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

1.1 Different Forms of the HART Protocol

The standard HART transmission is a frequency shift keyed (FSK) signal superimposed on the 4-20mA signal. The FSK bits are transmitted at 1200 bits per second (bps). FSK indicates that the bit signals are represented as two different frequency transmissions. Each frequency represents a digital 1 or digital 0.

An alternative to the standard FSK signal is a coherent 8-way phase-shift keyed (C8PSK) signal. This version of HART increases the digital transmission rate. This version uses a 3200-Hz carrier that has 8 different phases for information. Eight phases at 3200 Hz gives an effective bit transmission rate of 9600 bps. Similar to the standard FSK, C8PSK is compatible with the analog signaling on the 4-20 mA loop.

Additionally, HART protocol can be used in other signaling systems. Some manufacturers have developed devices using HART-based communication packets sent across RS-485.

WirelessHART® uses a 2.4 GHz time division multiple access (TDMA) communication built on the IEEE 802.15.4 standard to integrate wireless communication. This protocol was designed to work together with the existing HART protocol and HART devices. WirelessHART is basically a wireless mesh network used to connect field devices to a WirelessHART gateway.

HART-IP® uses internet protocol (IP) to send HART communication over Ethernet. HART-IP connects to individual devices and also wireless gateways to WirelessHART devices. HART-IP is basically HART using IP addressing. Ethernet connections use Ethernet-APL, which is a two-wire, loop-powered Ethernet physical layer used for rugged and hazardous conditions in process plants.

At the publication date of this application note, TI's existing HART capable devices feature the standard FSK HART. For this reason, only the HART FSK is discussed in this application note.