The Highway Addressable Remote Transducer (HART®) protocol is used in many factory automation and control systems. This protocol uses the established 4-20mA loop to send digital signals between a smart transmitter and a host for data that can be used for control, monitoring, or safety. This application note starts with an overview of 4-20mA systems and HART transmitters and discusses the protocols for developing HART enabled devices. This application note also discusses different TI devices with HART connectivity.
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The Highway Addressable Remote Transducer (HART) protocol is a backward-compatible enhancement to 4-20 mA instrumentation that allows two-way communication with smart, microprocessor-based field devices.
For factory automation and control (FA&C) applications, 4-20 mA current loop communications are a common method to send data from a remote transmitter measuring a factory variable back to a host that receives and processes the data. The value of current is the primary variable sent by the remote transmitter. This primary variable represents some measure of temperature, flow, or pressure.
The HART signal is superimposed onto this 4-20 mA current loop and is modulated for two-way digital communications. The 4-20 mA loop is already a widely used standard in FA&C applications. Because this technology is backwards compatible and can be used with already-existing infrastructure means that HART is easy to adopt and cost effective.
HART is a command/response protocol where a host sends commands and a remote transmitter returns standardized responses. The data received by the commands can communicate device status and diagnostics. Data can also include the device measurement floating-point digital values, the engineering units of the primary variable, and other information about the remote transmitter.
The HART protocol can also be used for standardized operating procedures such as testing the current loop, range setting of the current loop, and calibration information from the transmitter. HART modifies the 4-20 mA system that sends only the primary variable as a current value and adds digital communication with more functionality.
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.