1661899790626 Article 419 Hart

HART Communication – Dispelling the Myths: Part II

June 28, 2005
This "special to the web" article is designed to increase your knowledge about the HART Field Communications Protocol. Find out its benefits and dispel the myths surrounding this powerful technology.
By Ed T. Ladd Jr., HCFMYTH:  HART is an interim technology until the all-digital fieldbus arrives.FACT:  Digital fieldbus technology is already here, but newer technologies do not always automatically replace those that have been proven, tried and true. Industry consultants predict that the HART Protocol will be supported and remain a significant communication technology for the next 20-plus years.HART technology continues to be updated to address additional user needs as evidenced by the recent move of manufacturers to the HART 6 specification and the new graphical user interface enhancements being incorporated into HART Device Description Language (IEC 61804-2).These technology advancements open new doors for the continuing use of HART Communication today and in the future. However, to achieve maximum benefit from existing HART-enabled control systems, users should demand support for HART 6 on any new products being installed in their plants. HART 6 features and capabilities are designed to improve support for multi-variable and valve/actuator type devices; enhanced status and diagnostics, increase interoperability; extended commissioning/troubleshooting capabilities; and security measures to confirm any configuration change. HART 6 functionality includes:
  1. Device Variable Classification - Required
    Provides master applications with a simple mechanism to determine the number and type of process related variables (pressure, temperature, etc.) within a device. 

  2. Extended Device Status - Required
    One additional byte of well defined status information with Command 0 and new cyclic data access Command 9. Provides additional device status alerts such as “Device Needs Maintenance.”

  3. Device Variable Status - Required
    Enables field devices to self-validate and report on the quality of the data in the command response (good, poor, bad, fixed).

  4. Long Tag - Required
    This new Long Tag with international (ISO Latin 1) characters allows consistent implementation of the longer tag names required by many industry users. The specifications currently reflect the length of this tag to be 32 characters.

  5. Configuration Change Counter – Required
    Improved mechanism for master applications to determine that a field device configuration has been changed. Protects integrity of plant configuration databases.

  6. Device Families – Optional
    Establishes standard commands and status indicators for devices based on the type of process measurement. Initial Device Families include Temperature and PID Control. 

  7. Transducer Trim Commands – Optional
    New Common Practice commands for performing transducer trim (calibration) operations.

  8. Sub-Devices – Optional
    A simple mechanism using Common Practice Commands to support “HART device within a HART device” functionality. Potential uses include flow computers and multi-channel temperature devices.

  9. Block Data Transfer – Optional
    An updated mechanism to support the movement of large blocks of data, such as device configuration information, between masters and field devices. 

  10. Catch Device Variable – Optional
    A simple mechanism to support the sharing of process data between field devices on the same HART network. Allows a listening field device to capture process data from another field device to be used in calculations such as tank gauging, flow computers or PID control functions.

  11. Write Device Variable – Optional
    New command to support forcing the digital value for any Device Variable to a specific value to aid in commissioning and troubleshooting.

  12. Lock Device – Optional
    New commands allow a master application to lock the “local” front panel of a field device while performing remote configuration functions.

  13. Squawk and Find Device – Optional
    New commands to support commissioning and troubleshooting of HART devices in multi-drop and multi-pair cable installations.

The enhancements to HART Communication technology are bridging the gap between traditional analog devices and smart digital devices with intelligent field communications. This bridging provides a continuous forward migration path with minimal investment loss. This type migration path is vital to cost-conscious companies that work with continuous processes.

In the process industries, the complexity of new technologies or the lack of historical backing of a new technology has a tendency to slow adoption. This is especially true for companies that cannot afford to invest in and install new systems every time there is technology advancement. It is much wiser and more cost-effective to upgrade existing systems rather than throw away an investment made over many years.

Existing 4-20mA analog signal technology currently comprises the majority of all installed devices in the field (see Figure 1). For decades manufacturers have invested in systems that accept 4-20mA analog signals for control. Even now, instrument manufacturers worldwide continue to produce hundreds of thousands of instruments that utilize this established standard. Therefore, it is evident that use of the 4-20mA signal will continue far into the foreseeable future.

Because HART devices support two simultaneous communication channels on the same wire (4-20mA “current loop” analog and HART digital), HART Communication is the logical migration path for the millions of legacy systems still in use. All HART-enabled instruments produce or accept a 4-20mA analog control signal compatible with systems produced over the past 30 years. No loss of investment occurs when upgrading using the HART path. 

With HART Communication positioned to replace traditional analog transmitters, the technology’s growth is expected to continue over the next two decades. The expected growth rate for HART technology assures users that their investment is one that will continue to be used for many years to come. But, finding a place to begin integration of HART data is the key to future improvements in process performance. When control systems use only the 4-20mA analog communication channel, an “information gap” exists. Continuous real-time HART communication closes that information gap by providing a two-way exchange of process information.

Integrating HART Communication with plant systems is easy and cost effective. Get started today. Close the information gap and “See What You Can Do” when you use the “Power of HART”.

Upcoming HART Connection articles will provide information on applying HART technology, application notes to help you implement new HART strategies and what to expect from a HART-enabled device or host system. For more information, contact the HART Communication Foundation or visit the website at http://www.hartcomm.org.

  About the Author
Ed T. Ladd, Jr., is Director of Technology Programs for the HART Communication Foundation, Austin, Texas. Upcoming articles by Ladd will provide information on applying HART technology, application notes to help you implement new HART strategies and what to expect from a HART-enabled device or host system. For more information, visit theHART Communication Foundationwebsite.

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