HART is a master-slave communication protocol, which means that during normal operation, each slave (field device) communication is initiated by a master communication device. Two masters can connect to each HART loop. The primary master is generally a distributed control system (DCS), programmable logic controller (PLC), or a personal computer. Introduction HART is a master-slave communication protocol, which means that during normal operation, each slave (field device) communication is initiated by a master communication device. Two masters can connect to each HART loop. The primary master is generally a distributed control system (DCS), programmable logic controller (PLC), or a personal computer (PC). The secondary master can be a.
- Almost all HART host systems today are designed to communicate in poll / response mode, not burst mode. Some non-traditional hosts such as the Rosemount 333 Tri-Loop use burst mode to convert multi-variable information to 4-20mA analog signals. A wired HART Revision 5 or 6 device can be configured to burst all process variables.
- Www.rosemount.com Reference Manual -4530, Rev AA June 2007 Rosemount 5300 Series High Performance Guided Wave Radar.
Sometimes there's a problem getting a loop powered HART instrument to communicate when it's on the benchtop, but it communicated fine when wired in the field. Why is that? It's probably because when it's on the test bench, the loop doesn' t have enough resistance for the HART signal to be ‘seen' by the HART handheld or HART modem.
In a bench test situation, power is applied to the transmitter, so it fires up and runs fine. But there's no analog input in the loop as there is when the transmitter is wired in the field. What's missing is the dropping resistor on the analog input that supplies the loop resistance needed for the HART signal to develop.
4-20mA loops transmit the signal as electrical current (milliamps), but the receiver device with the analog input ‘reads' the signal as a voltage, whether the signal to be read is analog or HART. The current signal becomes a voltage drop when the current passes through a resistor (Ohms Law).
The HART signal is a 1200 baud signal superimposed on the loop's 4-20mA DC signal. 1200 baud is a relatively high frequency signal compared to the 4-20mA DC current signal.
Some minimum loop impedance is needed so that the HART signal can be ‘seen' or ‘read' by a HART master, like a HART communicator or HART modem. The analog inputs on the receiver device (DCS, PLC, RTU, PAC, controller, recorder, or indicator) have a precision shunt resistor installed at each analog input. It is usually 250 ohms, but not always.
The voltage (IR) drop needed for HART communications is produced by the resistance at the loop receiver's analog input. The loop's wire resistance contributes very little resistance. When a transmitter is powered up with just a DC power supply there isn't enough voltage from the 1200 baud HART signal for the for the HART master (handheld communicator or a HART modem) to ‘see' the signal. The internal resistance of a DC power supply is insufficient to develop a HART voltage drop, and its filter capacitors act as a low pass filter to squelch the relatively high frequency 1200 baud HART signal.
In the absence of a receiver device with its analog input resistance being part of the loop, there is insufficient loop resistance for a HART master device to ‘see' the FSK HART signal and HART communications fails.
The good news is that a resistor can be inserted into the loop and the HART signal will develop a voltage drop and the HART master can ‘see' the HART signal.
The HART Foundation's website has a technical specification that states that the minimum loop resistance needed is 230 ohms. A 250 ohm resistor is commonly used because 250 ohm resistors work for that purpose and are typically available in the instrument shops where these types of bench tests are done. The resistor need not be a precision resistor.
Connect a 250 ohm resistor with alligator clips, like this one, to a wiring terminal and a loose wire in a loop circuit to test HART communications at the benchtop.
HART handheld communicators (275/375/475, Meriam 5150) usually have a pair of banana jacks on the handheld for the purpose of connecting a plug adapter with a 249 or 250 ohm resistor to provide the needed loop resistance. If a HART communicator or HART modem fails to establish communications, it is advisable to insert a 250 ohm resistor in series in the loop (or use the banana plug adapter with a communicator).
| HART Feature Summary | HART Protocol Revision | ||
|---|---|---|---|
| Feature | 5 | 6 | 7 |
| PV with Status | m | m | m |
| Device Status | m | m | m |
| Broadcast Messaging | m | m | m |
| Device Configuration | m | m | m |
| 4-20 mA Analog Loop Check | m | m | m |
| Multi-Variable Reads | o | m | m |
| 32 Character Tag | - | m | m |
| All Variables with Status | - | m | m |
| Digital Loop Check | - | m | m |
| Enhanced Multi-Variable Support | - | m | m |
| Local Interface Lock | - | o | o |
| Manual ID of Device by Host | - | m | m |
| Visual ID of Device | - | m | m |
| Peer-to-Peer Messages | - | o | o |
| Report by Exception | - | - | o |
| Synchronized Sampling | - | - | o |
| Time or Condition based Alerts | - | - | o |
| Time Stamp | - | - | m |
| PV Trends | - | - | o |
| Wireless Co-Existence | - | - | o |
| Wireless Diagnostics | - | - | o |
| Wireless Mesh & Star Topologies | - | - | o |
| Wireless Message Routing | - | - | o |
| Wireless Security | - | - | o |
m = Mandatory o = Optional
- Device Variable Classification - Required
Provides master applications with a simple mechanism to determine the number and type of process-related variables (pressure, temperature, etc.) that are available within a device. A key enabler for many HART 6 improvements, this mechanism provides master applications with considerable information about device capabilities. (Commands 0, 8)
- Extended Device Status - Required
One additional byte of well-defined status information with Command 0 and new cyclic data access Command 9. This new Extended Device Status byte is in addition to the current device status byte returned with each command response. Provides additional device status alerts such as 'Device Needs Maintenance' plus more. (Commands 0, 9)
- Device Variable Status - Required
One byte of well-defined status for each Device Variable returned by new cyclic data access Command 9. Enables field devices to self-validate and report on the quality of the data in the command response (good, poor, bad, fixed) plus more. (Command 9)
- Long Tag - Required
This new Long Tag with international (ISO Latin 1) characters allows the 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 as outlined in the Long Tag proposal approved by HCF members in January 1998. (Commands 20, 21, 22)
- Configuration Change Counter – Required
Improved mechanism for master applications to determine that a field device configuration has been changed. Protects the integrity of plant configuration databases. (Command 0)
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- Expanded Manufacturer Codes - Required
Widespread growth and adoption of the HART Protocol has resulted in the available code space for 8-bit Manufacturer ID Codes being exhausted. Resolution requires the expansion of Manufacturer ID Codes from 8-bits to 16-bits. Previous addendums to the Data Link Layer Specification, Command Summary Specification and Universal Command Specification that are proven and already in use have been incorporated into the HART 7 revision of the HART Protocol Specifications.
For existing 8-bit manufacturer ID's:
8-bit manufacturer IDs will be padded with 0 for the MSB. The expanded device type will be the 8-bit manufacturer ID and the device type assigned according to the rules of HCF-99.
(Command 0, Command 11, Command 21)
- Private Label Code and Device Profile
Command 0 now contains the private label code and the device profile in addition to the other changes outlined above. Command 15 now returns value 250 in place of the private label code.
The Private Label code in command 0 will be set to the manufacturer ID if the manufacturer and distributor are the same.
The Device Profile uses enumerations from the Common Tables.
(Command 0, Command 15)
- Time Stamped Data - Required
New Data Type (Time) for Time Stamping of Device Variable Values in Universal Command 9 response. This new capability enables the Time Stamping of process variable values at the Field Device and their reporting to host applications via Command 9 with the Time Stamp or time interval from the previous value. Also, Common Practice Commands to set the device time of day have been added.
The time stamp does not have to be a real time clock. The time stamp could be a monotonic counter that resets every 24 hours.
(Command 9)
- Expanded number of device variables in Command 9 – Required
Command 9 previously only allowed up to 4 Device Variables to be accessed in a single read. Command 9 can now include up to 8 device variables. This provides the opportunity to access more data with a single transaction. (Command 9)
- Support of 6 device variables – Required
Mandatory Device Variables are now defined:
244 Percent Range
245 Loop Current
246 Primary Variable
247 Secondary Variable
248 Tertiary Variable
249 Quaternary Variable
The 6 device variables listed above must be implemented in every HART 7 devices. These device variables are not reported in the Command 0 response. Please note that if the device does not support device variables, the command 0 response will indicate 0 for the last device variable code supported.
The device is not required to implement more than these 6 device variables. If the device does not implement device variables other the ones listed above, the device will also need to support PV for device variable 0, SV for device variable 1, TV for device variable 2, and QV for device variable 3.
When a Device Variable requested (or dynamic variable) is not supported in the Field Device, then the following must be reported for the listed information:
Value must be set to '0x7F, 0xA0, 0x00, 0x00'
Status must be set to 0x30, (i.e., Status = 'Bad' and Limit = 'Constant')
Units Code must be set to '250' Not Used
Device Variable Classification set to '0', Not Yet Classified.
There is a device variable required for wireless devices in addition to the ones already listed:
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- Command 38 behavior modified – Required
Command 38 now returns the configuration change counter. Upon receiving this command the device now compares the Configuration Change Counter received in this command request with the device's current value. If they do not match then the device must return 'Configuration Change Counter Mismatch' and not reset the Configuration Changed bit.
Field devices receiving Command 38 with no data bytes must assume the Master is HART Revision 6 (or earlier) and reset the appropriate configuration changed bit. (Command 38)
- Upgrade of Command 38 & 48 to Mandatory - Required
These Common Practice Commands for Device Specific Status and reset of the Configuration Change Flag are upgraded to 'Universal' status with HART 7. (Command 38 and Command 48)
Command 48 can reset the more status available bit if the device has had no other changes since the last report.
- Device Families – Optional
Establishes standard commands and status indicators for devices based on the type of process connection. Standard commands allow simple master applications to more fully communicate and configure devices without having to rely on the complexity of DDL. Initial Device Families for Temperature and PID Control are included in the specification package. The Valve Positioner / Actuator Device Family is in final preparation.
- Transducer Trim Commands – Optional
New Common Practice commands for performing transducer trim (calibration) operations. Reduces need for device specific commands to perform this common function. Standard commands make it easier for master applications such as field instrument calibrators to perform calibration functions on all HART devices.
- Delayed Response – Optional
Provides master applications with a simple mechanism to determine the number and type of process-related variables (pressure, temperature, etc.) that are available within a device. A key enabler for many HART 6 improvements, this mechanism provides master applications with considerable information about device capabilities. (Commands 0, 8)
- Extended Device Status - Required
One additional byte of well-defined status information with Command 0 and new cyclic data access Command 9. This new Extended Device Status byte is in addition to the current device status byte returned with each command response. Provides additional device status alerts such as 'Device Needs Maintenance' plus more. (Commands 0, 9)
- Device Variable Status - Required
One byte of well-defined status for each Device Variable returned by new cyclic data access Command 9. Enables field devices to self-validate and report on the quality of the data in the command response (good, poor, bad, fixed) plus more. (Command 9)
- Long Tag - Required
This new Long Tag with international (ISO Latin 1) characters allows the 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 as outlined in the Long Tag proposal approved by HCF members in January 1998. (Commands 20, 21, 22)
- Configuration Change Counter – Required
Improved mechanism for master applications to determine that a field device configuration has been changed. Protects the integrity of plant configuration databases. (Command 0)
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- Expanded Manufacturer Codes - Required
Widespread growth and adoption of the HART Protocol has resulted in the available code space for 8-bit Manufacturer ID Codes being exhausted. Resolution requires the expansion of Manufacturer ID Codes from 8-bits to 16-bits. Previous addendums to the Data Link Layer Specification, Command Summary Specification and Universal Command Specification that are proven and already in use have been incorporated into the HART 7 revision of the HART Protocol Specifications.
For existing 8-bit manufacturer ID's:
8-bit manufacturer IDs will be padded with 0 for the MSB. The expanded device type will be the 8-bit manufacturer ID and the device type assigned according to the rules of HCF-99.
(Command 0, Command 11, Command 21)
- Private Label Code and Device Profile
Command 0 now contains the private label code and the device profile in addition to the other changes outlined above. Command 15 now returns value 250 in place of the private label code.
The Private Label code in command 0 will be set to the manufacturer ID if the manufacturer and distributor are the same.
The Device Profile uses enumerations from the Common Tables.
(Command 0, Command 15)
- Time Stamped Data - Required
New Data Type (Time) for Time Stamping of Device Variable Values in Universal Command 9 response. This new capability enables the Time Stamping of process variable values at the Field Device and their reporting to host applications via Command 9 with the Time Stamp or time interval from the previous value. Also, Common Practice Commands to set the device time of day have been added.
The time stamp does not have to be a real time clock. The time stamp could be a monotonic counter that resets every 24 hours.
(Command 9)
- Expanded number of device variables in Command 9 – Required
Command 9 previously only allowed up to 4 Device Variables to be accessed in a single read. Command 9 can now include up to 8 device variables. This provides the opportunity to access more data with a single transaction. (Command 9)
- Support of 6 device variables – Required
Mandatory Device Variables are now defined:
244 Percent Range
245 Loop Current
246 Primary Variable
247 Secondary Variable
248 Tertiary Variable
249 Quaternary Variable
The 6 device variables listed above must be implemented in every HART 7 devices. These device variables are not reported in the Command 0 response. Please note that if the device does not support device variables, the command 0 response will indicate 0 for the last device variable code supported.
The device is not required to implement more than these 6 device variables. If the device does not implement device variables other the ones listed above, the device will also need to support PV for device variable 0, SV for device variable 1, TV for device variable 2, and QV for device variable 3.
When a Device Variable requested (or dynamic variable) is not supported in the Field Device, then the following must be reported for the listed information:
Value must be set to '0x7F, 0xA0, 0x00, 0x00'
Status must be set to 0x30, (i.e., Status = 'Bad' and Limit = 'Constant')
Units Code must be set to '250' Not Used
Device Variable Classification set to '0', Not Yet Classified.
There is a device variable required for wireless devices in addition to the ones already listed:
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- Command 38 behavior modified – Required
Command 38 now returns the configuration change counter. Upon receiving this command the device now compares the Configuration Change Counter received in this command request with the device's current value. If they do not match then the device must return 'Configuration Change Counter Mismatch' and not reset the Configuration Changed bit.
Field devices receiving Command 38 with no data bytes must assume the Master is HART Revision 6 (or earlier) and reset the appropriate configuration changed bit. (Command 38)
- Upgrade of Command 38 & 48 to Mandatory - Required
These Common Practice Commands for Device Specific Status and reset of the Configuration Change Flag are upgraded to 'Universal' status with HART 7. (Command 38 and Command 48)
Command 48 can reset the more status available bit if the device has had no other changes since the last report.
- Device Families – Optional
Establishes standard commands and status indicators for devices based on the type of process connection. Standard commands allow simple master applications to more fully communicate and configure devices without having to rely on the complexity of DDL. Initial Device Families for Temperature and PID Control are included in the specification package. The Valve Positioner / Actuator Device Family is in final preparation.
- Transducer Trim Commands – Optional
New Common Practice commands for performing transducer trim (calibration) operations. Reduces need for device specific commands to perform this common function. Standard commands make it easier for master applications such as field instrument calibrators to perform calibration functions on all HART devices.
- Delayed Response – Optional
The Delayed Resposne Mechanism(DRM) enables the slave to indicate to a master that it received the request but is not able to formulate a reply in the time allowed by the Data Link Layer. This mechanism provides an informative and flexible convention for slave devices needing additional time to perform relatively infrequent operations like self diagnostics, calibration, or configuration. Unlike the Response Code BUSY, the master knows that the slave has understood the command is still communicating. This is optional unless the HART 5 product used busy. If the HART 5 device used busy, then DRM is mandatory (No commands necessary)
- 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.
- Block Data Transfer – Optional
An updated mechanism to support the movement of large blocks of data between masters and field devices. Potential uses include the movement of diagnostic information stored in the field device and upload/download of device configurations.
- Catch Device Variable – Optional
A simple mechanism using Common Practice Commands 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.
- Write Device Variable – Optional
New Common Practice Commands to support forcing the digital value for any Device Variable to a specific value to aid in commissioning and troubleshooting.
- Lock Device – Optional
New Common Practice Commands allow a master application to lock the 'local' front panel of a field device while performing remote configuration functions.
- Squawk and Find Device – Optional
New Common Practice commands to support commissioning and troubleshooting of HART devices in multi-drop and multi-pair cable installations.
- Wireless – Optional
A major new enhancement to the HART technology providing additional capabilities to monitor the performance of plant assets in areas that were uneconomical or technically difficult with wired systems. This new enabling technology provides reliable, robust and secure wireless communication using standard IEEE 802.15.4 radios at 2.4 GHz for global application with Channel Hopping, Mesh Networking and robust Security appropriate for process applications.
- Enhanced Sub Device & I/O System Commands – Optional
New Common Practice Commands have been defined to support the set-up and operation of Wireless Adapters and I/O Systems. (Command 101, 102, 77, 84, 85, 86, 87, 88, 94)
- Enhanced Burst Mode including events, trends, and burst data – Optional
Burst mode communication has been enhanced to allow the bursting of more than one HART message and the ability to specify time interval or frequency for how often the message is to be sent (burst frequency). A form of event reporting is also defined enabling devices to automatically send message noting a change in Command 48 status or a change to the Configuration Change Counter. (Command 103, 104, 115, 116, 117, 118, 119)
- Flush Delayed Response – Optional
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If the product implements the Delayed Response Mechanism(DRM), the device must allow the master to flush the DRM buffers. The master may request the device to clear all pending delayed responses for the Master that issues the command. Delayed responses currently running that must not be interrupted or aborted may be completed. (Command 106)
- Read Trend Command – Optional
New Common Practice Command to read multiple values (or the trend) for a specified Device Variable. (Command 91, 92, 93)
- Aggregated Read Command – Optional
New Common Practice Command to support expedited upload of Device Configuration enabling more than one command response to be read at a time. (Command 78)
- Synchronized Actions – Optional
Synchronous Actions are used to defer a device activity or action to a specified, future time. The Action could be to simply synchronously sample a single measurement (i.e., a Device Variable), triggering a (e.g., vibration) waveform acquisition, an automatic calibration cycle, or some device specific procedure. In addition, this allows measurements or other operations performed by multiple devices to be synchronized. (Command 89, 90, 96, 97, 98, 99)
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- Common Tables Additions – Optional
New Common Tables Additions to support Wireless and all other enhanced functionality.
- Discrete Applications – Optional
This new Specification establishes the framework for defining the application of HART Communication to devices for point detection or discrete on/off type functionality.
