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

1.2.1 Adding HART to the 4-20mA Loop

Starting with the original diagram for the 4-20mA loop in Figure 1-1, HART can be added to the transmitter and receiver. HART communication uses the original 4-20mA loop and adds a two-way digital signal to the loop using HART modems. Again, this backwards compatibility makes HART any easy add-on to existing infrastructure. Figure 1-2 shows how HART is added to the basic 4-20mA loop.

Adding HART to the 4-20mA loop Figure 1-2 Adding HART to the 4-20mA loop

The HART FSK is added to the signal controlling the current in the loop. In the transmitter, the HART modem receives the signal from the loop. The modem capacitively couples the input voltage to receive the HART signal. The modem also modulates the current to send the HART signal. This transmitted signal is superimposed onto the current representing the primary variable as measured by the sensor.

At the same time, the receiver measures the voltage across the resistor to measure the primary variable. In the HART-enabled receiver, a HART modem is used to both transmit commands and receive HART communication from the transmitter. However, the primary variable measurement requires a low pass filter to filter out the HART signal. A resistor is used to measure the current in the loop which represents the primary variable. The minimum resistance for HART communication is 230Ω, and 250Ω is often used.