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

1.3 The HART FSK Signal

The HART FSK digital signal is a sinusoid modulated on the 4-20mA loop. Nominally, the HART FSK is a 1-mAPP sinusoid. Figure 1-3 shows a representation of the instantaneous current in the loop with a HART modulated signal. The 4-20mA current represents the primary variable.

GUID-20231026-SS0I-QNZ6-WMNS-DDD1QXFJSBCZ-low.svg Figure 1-3 HART Transmissions on a 4-20 mA Loop

As previously mentioned, bits are represented as two different FSK signals. A 1200-Hz signal is a digital 1 and a 2200-Hz signal is a digital 0. The data is sent at 1200 baud and each bit is 833 µs long.

Note that many transmitters operate off of the loop power. This means that power applied to the loop is also used to power the transmitter. Because there is no other source of power and the loop’s zero scale is 4 mA, the transmitter must operate with a current budget of under this amount of current. In some cases, a current of 3.5 mA can be used as an error signal, so the transmitter must effectively operate with a maximum budget of 3 mA.

The primary variable and the HART signal share the same transmission and each signal must be filtered to be received. Figure 1-4 shows the frequency content of the primary variable and the HART signal.

GUID-20231026-SS0I-JMBJ-XDK4-98JR76HLJ8HX-low.svg Figure 1-4 Filtering for the Primary Variable and HART FSK

In the HART-enabled receiver, the primary variable is read using a low-pass filter to measure the voltage across a resistor. This signal is generally under 20 Hz and the low-pass filter can have a cutoff frequency of 25 Hz. The HART transmission uses higher frequencies, with the FSK bits at 1200 Hz and 2200 Hz. This HART signal is received using a band-pass filter that typically operates from 500 Hz to 10 kHz.