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

1.1 The 4-20mA Loop

Understanding the 4-20mA loop is essential to explaining how HART is implemented in a field transmitter. The 4-20mA loop is a standard signaling method often used in factory automation and control to transmit information on a current loop. A sensor at a remote location measures a process variable and sets the current through the loop to indicate the measurement value. Figure 1-1 shows basic diagram of the loop.

Components of the 4-20mA loop Figure 1-1 Components of the 4-20mA loop

The 4-20mA loop has four basic components. The first component is the loop. Each transmitter operates on a separate loop and a receiver measures the current in the loop across a resistor. The loop is simple, durable, and easy to debug. The loop is robust against electrical noise, making the loop reliable for long-distance transmissions. The 4-20mA loop can be extended to 500 meters or longer. The loop is also cost-effective and already commonly exists in many factory automation and control systems.

Second, the loop has a transmitter with a sensor used in process control. This sensor measures a parameter such as pressure, temperature, flow, or any other input for an industrial process or factory floor. The transmitter converts the measurement into the current value in the loop. For example, a transmitter measures the temperature of an oven from between 100°C to 500°C. In the 4-20mA loop, 4mA is used as the minimum value of the measurement and 20mA is used as the maximum value of the measurement. In that case, 4mA translates to 100°C and 20mA translates to 500°C. All measurements are linearly converted to this current value. In some systems, loop currents under 4mA and currents over 20mA can be used to indicate some error from the transmitter. For transmitters compliant to the NAMUR NE 43 standard, currents below 3.6mA or above 21mA are interpreted as a sensor fault. For designers of these systems, noise, resolution, and linearity are all important parameters for transmitter design.

Next, the loop has a power supply capable of sourcing at least 20mA. If NAMUR fault levels are supported by the transmitter, then higher levels of current are needed to indicate an error. The power supply is often a standard voltage of 24V. However, voltages of 36V, 15V, 12V or other voltage can be seen depending on the system. This power supply sends current through the loop and often also powers the transmitter.

Finally, there is a receiver that measures the current on the loop. The receiver measures the voltage across a series resistance. The current is calculated and converted to the primary variable. A load resistance of typically 250Ω is measured with an ADC to convert the primary variable back to the measured result from the transmitter.