate detection of loop-integrity and/or device problems.

• Device malfunction (check device immediately)
• Configuration change (somebody reconfigured the device, go see if it was done right)
• Cold start or reset (there may be power supply problems, check it out)
• More status available (here is where HART’s native EDDL capability permits a DD enabled host application to retrieve additional device specific diagnostics)
• Loop current fixed (the loop current should not be used for control—this may or may not be an error, go see)
• Loop current saturate (the loop current range has been exceeded, use the digital value for measurement, go see why the device is overranged)
• Non PV out of limits (this varies by device type. For example, a multivariable transmitter with a temperature measurement may have exceeded operating range—go verify the device is working properly)
• PV out of limits (go find out why the device is over-ranging or under-ranging)

Every HART device conforms to this universal diagnostics set, meaning that diagnosing problems on any HART device from any manufacturer works the same way.

Interoperability = Integration Choice

The most important feature of HART 7 is the integration versatility of the technology. Wired or wireless, HART offers unparalleled ease in integration with systems. Using EDDL, OPC, and XML, HART 7 communicates with any advanced control, simulation, asset management or enterprise integration system.

All major control system suppliers offer HART-enabled I/O as standard equipment. Almost all major field device manufacturers offer HART-enabled devices.

A significant number of third-party products support integration with legacy control systems using a variety of devices such as Multiplexers (HART to RS-485) and Gateways (HART to Ethernet, HART to Modbus, HART to Profibus, etc.) and Single-Loop Monitors (devices that convert digital values and alerts to analog signals and contact closures that older DCS and SCADA systems can use)... and of course, there’s wireless.

HART 7 Communications: Interfacing with the rest of the world

Wired or wireless, HART 7 easily interfaces with the rest of the world. “One of the many strengths of a HART device,” says MACTek’s Holmes, “is that the 26-plus million installed devices work on any analog or digital control system or PLC.”

“Clear end-user advantages also result,” says Emerson’s vice president of wireless, Bob Karshnia, “since HART 7 didn’t change what didn’t need to be changed. Users with working HART systems can continue to use HART 5 host system products and add supplemental HART 7 functionality to the host when ready, and then continue to add HART 5 or HART 7 products going forward.”