SLAAEH8 October   2024 AFE781H1 , AFE782H1 , AFE881H1 , AFE882H1 , DAC8740H , DAC8741H , DAC8742H

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 The 4-20mA Loop
    2. 1.2 The HART Protocol
      1. 1.2.1 Adding HART to the 4-20mA Loop
      2. 1.2.2 HART FSK
  5. 2AFE881H1 HART Modem
    1. 2.1 AFE881H1 HART Transmitter
    2. 2.2 Detailed Schematic
      1. 2.2.1 Input Protection
      2. 2.2.2 Startup Circuit
      3. 2.2.3 Voltage-to-Current Stage
      4. 2.2.4 Voltage-to-Current Calculation
      5. 2.2.5 HART Signal Transmission
      6. 2.2.6 HART Input Protection
      7. 2.2.7 Current Consumption
      8. 2.2.8 HART Transmitter Board
      9. 2.2.9 HART Protocol Stack
  6. 3HART Testing and Registration
    1. 3.1  HART History and the FieldComm Group
    2. 3.2  HART Testing Overview
      1. 3.2.1 HART Protocol Specifications
      2. 3.2.2 HART Protocol Test Specifications
      3. 3.2.3 Remote Transmitter Device Testing
    3. 3.3  HART Test Equipment
    4. 3.4  HART Physical Layer Testing
      1. 3.4.1 FSK Sinusoid Test
      2. 3.4.2 Carrier Start and Stop Time Tests
      3. 3.4.3 Carrier Start and Stop Transient Tests
      4. 3.4.4 Output Noise During Silence
      5. 3.4.5 Analog Rate of Change Test
      6. 3.4.6 Receive Impedance Test
      7. 3.4.7 Noise Sensitivity Test
      8. 3.4.8 Carrier Detect Test
    5. 3.5  Data Link Layer Tests
      1. 3.5.1 Data Link Layer Test Specifications
      2. 3.5.2 Data Link Layer Test Logs
    6. 3.6  Universal Command Tests
    7. 3.7  Common-Practice Command Tests
    8. 3.8  Device Specific Command Tests
    9. 3.9  HART Protocol Test Submission
    10. 3.10 HART Registration
  7. 4Other TI HART Modem Designs
  8. 5Summary
  9. 6Acknowledgments
  10. 7References

3.3 HART Test Equipment

Below are some of the equipment needed to run HART testing for your field transmitter.

  • A test load resistor used in the loop. This is typically 500Ω depending on the particular test.
  • An oscilloscope is required to observe the HART signals. The physical layer tests require the FSK to have a certain waveshape and magnitude, that have specific rise and fall times.
  • A frequency counter is needed to observe the frequency of the FSK bits. In these tests, an oscilloscope is used to verify the frequency.
  • A DC power source of about 40V maximum powers the loop.
  • A signal generator is used for generate interference noise of different frequencies and amplitude to verify HART communications in presence of noise.

There are also a few specialized pieces of equipment required to run the HART physical layer tests. These devices are shown in Figure 3-1.

HART Test System Shown With an HCF_TOOL-31, HCF_TOOL-32, and an FSK Physical Test Interface Figure 3-1 HART Test System Shown With an HCF_TOOL-31, HCF_TOOL-32, and an FSK Physical Test Interface

A HART test system is required for the HART protocol testing. While this system is mostly required for the data link layer and command tests, the test system can be used to generate the HART responses from the transmitter to view the specific FSK bits and responses. The HART test system is Linux® based, and runs tests with a set of commands designed by FieldComm. The HART test system requires two HART registered RS-232 modems.

The HCF_TOOL-31 filter is needed for the physical layer tests. This filter is a passband Butterworth filter that passes the HART FSK digital signal. The filter has a 2nd order roll-off below 500Hz, 1st order roll-off above 10kHz, and a passband gain of 10. This filter passes the HART FSK signal but removes the lower frequencies of the primary variable and any noise from higher frequencies.

The HCF_TOOL-32 filter is also required. This filter is a low-pass Butterworth filter with a 2nd order roll-off above 25Hz and a passband gain of 10. This low-pass filter passes the analog primary variable from a field transmitter and rejects the HART FSK signal and higher frequencies.

Finally, the FSK Physical Layer Test interface is used to increase and decrease the HART sinusoids from the HART test system in the transmission. This device is used to determine the sensitivity of the field transmitter for detecting the HART signal.

The HART test system, the HCF_TOOL-31 filter, the HCF_TOOL-32 filter, and the FSK Physical Layer Test interface can be purchased from FieldComm.