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  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 HART Data Frame Structure

HART uses a specific data frame structure for communication. The host sends a command and the remote transmitter device sends a frame with a specific structure as a response. The HART frame has 9 sections of HART bytes that are sent in a specific order.

Table 1-1 shows the different fields in the HART communication packet.

Table 1-1 Fields in the HART Data Frame Structure
Field Name Length (in bytes) Purpose
Preamble 5 - 20 The HART frame starts with a preamble of 5 to 20 bytes. This is the beginning of transmission and uses a consecutive set of 0xFF bytes. This preamble transmission establishes the carrier detect and synchronization for the frame.
Start byte 1 The start byte (also known as the delimiter) is used at the start of the HART data frame to indicate where the packet originated. The start byte can indicate the host number and the message and address format and also indicates if the packet is a response from a field device, or data from a field device in burst mode. The delimiter specifies address type, number of expansion bytes, the physical layer, and the origination of the data frame.
Address 1 or 5 The HART address is 1 or 5 bytes. The address indicates primary or secondary host, or 4-bit polling address from a device. The long frame uses five bytes total comprised of 14 bits used for the expanded device type and three additional bytes for a device ID number. The address also can indicate burst mode operation.
Expansion 0 - 3 Expansion bytes are used for potential expansion. This field is 0 to 3 bytes long and the length is indicated in the start byte. These bytes are reserved for protocol enhancements that can be implemented in the future.
Command 1 The command byte tells the device or host what information is being transferred or operation to perform. The byte indicates numerical value for the command to be executed.
Byte count 1 This byte indicates the number of data bytes in the data frame. This value describes the size of the data field including the status bytes
Status bytes 2 The status bytes are used for device transmissions to the host and contains information on communication errors, status of commands, and device information.
Data field 0 - 253 This section is from 0 to 253 bytes of data transferred between the field transmitter device and the host and vice versa based on the command. The byte count field mentioned previously is indicated by 1 byte, which counts a maximum of 255. The byte count includes both the status and the data bytes. Universal and common-practice commands are up to 33 bytes of data long.
Checksum 1 The HART frame concludes with a checksum. This single byte is the XOR of all bytes from the start byte to last byte of data.

The following sections briefly describe some sections of the HART data frame.