Rerange Procedures
These Common Practice Commands support two methods for setting the Primary Variable URV and LRV.
• The first technique uses Command 35, Write Primary Variable Range Values, to set the URV and LRV. The engineering units need not be the same as the Primary Variable units, and this command does not change the Primary Variable Units.
• The second technique re-ranges the Field Device based on process conditions:
1. Adjust the process until the Primary Variable matches the desired LRV (the zero). Use Command 37, Set Primary Variable Lower Range Values, to set the LRV.
2. Adjust the process until the Primary Variable matches the desired URV (the span). Use Command 36, Set Primary Variable Upper Range Value, to set the URV.
Loop Current Trim Procedure
Procedure for Transmitters
In this procedure, the transmitter controls the Loop Current generally using a DAC. A suitable reference, like a digital multi-meter, is used to calibrate the transmitter’s output. The Master’s Loop Current measurement could be used as the reference.
1. Use Command 40, Enter/Exit Fixed Current Mode, to set the current to the device’s minimum value. 4.00mA is usually used as the zero trim point.
2. Using the reference instrument’s measured value, set the zero trim of the device using Command 45, Trim Loop Current Zero. The device trims its calibration and returns its Loop Current value. This may be slightly different from the value the Master sent the device due to rounding or truncation.
3. Use Command 40, Enter/Exit Fixed Current Mode, to set the current to the device’s maximum value. 20.00mA is normally used as the span trim point.
4. Using the value measured by the reference, trim the span of the device with Command 46, Trim Loop Current Gain.
5. Repeat steps 1-4 as needed to gain the accuracy desired. Once the Loop Current is calibrated, return the device to normal operation by issuing Command 40, Enter/Exit Fixed Current Mode with a value of 0.0. This takes the device out of fixed current mode.
Procedure for Actuators
In this procedure, the Master or a reference controls the Loop Current and the actuator measures that current. A suitable reference, like a digital multi-meter, is used to calibrate the actuator input. Alternatively, the Master’s Loop Current value could be used as the reference.
1. Using the appropriate Loop Current source, set the current to the device’s minimum value, usually 4.00mA as the zero trim point.
2. Using the reference instrument’s measurement value, set the zero trim of the device using Command 45, Trim Loop Current Zero. The device trims its calibration and returns its Loop Current value. This may be slightly different from the value the Master sent the device due to rounding or truncation.
3. Using the appropriate Loop Current source, set the current to the device’s maximum value, normally 20.00mA is used as the span trim point.
Performing Self Test
Occasionally an operator may want to perform a self test on a device to confirm the devices integrity. The procedure is:
1. The Host sends Command 41, Perform Self Test, to initiate the self test. The Slave must answer within the Slave Time Out.
2. The Slave must answer Command 41, Perform Self Test, and begin its self test. Self test may take a relatively long time to complete. As a result, during the self test, the Slave may not answer any HART commands. The Master must not disconnect from the device as the result of issuing Command 41.
3. Once the Master has confirmed the completion of the self test, the Master should send a Command 48, Read Additional Device Status, to return diagnostics information generated by the Self Test.
A Master must be prepared for the device’s reaction to this command. The Field Device may not respond during the Self Test. This may look like communications with the Field Device was lost. In addition, the Loop Current may not reflect the process while the Self test is executing. A master must not generate spurious error messages or disconnect from the Field Device while the Self Test is in progress.
Locking The Device To Allow Exclusive Access
In some cases, technicians have been using the local panel on a Field Device simultaneous to a HART Master configuring the Field Devices. When this happens, the HART Master cannot guarantee the accuracy of the data items presented to the user. This command allows a HART Master to have exclusive access while configuring or calibrating a Field Device. The normal use of Command 71, Lock Device is:
1. Issue Command 71 to ensure exclusive access during configuration.
2. Configure the device as needed. While locked the device returns Response Code 16, Access Restricted, to any write commands from the other Master.
3. Issue Command 38, Reset Configuration Changed Flag. This will allow the Master to easily determine if the device configuration is ever changed by monitoring the Device Status Byte.
4. Issue Command 71 to restore access to the other Master and the device’s front panel.
Transducer Trim Procedure
Adjustment of a Device Variable reading is one of the most common functions that instruments must support. The commands in this section constitute a trim procedure that is applicable to a variety of instruments, both transmitters and actuators.
1. Issue Command 81, Read Device Variable Trim Guidelines, to determine the number of applicable trim points and their acceptable limits.
2. Issue Command 80, Read Device Variable Trim Points, to retrieve the last trim points used. These should be used as default values for a new trim operation. If the value supports a two point trim, then perform the low trim first (step 3-6).
3. Prompt the user to set the variable input to a value within acceptable limits for the trim point.
4. Once set and stable, obtain the exact process variable value from either a calibrator or the user.
5. Issue Command 82, Write Device Variable Trim Point. Inform user of any errors.
6. If a DR_INITIATE response is received, then resend the identical trim command until the operations is completed. Once completed, inform the user of the results.
7. If the variable supports a two-point trim and the low trim is completed successfully, repeat steps 3 through 6 for the upper trim point
Configuring A Device For Burst Mode Operation
The procedure a Master should follow to place a Field Device into burst mode is as follows:
1. If Command 9 is the command selected to burst, then use Command 107, Write Burst Device Variables, to assign Device Variables to the four response data slots.
2. The command response is configured using Command 108, Write Burst Mode Command Number. Commands 1, 2, 3, and 9 are always available and Command 33 is normally available as well.
3. Issuing Command 109, Burst Mode Control, will enter or exit Burst Mode. While in Burst Mode, the Slave will begin transmitting the responses to the command number set by Command 108.
4. Once the device is in burst mode, Command 108 is used to change the burst command response. A device may take one burst response before the response changes to the new command number.
Using Analog Trim Commands
Some devices support more Analog Channels then just the Loop Current. The Analog Channel may be an input to or output from the Field Device. For these devices, the Analog Trim Commands allow Master to calibrate individual Analog Channels. The trim procedure is similar to the one used in Section 6.3.1.
1. Use Command 66, Enter/Exit Fixed Analog Output Mode, to set the analog output to the lower endpoint value.
2. Command 67, Trim Analog Output Zero, can then be used to send the Zero value. 3. Use Command 66 to set the analog output to the upper end point value.
4. Command 68, Trim Analog Output Gain, can then be used to send the gain value.
5. Exit Fixed Output Mode by resending Command 66 with the analog output value of “0x7F, 0xA0, 0x00, 0x00” and any Units Code.