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.
Figure 1-1 shows a basic block diagram of a remote transmitter installed on a 4-20mA loop.
The remote transmitter takes measurements with a sensor and translates that current on the 4-20 mA loop. The measurement data can be a variable for industrial control: temperature, pressure, flow, or any other measurement required on a factory floor. This measurement is converted to a primary variable, which has a defined full-scale range. This primary variable is converted into a value that is proportional to the signal in the 4-20 mA loop. For example, 4 mA of loop current represents the zero-scale, and 20 mA of loop current represents the full-scale. If a temperature measurement from an oven has a full range of 0°C to 1000°C, then 4 mA of current in the loop represents 0°C and 20 mA of current in the loop represents 1000°C. Intermediate values are linearly translated to this scale. The value of the primary variable is measured by a receiver connected to a host. Recovering the primary variable involves using an analog-to-digital converter (ADC) to measure a resistor to determine the current in the loop.
As mentioned previously, HART communication uses the 4-20mA loop and adds a digital signal to the loop using a HART modem. The HART communication uses an FSK signal to modulate digital bits in the communication. The modem signaling sends two different frequencies that act as the 0 or 1 in the digital communications. Figure 1-2 shows the addition of HART to the transmitter.
The transmitter incorporates a HART modem with a transmission shown as TX. The modem modulates the current in the loop to transmit the digital signal. The HART signal is added to the current value used to represent the primary variable. The modem also capacitively couples the voltage signal to receive the digital signal. This part of the modem is shown as receiver (RX) inside the remote transmitter.
Another HART enabled receiver connected to the host measures the voltage across the resistor in the loop to determine the primary variable. The host receives the primary variable measurement using a low pass filter to filter out the HART FSK signal. The resistor range for communication is from 230 Ω to 600 Ω, and 250 Ω is the typical resistance used in HART applications. At the same time, the host receives the HART FSK digital signal using a bandpass filter. Both the host and the field transmitter can send and receive data relating to the sensor and the HART enabled transmitter. This HART signal must be band pass filtered to be received by the remote transmitter.