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

3.4.1 FSK Sinusoid Test

Figure 2-9 shows a block diagram of the HART FSK sinusoid test. Most HART tests require a similar setup with the transmitter as the device under test. The FSK sinusoid test verifies that the HART signal have the correct frequencies and wave shape for transmission.

HART FSK Sinusoid Test Block Diagram Figure 3-2 HART FSK Sinusoid Test Block Diagram

A Keysight E36313A is used as the 24V power supply. The power supply powers the loop and the transmitter sets the current through the loop. A series resistor is used to measure the current through the loop, but also attaches to the HART test system through a modem.

For the HART FSK sinusoid test, an oscilloscope measures the HART transmitted signals. The HART tester sends commands and the oscilloscope plots the responses to find a series of 1s and 0s to measure the HART bits. A frequency counter can be used to measure the frequency of the FSK bits, but an oscilloscope can also be used to verify the timing.

Figure 3-3 and Figure 3-4 define the tolerance of the HART FSK signals. As previously mentioned, the two FSK signals are 1200Hz and 2200Hz. However, there is some acceptable variation in the signal.

HART 1200Hz FSK Waveform Limits Figure 3-3 HART 1200Hz FSK Waveform Limits
HART 2200Hz FSK Waveform Limits Figure 3-4 HART 2200Hz FSK Waveform Limits

These measurements define the FSK frequency variation allowed, the minimum and maximum voltages, and the rise times needed for the HART signal. In addition to the waveform requirements the two FSK frequencies, there is a transition requirement for the waveform shown in Figure 3-5.

HART Sinusoidal Signaling Transition Figure 3-5 HART Sinusoidal Signaling Transition

These diagrams show the tolerance for amplitude of the HART signal, the frequency variation, and the rise time of the signals. To pass the tests, the HART sinusoid must fit within these trapezoidal windows shown in the figures.

The waveforms are taken from the AFE881H1 transmitter test setup using an oscilloscope. Figure 3-6 shows the oscilloscope photos taken for the two HART FSK sinusoids and Table 3-1 shows the test data taken from the measurements.

FSK Sinusoid Test Oscilloscope Results Figure 3-6 FSK Sinusoid Test Oscilloscope Results
Table 3-1 FSK Sinusoid Waveform Test Data
FSK FREQUENCY TEST MEASURED MINIMUM MAXIMUM RESULT
1200Hz Frequency 1202Hz 1188Hz 1212Hz Pass
Amplitude 454mVPP 400mVPP 600mVPP Pass
Waveform rise time 130µs 80µs 200µs Pass
Waveform fall time 130µs 80µs 200µs Pass
2200Hz Frequency 1202Hz 1188Hz 1212Hz Pass
Amplitude 472mVPP 400mVPP 600mVPP Pass
Waveform rise time 80µs 75µs 200µs Pass
Waveform fall time 80µs 75µs 200µs Pass

While a 250Ω resistor is commonly used in 4-20mA loops, a 500Ω current sense resistor is required for this and the other physical layer tests.