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

2.1 The OSI Protocol Model

The HART protocol follows the seven-layer Open Systems Interconnection (OSI) protocol model. The OSI model describes seven layers that computer systems use to communicate over a network. Figure 2-1 shows a comparison between the OSI protocol model and the HART protocol.

GUID-20231026-SS0I-MHBX-TM3G-796GCWKHXQ8T-low.svg Figure 2-1 HART Protocol Compared to the OSI Protocol Model

Of the seven layers of the OSI model, the HART protocol uses three layers to describe network communication.

At the bottom, there is a HART Physical Layer. This is the FSK format for the HART transmission based on the Bell 202 standard. This signal sends data at 1200 baud and is superimposed on the 4-mA to 20-mA analog measurement signal.

Next, the HART Data Link Layer (DLL) defines the communication format and timing rules between the host and the transmitter device. There can be two different hosts on a bus, and in multi-drop mode, there can be as many as 15 devices (HART versions 3 to 5) or 62 devices (HART version 6 or later) connected in parallel.

Finally, there is an Application Layer that has some human or computer interaction that define commands, responses, and data formats that are used in the protocol. As previously mentioned, commands are broken up into several categories for the application layer. These commands are the universal commands, the common-practice commands, and device-specific commands.

Each of the layers use tests to verify different rules in the HART protocol. Testing for HART registration is divided into the three layers, covering the physical layer, the data link layer, and the application layer commands.