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

1.5.3.1 HART Start Byte

After the a transmission of the HART preamble (several bytes of 0xFF), the start byte delimiter is sent. The delimiter describes information within the frame being sent. Figure 1-10 shows the structure of the delimiter.

GUID-20231026-SS0I-JWHR-SJKD-ZN85ZRZJLTSB-low.svg Figure 1-10 HART Start Byte

The MSB of the delimiter indicates the type of address used for the frame. If the bit is zero, the address is the 4-bit polling address. If the bit is one, the address uses the unique device ID to indicate descibe the device. Next, two bits are used for HART expansion. These bits are normally zeros.

Two bits describe the type of physical layer used for the HART transmission. Bits 00b describe an asynchronous FSK transmission, while bits 11b describe a synchronous C8PSK transmission.

The final three bits describe the type of frame and origin of the transmission. Bits 001b indicate that the frame is a transmission of a device in burst mode. Bits 010b indicate the frame is a transmission from a host to a field device. Bits 110b indicate the frame is a transmission from a field device to a host.