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

3.4.8 Carrier Detect Test

The last of the physical layer tests are the carrier detect tests. During these tests, the modem is replaced with the HCF_TOOL-35 physical layer test interface. This tool is a HART modem interface with an adjustable output that can vary the amplitude of the tester HART signals. Figure 3-20 shows the HART carrier detect test setup.

HART Carrier Detect Test Setup Figure 3-20 HART Carrier Detect Test Setup

The carrier detect tests verify what minimum amplitude of transmission is needed to identify the signal as a HART signal. The test also identifies the maximum in-band signal that is not identified as a HART signal. An oscilloscope is used to verify the amplitude of the signal. The test also verifies the time required for a start and stop with a HART minimum amplitude transmission.

For the carrier detect tests, the signal level is dropped from the typical 500mVPP level to 120mVPP. The HART modem communication must still be received by the transmitter. After completion, the signal level is dropped even further to 80mVPP, where the transmitter must reject any commands. Finally, the signal level is raised back to 120mVPP. The device must again receive HART communication from the test system. Each of these tests are performed while sending 100 HART commands to the transmitter.

After these tests are completed the start and stop time are re-tested with the 120mVPP signal to verify transmission. Table 3-8 reports the results for the Carrier Detect tests

Table 3-8 Carrier Detect Test Results
TEST SIGNALING AMPLITUDE MEASUREMENT MAXIMUM RESULT
Successful error-free communications 120mVPP 100 communication attempts 0 errors Pass
Unsuccessful Communications 80mVPP 100 communication attempts 0 successful communications Pass
Successful error-free communications (return) 120mVPP 100 communication attempts 0 errors Pass

This test uses a 250Ω current sense resistor. Carrier start and stop times must be less than 5ms (less than 6 bit times).