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HART Communication – Dispelling the Myths: Part II

ControlGlobal.com

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., HCF

MYTH:  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.