sww - 2008-8-26 11:22:01
[table=98%][tr][td=1,1,234][table=98%][tr][td=1,1,234][/td][/tr][/table][/td][td=1,1,774][table=98%][tr][td=1,1,774][/td][/tr][tr][td][table=98%][tr][td=1,1,664]HART Communication: Driving New Product Developments[/td][td=1,1,110][table=98%][tr][td=1,1,110][/td][/tr][/table][/td][/tr][/table][/td][/tr][tr][td]
Since the first open communications protocols were introduced, there has been an accelerating industry move from traditional analog to intelligent instrumentation. The HART® Field Communications Protocol dominates this smart instrumentation marketplace. More than 14 million HART-capable devices are installed worldwide. Seventy percent of all smart process measurement and control instruments installed each year communicate using HART technology.
The impact of HART Communication on the process automation industry is immeasurable. No other field communication technology comes close—in size or scope of installation and overall effectiveness. HART is the industry’s most cost-effective, easy-to-use and low-risk communication solution and a key enabler for asset management and process improvement.
Figure 1 – The Market Growth Outlook is Intelligent Instruments with Open Communication Protocols – HART Being the Biggest
Driving Development
The world’s leading process automation control systems and instrumentation suppliers are investing heavily in HART technology. Most suppliers now offer control system interfaces, remote I/O systems and PC-based software applications that leverage the intelligence in HART-smart field devices to deliver continuous, real-time device diagnostics, multi-variable process information and much more.
Integrating this information into DCS architectures enables users to get the full benefit from intelligent devices making HART Communication an important part of plant applications for control, safety, asset productivity and more. Continuous communication between the field device and control system enables problems with the device, its connection to the process, or inaccuracies in the 4-20mA control signal to be detected automatically so that corrective action can be taken to avoid process disruptions.
HART Communication Basics
HART Communication is a backward compatible enhancement for 4-20mA instrumentation equipment that enables remote, two-way, digital communication with smart microprocessor-based field devices. Primary characteristics include: two simultaneous communication channels for exchange of process variables, device status and diagnostics information between field devices and process automation systems; an extensive application layer with standard commands for real-time transfer of standardized data; and standard operating procedures for efficient device commissioning and calibration.
The two communication channels supported by HART-enabled devices are the 4-20mA “current loop” analog communication channel and the HART digital communication channel. The 4-20mA analog communication channel is significant because it ensures compatibility with legacy systems. Plus, the continuous 4-20mA analog current loop is the fastest way to pass a single control variable value to/from the process connection (field device) and controller. No digital signal can be as fast.
The HART digital communication channel transmits simultaneously on the same wire and without disruption to the 4-20mA analog channel. This bi-directional communication channel provides real-time access to the rich set of data items available in all HART smart devices—process variables, device status, diagnostics, device identification, and calibration/sensor information. Simultaneous and complementary real-time use of both communication channels provides a high level of control security and loop integrity far beyond what is achievable by using either channel alone.
Figure 2 – HART Devices Have Two Communication Channels
Figure 3 – Simultaneous 4-20mA Current Loop and HART Communication
The Smart Part of HART
All HART-enabled field devices, regardless of manufacture, contain 35 to 40 data items of rich information for improving plant operations and managing assets. The inherent intelligence of these devices allows them to perform internal diagnostic checks and communicate information regarding their status continuously. The extensive array of device status, diagnostic and process related data items in every HART device are continually updated and readily available for real-time use by plant control, safety and asset management systems.
Standard HART Universal and Common Practice Commands make it easy for systems to access the real-time data in HART devices. Each Command defines unique request and response data packets for communication over the HART digital channel. Device status information is transmitted with every response packet from the field device.
Control systems use standard HART commands to access multi-variable process data and detect any problems impacting the device or the integrity of the 4-20mA current loop in real-time. The 4-20mA analog communication channel provides additional security as serious device failures are also indicated by fail-safe actions of the current loop - full upscale (> 20mA), full downscale (< 4mA).
Extensive Device Status
The HART protocol specifications define a rich set of device status indicators for implementation in all HART-enabled field devices. The status indicators are designed to support easy detection of a particular device problem or event. Devices transmit status information pertaining to the health of the field device with every response message. Tables 1 and 2 summarize the status, diagnostic, and data quality indicators available in HART devices.
Table 1. HART Device Status Indicators
[table][tr][td=1,1,157][b]Status[/b]
[/td][td=1,1,505][b]Description[/b]
[/td][/tr][tr][td=1,1,157][b]Device Malfunction[/b]
[/td][td=1,1,505]Indicates an error that prevents proper operation of the field device has been detected.
Hosts should consider this an alarm condition and refrain from using the device for process control applications.
[/td][/tr][tr][td=1,1,157][b]Device Variable Alert[/b]
[/td][td=1,1,505]This bit is set if any Device Variable is in an Alarm or Warning State.
Allows detection of process data quality problems even when the suspect Device Variable is not being actively monitored.
[/td][/tr][tr][td=1,1,157][b]Cold Start[/b]
[/td][td=1,1,505]This status is set when the field device is powered up and after a Device Reset.
It is automatically reset by the first command received from the host. Hosts should use this to detect a power loss at the Field Device.
[/td][/tr][tr][td=1,1,157][b]Configuration Changed[/b]
[/td][td=1,1,505]This bit will be set any time any configuration item in a field device is modified.
The Configuration Changed status is maintained even through power failures and can only be reset by a Host issuing Command 38. This status is used along with the [b]Configuration Change Counter[/b] to allow host to monitor the configuration of the field device.
[/td][/tr][tr][td=1,1,157][b]Configuration Change Counter[/b]
[/td][td=1,1,505]A 16 bit counter incremented once for every operation that changes the devices configuration.
This value cannot be reset and is maintained even if power is removed from the device or a Device Reset is performed.
[/td][/tr][tr][td=1,1,157][b]Maintenance Required[/b]
[/td][td=1,1,505]This bit is set to indicate that, while the device has not malfunctioned, the Field Device requires maintenance.
[/td][/tr][tr][td=1,1,157][b]More Status Available[/b]
[/td][td=1,1,505]More Status Available indicates that additional device status currently has status set that may be useful to the host.
When More Status Available is set, the measurements may still be correct and suitable for use by control systems.
[/td][/tr][tr][td=1,1,157][b]Non-PV Out of Limits[/b]
[/td][td=1,1,505]This status is set when any Device Variable other than the one mapped to the current loop reaches its Upper or Lower Transducer Limit.
Limits may be electrical (e.g., the sensor in a transmitter) or mechanical (e.g., actuator at its travel limit as indicated by a limit switch).
[/td][/tr][tr][td=1,1,157][b]PV Out of Limits[/b]
[/td][td=1,1,505]When the Device Variable (input or output) mapped to the loop current reaches its Upper or Lower Transducer Limit, the "PV Out of Limits" status is set.
[/td][/tr][tr][td=1,1,157][b]Loop Current Fixed[/b]
[/td][td=1,1,505]Indicates that the Loop Current is fixed and not responding to process changes.
Hosts should not use the Loop Current signal for control purposes.
[/td][/tr][tr][td=1,1,157][b]Loop Current Saturated[/b]
[/td][td=1,1,505]HART devices continuously calculate digital values for their Device Variables.
These calculations a valid between the corresponding upper and lower limits.
In most cases the loop current signal represents only a portion of those range of value.
When the digital value sufficiently exceeds the designated range the loop current will saturate.
This status is set when that condition is present.
[/td][/tr][/table]
Table 2. Process Data and Limit Status Indicators
[table][tr][td=1,1,157][b]Indicator Value[/b]
[/td][td=1,1,545][b]Description[/b]
[/td][/tr][tr][td=1,1,157][b]3
Good [/b]
[/td][td=1,1,545]The process value provided by the Field Device is estimated to be within the manufacturers specified accuracy range.
[/td][/tr][tr][td=1,1,157][b]2
Poor Accuracy[/b]
[/td][td=1,1,545]The accuracy of the process value has been degraded due to an internal or external factor.
Devices may set this status level when their internal algorithms detect measurement or control problems.
[/td][/tr][tr][td=1,1,157][b]1
Manual/Fixed[/b]
[/td][td=1,1,545]The process value does not reflect actual process conditions.
This status level is indicated when the process value is being simulated or is manually overridden.
This may not be the result of an error condition
[/td][/tr][tr][td=1,1,157][b]0
Bad[/b]
[/td][td=1,1,545]An error has been detected that prevent the generation of a valid process value.
While a process value is still provided by the Field Device, the process value should be used for diagnostic purposes only.
[/td][/tr][tr][td=1,1,157][b]3
Constant [/b]
[/td][td=1,1,545]The associated process value is frozen and unable to vary.
[/td][/tr][tr][td=1,1,157][b]2
Low Limited[/b]
[/td][td=1,1,545]The associated process value has reached its lower transducer limit and is no longer varying.
[/td][/tr][tr][td=1,1,157][b]1
High Limited[/b]
[/td][td=1,1,545]The associated process value has reached its upper transducer limit and is no longer varying.
[/td][/tr][tr][td=1,1,157][b]0
Not Limited[/b]
[/td][td=1,1,545]The associated process value can vary an respond as required by the system.
[/td][/tr][/table]
Device Specific Diagnostics
In addition to the standardized indicators for device status and process variable quality, the HART Protocol defines an efficient mechanism for the reporting of device specific diagnostic information in Command 48. This standard HART command provides status information for up to 136 device specific diagnostic parameters in a single command response.
HART-enabled control systems should interrogate the device with Command 48 any time the "More Status Available" status bit is set. The status information returned with this command response is device specific in nature and is typically described by the device manufacturer in the HART Device Description (DD) for that particular device.
Control systems with DD capability or those with special drivers will be able to decode the information and display the specific device problem directly. Control systems unable to decode the specific meaning should perform the following: (1) monitor the Command 48 status response and provide an alert should any status indicator change; (2) display and provide listing of the command response details. This procedure enables early detection of any device problems and facilitates follow-up diagnosis / troubleshooting with DD-enabled tools or manufacturer’s technical support.
Control Loop Validation
Clearly reliable, continuous communication is critical to making good decisions. Likewise the quality, integrity, and accuracy of the 4-20mA current loop signal is essential for good control. Real-time integration of the intelligent information in HART-devices with control, safety and asset management systems provides tremendous benefits as it provides early warning to device problems and continuously validates the accuracy of data exchange between the control system and field device.
In most measurement applications, the field device transmits the signal value for the Primary Control Variable on the 4-20mA analog loop current. The value of the Primary Control Variable is mapped to the 4-20mA analog current signal by the device based on the Upper and Lower Range values and the designated Transfer Function (e.g., linear, square root, etc.).
At the other end of the wire, a similar conversion is also performed at the control system I/O. If everything is working properly the value for the Primary Control Variable derived by the control system I/O and the Field Device will agree. HART Communication enables control systems to continuously monitor and validate the integrity of the 4-20mA current loop. Performing the following checks at the control system I/O level allows immediate detection of any disagreement that could invalidate the process data value being transmitted by the 4-20mA signal.
[list][*]The loop current being transmitted by the Field Device can be read (using Command 2). Should this value differ from that received at the control system I/O, the integrity of the 4-20mA current loop is suspect.[*]The Configuration Changed bit should be monitored by the system. If this status bit is set, the Upper and Lower Calibrated Ranges Values for the device should be read (using Command 15) and compared with the system configured range values to ensure that the device and system still agree on the meaning of the 4-20mA loop current.[*]The device Primary Variable value should be read periodically (using Command 1, 3 or 9) for comparison with the digital value for the variable derived by the control system I/O. This allows a front-to-back verification of the loop current integrity.[*]Loop tests can be automatically performed to confirm loop current calibration. The HART Protocol provides standard commands and procedures allowing agreement between the I/O and Field Device to be confirmed.[/list]Getting Connected
Today, most process automation system suppliers offer a variety of HART interface solutions to support integration with their control systems. Many have intelligent remote I/O subsystems and system interfaces for direct connection to HART devices. Most allow the HART data to be used in real-time for operator display, alarm and control functions. Third-party I/O systems and interface products are available to support integration with legacy control systems that might not be easily upgraded for HART Communication. Gateway interface solutions are also available for linking HART devices to systems based on network protocols such as Ethernet, Modbus, and Profibus.
Real-time HART integration with plant control, safety and asset management systems unlocks the value of connected devices and extends the capability of systems to detect any problems with the device, its connection to the process or interference with accurate communication between the device and system. The justification is two-fold: (1) cost reduction due to improved operations and increased efficiencies; and (2) cost avoidance as the early warning to impending problems enables the high cost of process disruptions and unplanned shutdowns to be averted.
HART Communication Revisited
A Different Point of View
Standard Data (You Probably Know)
[list][*]Most smart instruments use HART Communication[*]HART smart field devices are used interchangeably with traditional 4-20mA analog only units[*]Handheld communicators are frequently used to perform device set-up, calibration, commissioning and periodic maintenance functions[*]PC-based instrument management tools remotely communicate with HART devices to manage calibration and other instrument parameters from central locations[*]HART Communication reduces installation, commissioning and maintenance costs[/list]Bonus Data (You Might NOT Know)
[list][*]All HART smart devices provide process data for plant operation continuously (24 hours a day, 7 days a week)[*]HART Communication allows access to all device data while the 4-20mA analog signal is being used for control[*]Information about the status (health) of the field device and quality of the 4-20mA signal is communicated routinely (every message)[*]The Primary Variable for control is transmitted on the 4-20mA current loop and also as a digital value[*]Many HART devices are multivariable with Secondary process variables (measured or calculated) in addition to the Primary variable.[/list]There are many cost-effective interface solutions to support HART integration with plant control and safety systems
2004 and Beyond
The HART Communication Foundation (HCF), an independent not-for-profit organization, is the technology owner and standards setting body for the HART Protocol. The Foundation provides worldwide support for application of the HART Protocol and ensures that the technology is openly available for the benefit of the industry.
The Foundation continues to enhance the HART technology to ensure it serves evolving needs for smart instrumentation today and in the future.
Additional diagnostics and new capabilities were incorporated with HART 6, a major enhancement to the HART technology, to improve integration with plant control and safety systems. The Device Description Language (DDL) is also being enhanced with new capabilities from the joint HART, Fieldbus Foundation and PROFIBUS cooperation; new tools for efficient development, testing and maintenance of Device Descriptions; and new technology to facilitate the development of DD-capable host applications.
HART is the global standard for smart process instrumentation communication. With its huge installed base, ease of use, global acceptance, and the support of major process automation suppliers, HART Communication will continue to lead the world market well beyond the next decade—creating an ongoing demand for development of new HART-enabled products and advanced applications.
[/td][/tr][/table][/td][/tr][/table]
Since the first open communications protocols were introduced, there has been an accelerating industry move from traditional analog to intelligent instrumentation. The HART® Field Communications Protocol dominates this smart instrumentation marketplace. More than 14 million HART-capable devices are installed worldwide. Seventy percent of all smart process measurement and control instruments installed each year communicate using HART technology.
The impact of HART Communication on the process automation industry is immeasurable. No other field communication technology comes close—in size or scope of installation and overall effectiveness. HART is the industry’s most cost-effective, easy-to-use and low-risk communication solution and a key enabler for asset management and process improvement.
Figure 1 – The Market Growth Outlook is Intelligent Instruments with Open Communication Protocols – HART Being the Biggest
Driving Development
The world’s leading process automation control systems and instrumentation suppliers are investing heavily in HART technology. Most suppliers now offer control system interfaces, remote I/O systems and PC-based software applications that leverage the intelligence in HART-smart field devices to deliver continuous, real-time device diagnostics, multi-variable process information and much more.
Integrating this information into DCS architectures enables users to get the full benefit from intelligent devices making HART Communication an important part of plant applications for control, safety, asset productivity and more. Continuous communication between the field device and control system enables problems with the device, its connection to the process, or inaccuracies in the 4-20mA control signal to be detected automatically so that corrective action can be taken to avoid process disruptions.
HART Communication Basics
HART Communication is a backward compatible enhancement for 4-20mA instrumentation equipment that enables remote, two-way, digital communication with smart microprocessor-based field devices. Primary characteristics include: two simultaneous communication channels for exchange of process variables, device status and diagnostics information between field devices and process automation systems; an extensive application layer with standard commands for real-time transfer of standardized data; and standard operating procedures for efficient device commissioning and calibration.
The two communication channels supported by HART-enabled devices are the 4-20mA “current loop” analog communication channel and the HART digital communication channel. The 4-20mA analog communication channel is significant because it ensures compatibility with legacy systems. Plus, the continuous 4-20mA analog current loop is the fastest way to pass a single control variable value to/from the process connection (field device) and controller. No digital signal can be as fast.
The HART digital communication channel transmits simultaneously on the same wire and without disruption to the 4-20mA analog channel. This bi-directional communication channel provides real-time access to the rich set of data items available in all HART smart devices—process variables, device status, diagnostics, device identification, and calibration/sensor information. Simultaneous and complementary real-time use of both communication channels provides a high level of control security and loop integrity far beyond what is achievable by using either channel alone.
Figure 2 – HART Devices Have Two Communication Channels
Figure 3 – Simultaneous 4-20mA Current Loop and HART Communication
The Smart Part of HART
All HART-enabled field devices, regardless of manufacture, contain 35 to 40 data items of rich information for improving plant operations and managing assets. The inherent intelligence of these devices allows them to perform internal diagnostic checks and communicate information regarding their status continuously. The extensive array of device status, diagnostic and process related data items in every HART device are continually updated and readily available for real-time use by plant control, safety and asset management systems.
Standard HART Universal and Common Practice Commands make it easy for systems to access the real-time data in HART devices. Each Command defines unique request and response data packets for communication over the HART digital channel. Device status information is transmitted with every response packet from the field device.
Control systems use standard HART commands to access multi-variable process data and detect any problems impacting the device or the integrity of the 4-20mA current loop in real-time. The 4-20mA analog communication channel provides additional security as serious device failures are also indicated by fail-safe actions of the current loop - full upscale (> 20mA), full downscale (< 4mA).
Extensive Device Status
The HART protocol specifications define a rich set of device status indicators for implementation in all HART-enabled field devices. The status indicators are designed to support easy detection of a particular device problem or event. Devices transmit status information pertaining to the health of the field device with every response message. Tables 1 and 2 summarize the status, diagnostic, and data quality indicators available in HART devices.
Table 1. HART Device Status Indicators
[table][tr][td=1,1,157][b]Status[/b]
[/td][td=1,1,505][b]Description[/b]
[/td][/tr][tr][td=1,1,157][b]Device Malfunction[/b]
[/td][td=1,1,505]Indicates an error that prevents proper operation of the field device has been detected.
Hosts should consider this an alarm condition and refrain from using the device for process control applications.
[/td][/tr][tr][td=1,1,157][b]Device Variable Alert[/b]
[/td][td=1,1,505]This bit is set if any Device Variable is in an Alarm or Warning State.
Allows detection of process data quality problems even when the suspect Device Variable is not being actively monitored.
[/td][/tr][tr][td=1,1,157][b]Cold Start[/b]
[/td][td=1,1,505]This status is set when the field device is powered up and after a Device Reset.
It is automatically reset by the first command received from the host. Hosts should use this to detect a power loss at the Field Device.
[/td][/tr][tr][td=1,1,157][b]Configuration Changed[/b]
[/td][td=1,1,505]This bit will be set any time any configuration item in a field device is modified.
The Configuration Changed status is maintained even through power failures and can only be reset by a Host issuing Command 38. This status is used along with the [b]Configuration Change Counter[/b] to allow host to monitor the configuration of the field device.
[/td][/tr][tr][td=1,1,157][b]Configuration Change Counter[/b]
[/td][td=1,1,505]A 16 bit counter incremented once for every operation that changes the devices configuration.
This value cannot be reset and is maintained even if power is removed from the device or a Device Reset is performed.
[/td][/tr][tr][td=1,1,157][b]Maintenance Required[/b]
[/td][td=1,1,505]This bit is set to indicate that, while the device has not malfunctioned, the Field Device requires maintenance.
[/td][/tr][tr][td=1,1,157][b]More Status Available[/b]
[/td][td=1,1,505]More Status Available indicates that additional device status currently has status set that may be useful to the host.
When More Status Available is set, the measurements may still be correct and suitable for use by control systems.
[/td][/tr][tr][td=1,1,157][b]Non-PV Out of Limits[/b]
[/td][td=1,1,505]This status is set when any Device Variable other than the one mapped to the current loop reaches its Upper or Lower Transducer Limit.
Limits may be electrical (e.g., the sensor in a transmitter) or mechanical (e.g., actuator at its travel limit as indicated by a limit switch).
[/td][/tr][tr][td=1,1,157][b]PV Out of Limits[/b]
[/td][td=1,1,505]When the Device Variable (input or output) mapped to the loop current reaches its Upper or Lower Transducer Limit, the "PV Out of Limits" status is set.
[/td][/tr][tr][td=1,1,157][b]Loop Current Fixed[/b]
[/td][td=1,1,505]Indicates that the Loop Current is fixed and not responding to process changes.
Hosts should not use the Loop Current signal for control purposes.
[/td][/tr][tr][td=1,1,157][b]Loop Current Saturated[/b]
[/td][td=1,1,505]HART devices continuously calculate digital values for their Device Variables.
These calculations a valid between the corresponding upper and lower limits.
In most cases the loop current signal represents only a portion of those range of value.
When the digital value sufficiently exceeds the designated range the loop current will saturate.
This status is set when that condition is present.
[/td][/tr][/table]
Table 2. Process Data and Limit Status Indicators
[table][tr][td=1,1,157][b]Indicator Value[/b]
[/td][td=1,1,545][b]Description[/b]
[/td][/tr][tr][td=1,1,157][b]3
Good [/b]
[/td][td=1,1,545]The process value provided by the Field Device is estimated to be within the manufacturers specified accuracy range.
[/td][/tr][tr][td=1,1,157][b]2
Poor Accuracy[/b]
[/td][td=1,1,545]The accuracy of the process value has been degraded due to an internal or external factor.
Devices may set this status level when their internal algorithms detect measurement or control problems.
[/td][/tr][tr][td=1,1,157][b]1
Manual/Fixed[/b]
[/td][td=1,1,545]The process value does not reflect actual process conditions.
This status level is indicated when the process value is being simulated or is manually overridden.
This may not be the result of an error condition
[/td][/tr][tr][td=1,1,157][b]0
Bad[/b]
[/td][td=1,1,545]An error has been detected that prevent the generation of a valid process value.
While a process value is still provided by the Field Device, the process value should be used for diagnostic purposes only.
[/td][/tr][tr][td=1,1,157][b]3
Constant [/b]
[/td][td=1,1,545]The associated process value is frozen and unable to vary.
[/td][/tr][tr][td=1,1,157][b]2
Low Limited[/b]
[/td][td=1,1,545]The associated process value has reached its lower transducer limit and is no longer varying.
[/td][/tr][tr][td=1,1,157][b]1
High Limited[/b]
[/td][td=1,1,545]The associated process value has reached its upper transducer limit and is no longer varying.
[/td][/tr][tr][td=1,1,157][b]0
Not Limited[/b]
[/td][td=1,1,545]The associated process value can vary an respond as required by the system.
[/td][/tr][/table]
Device Specific Diagnostics
In addition to the standardized indicators for device status and process variable quality, the HART Protocol defines an efficient mechanism for the reporting of device specific diagnostic information in Command 48. This standard HART command provides status information for up to 136 device specific diagnostic parameters in a single command response.
HART-enabled control systems should interrogate the device with Command 48 any time the "More Status Available" status bit is set. The status information returned with this command response is device specific in nature and is typically described by the device manufacturer in the HART Device Description (DD) for that particular device.
Control systems with DD capability or those with special drivers will be able to decode the information and display the specific device problem directly. Control systems unable to decode the specific meaning should perform the following: (1) monitor the Command 48 status response and provide an alert should any status indicator change; (2) display and provide listing of the command response details. This procedure enables early detection of any device problems and facilitates follow-up diagnosis / troubleshooting with DD-enabled tools or manufacturer’s technical support.
Control Loop Validation
Clearly reliable, continuous communication is critical to making good decisions. Likewise the quality, integrity, and accuracy of the 4-20mA current loop signal is essential for good control. Real-time integration of the intelligent information in HART-devices with control, safety and asset management systems provides tremendous benefits as it provides early warning to device problems and continuously validates the accuracy of data exchange between the control system and field device.
In most measurement applications, the field device transmits the signal value for the Primary Control Variable on the 4-20mA analog loop current. The value of the Primary Control Variable is mapped to the 4-20mA analog current signal by the device based on the Upper and Lower Range values and the designated Transfer Function (e.g., linear, square root, etc.).
At the other end of the wire, a similar conversion is also performed at the control system I/O. If everything is working properly the value for the Primary Control Variable derived by the control system I/O and the Field Device will agree. HART Communication enables control systems to continuously monitor and validate the integrity of the 4-20mA current loop. Performing the following checks at the control system I/O level allows immediate detection of any disagreement that could invalidate the process data value being transmitted by the 4-20mA signal.
[list][*]The loop current being transmitted by the Field Device can be read (using Command 2). Should this value differ from that received at the control system I/O, the integrity of the 4-20mA current loop is suspect.[*]The Configuration Changed bit should be monitored by the system. If this status bit is set, the Upper and Lower Calibrated Ranges Values for the device should be read (using Command 15) and compared with the system configured range values to ensure that the device and system still agree on the meaning of the 4-20mA loop current.[*]The device Primary Variable value should be read periodically (using Command 1, 3 or 9) for comparison with the digital value for the variable derived by the control system I/O. This allows a front-to-back verification of the loop current integrity.[*]Loop tests can be automatically performed to confirm loop current calibration. The HART Protocol provides standard commands and procedures allowing agreement between the I/O and Field Device to be confirmed.[/list]Getting Connected
Today, most process automation system suppliers offer a variety of HART interface solutions to support integration with their control systems. Many have intelligent remote I/O subsystems and system interfaces for direct connection to HART devices. Most allow the HART data to be used in real-time for operator display, alarm and control functions. Third-party I/O systems and interface products are available to support integration with legacy control systems that might not be easily upgraded for HART Communication. Gateway interface solutions are also available for linking HART devices to systems based on network protocols such as Ethernet, Modbus, and Profibus.
Real-time HART integration with plant control, safety and asset management systems unlocks the value of connected devices and extends the capability of systems to detect any problems with the device, its connection to the process or interference with accurate communication between the device and system. The justification is two-fold: (1) cost reduction due to improved operations and increased efficiencies; and (2) cost avoidance as the early warning to impending problems enables the high cost of process disruptions and unplanned shutdowns to be averted.
HART Communication Revisited
A Different Point of View
Standard Data (You Probably Know)
[list][*]Most smart instruments use HART Communication[*]HART smart field devices are used interchangeably with traditional 4-20mA analog only units[*]Handheld communicators are frequently used to perform device set-up, calibration, commissioning and periodic maintenance functions[*]PC-based instrument management tools remotely communicate with HART devices to manage calibration and other instrument parameters from central locations[*]HART Communication reduces installation, commissioning and maintenance costs[/list]Bonus Data (You Might NOT Know)
[list][*]All HART smart devices provide process data for plant operation continuously (24 hours a day, 7 days a week)[*]HART Communication allows access to all device data while the 4-20mA analog signal is being used for control[*]Information about the status (health) of the field device and quality of the 4-20mA signal is communicated routinely (every message)[*]The Primary Variable for control is transmitted on the 4-20mA current loop and also as a digital value[*]Many HART devices are multivariable with Secondary process variables (measured or calculated) in addition to the Primary variable.[/list]There are many cost-effective interface solutions to support HART integration with plant control and safety systems
2004 and Beyond
The HART Communication Foundation (HCF), an independent not-for-profit organization, is the technology owner and standards setting body for the HART Protocol. The Foundation provides worldwide support for application of the HART Protocol and ensures that the technology is openly available for the benefit of the industry.
The Foundation continues to enhance the HART technology to ensure it serves evolving needs for smart instrumentation today and in the future.
Additional diagnostics and new capabilities were incorporated with HART 6, a major enhancement to the HART technology, to improve integration with plant control and safety systems. The Device Description Language (DDL) is also being enhanced with new capabilities from the joint HART, Fieldbus Foundation and PROFIBUS cooperation; new tools for efficient development, testing and maintenance of Device Descriptions; and new technology to facilitate the development of DD-capable host applications.
HART is the global standard for smart process instrumentation communication. With its huge installed base, ease of use, global acceptance, and the support of major process automation suppliers, HART Communication will continue to lead the world market well beyond the next decade—creating an ongoing demand for development of new HART-enabled products and advanced applications.
[/td][/tr][/table][/td][/tr][/table]