SLAAEH0 November   2023 AFE781H1 , AFE782H1 , AFE881H1 , AFE882H1 , DAC161P997 , DAC161S997 , DAC7750 , DAC7760 , DAC8740H , DAC8741H , DAC8742H , DAC8750 , DAC8760 , DAC8771 , DAC8775

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction to the HART Protocol
    1. 1.1 Different Forms of the HART Protocol
    2. 1.2 HART as an Enhancement to the 4-20 mA Loop
    3. 1.3 The HART FSK Signal
    4. 1.4 HART Configurations
    5. 1.5 HART Protocol Structure
      1. 1.5.1 HART Communication
      2. 1.5.2 HART Bytes
      3. 1.5.3 HART Data Frame Structure
        1. 1.5.3.1 HART Start Byte
        2. 1.5.3.2 HART Device Addressing
        3. 1.5.3.3 HART Commands
  5. 2HART Protocol and Test Specifications
    1. 2.1 The OSI Protocol Model
    2. 2.2 HART Protocol Specifications
    3. 2.3 HART Test Specifications
  6. 3TI HART Enabled Devices
    1. 3.1 TI DACs with HART Connections
    2. 3.2 TI HART Modems
  7. 4Conclusion
  8. 5References

1.2 HART as an Enhancement to the 4-20 mA Loop

Figure 1-1 shows a basic block diagram of a remote transmitter installed on a 4-20mA loop.

GUID-20231026-SS0I-ZB7F-P8ZV-BFC62GH26P5T-low.svg Figure 1-1 A Remote Transmitter on a 4-20 mA Loop

The remote transmitter takes measurements with a sensor and translates that current on the 4-20 mA loop. The measurement data can be a variable for industrial control: temperature, pressure, flow, or any other measurement required on a factory floor. This measurement is converted to a primary variable, which has a defined full-scale range. This primary variable is converted into a value that is proportional to the signal in the 4-20 mA loop. For example, 4 mA of loop current represents the zero-scale, and 20 mA of loop current represents the full-scale. If a temperature measurement from an oven has a full range of 0°C to 1000°C, then 4 mA of current in the loop represents 0°C and 20 mA of current in the loop represents 1000°C. Intermediate values are linearly translated to this scale. The value of the primary variable is measured by a receiver connected to a host. Recovering the primary variable involves using an analog-to-digital converter (ADC) to measure a resistor to determine the current in the loop.

As mentioned previously, HART communication uses the 4-20mA loop and adds a digital signal to the loop using a HART modem. The HART communication uses an FSK signal to modulate digital bits in the communication. The modem signaling sends two different frequencies that act as the 0 or 1 in the digital communications. Figure 1-2 shows the addition of HART to the transmitter.

GUID-20231026-SS0I-3R9S-D1B3-F2KG4VRSGG55-low.svg Figure 1-2 Adding HART to the 4-20 mA Loop

The transmitter incorporates a HART modem with a transmission shown as TX. The modem modulates the current in the loop to transmit the digital signal. The HART signal is added to the current value used to represent the primary variable. The modem also capacitively couples the voltage signal to receive the digital signal. This part of the modem is shown as receiver (RX) inside the remote transmitter.

Another HART enabled receiver connected to the host measures the voltage across the resistor in the loop to determine the primary variable. The host receives the primary variable measurement using a low pass filter to filter out the HART FSK signal. The resistor range for communication is from 230 Ω to 600 Ω, and 250 Ω is the typical resistance used in HART applications. At the same time, the host receives the HART FSK digital signal using a bandpass filter. Both the host and the field transmitter can send and receive data relating to the sensor and the HART enabled transmitter. This HART signal must be band pass filtered to be received by the remote transmitter.