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The HART Digital/Analog Hybrid Standard

A technological advance introduced in the late 1980’s was HART, an acronym standing for Highway Addressable Remote Transmitter. The purpose of the HART standard was to create a way for instruments to digitally communicate with one another over the same two wires used to convey a 4-20 mA analog instrument signal. In other words, HART is a hybrid communication standard, with one variable (channel) of information communicated by the analog value of a 4-20 mA DC signal, and another channel for digital communication whereby many other variables could be communicated using pulses of current to represent binary bit values of 0 and 1. Those digital current pulses are superimposed upon the analog DC current signal, such that the same two wires carry both analog and digital data simultaneously.

The HART standard was developed with existing installations in mind. The medium for digital communication had to be robust enough to travel over twisted-pair cables of very long length and unknown characteristic impedance. This meant that the data communication rate for the digital data had to be very slow, even by 1980’s standards. The HART standard is concerned only with layers 1 (FSK modulation, ±0.5 mA signaling), 2 (Master-slave arbitration, data frame organization), and 7 (specific commands to read and write device data) of the OSI Reference model. Layers 3 through 6 are irrelevant to the HART standard.

Digital data is encoded in HART using the Bell 202 modem standard: two audio-frequency “tones” (1200 Hz and 2200 Hz) are used to represent the binary states of “1” and “0,” respectively, transmitted at a rate of 1200 bits per second. This is known as frequency-shift keying, or FSK. The physical representation of these two frequencies is an AC current of 1 mA peak-to-peak superimposed on the 4-20 mA DC signal. Thus, when a HART-compatible device “talks” digitally on a two-wire loop circuit, it produces tone bursts of AC current at 1.2 kHz and 2.2kHz. The receiving HART device “listens” for these AC current frequencies and interprets them as binary bits.

An important consideration in HART current loops is that the total loop resistance (precision resistor values plus wire resistance) must fall within a certain range: 250 ohms to 1100 ohms. Most 4-20 mA loops (containing a single 250 ohm resistor for converting 4-20 mA to 1-5 V) measure in at just over 250 ohms total resistance, and work quite well with HART. Even loops containing two 250 ohm precision resistors meet this requirement. Where technicians often encounter problems is when they set up a loop-powered HART transmitter on the test bench with a lab-style power supply and no 250 ohm resistor anywhere in the circuit:


Connecting a HART transmitter without a 250 ohm resistor

The HART transmitter may be modeled as two parallel current sources: one DC and one AC. The DC current source provides the 4-20 mA regulation necessary to represent the process measurement as an analog current value. The AC current source turns on and off as necessary to “inject” the 1 mA P-P audio-frequency HART signal along the two wires. Inside the transmitter is also a HART modem for interpreting AC voltage tones as HART data packets. Thus, data transmission takes place through the AC current source, and data reception takes place through a voltage-sensitive modem, all inside the transmitter, all “talking” along the same two wires that carry the DC 4-20 mA signal.

For ease of connection in the field, HART devices are designed to be connected in parallel with each other. This eliminates the need to break the loop and interrupt the DC current signal every time we wish to connect a HART communicator device to communicate with the transmitter. A typical HART communicator may be modeled as an AC voltage source1 (along with another HART voltage-sensitive modem for receiving HART data). Connected in parallel with the HART transmitter, the complete circuit looks something like this:

 

HART transmitter connected in parallel with a HART Communicator


The actual hand-held communicator may look like one of these devices:

Handheld HART Communicators

 

With all these sources in the same circuit, it is advisable to use the Superposition Theorem for analysis. This involves “turning off” all but one source at a time to see what the effect is for each source, then superimposing the results to see what all the sources do when all are working simultaneously.

We really only need to consider the effects of either AC source to see what the problem is in this circuit with no loop resistance. Consider the situation where the transmitter is sending HART data to the communicator. The AC current source inside the transmitter will be active, injecting its 1 mA P-P audio-frequency signal onto the two wires of the circuit. The AC voltage source in the communicator will disconnect itself from the network, allowing the communicator to “listen” to the transmitter’s data.

To apply the Superposition Theorem, we replace all the other sources with their own equivalent internal resistances (voltage sources become “shorts,” and current sources become “opens”):

 

Applying the Superposition theorem in HART Loop

 

The HART communicator is “listening” for those audio tone signals sent by the transmitter’s AC source, but it “hears” nothing because the DC power supply’s equivalent short-circuit prevents any significant AC voltage from developing across the two wires. This is what happens when there is no loop resistance: no HART device is able to receive data sent by any other HART device. The solution to this dilemma is to install a resistance of at least 250 ohms but not greater than 1100 ohms between the DC power source and all other HART devices, like this:

 

Putting Loop Resistance to enable one HART device to listen to another HART device

 

Loop resistance must be at least 250 ohms to allow the 1 mA P-P AC signal to develop enough voltage to be reliably detected by the HART modem in the listening device. The upper limit (1100 ohms) is not a function of HART communication so much as it is a function of the DC voltage drop, and the need to maintain a minimum DC terminal voltage at the transmitter for its own operation. If there is too much loop resistance, the transmitter will become “starved” of voltage and act erratically. In fact, even 1100 ohms of loop resistance may be too much if the DC power supply voltage is insufficient.

Loop resistance is also necessary for the HART transmitter to receive data signals transmitted by the HART communicator. If we analyze the circuit when the HART communicator’s voltage source is active, we get this result:

 

Loop Resistance is Necessary for the HART transmitter to receive data transmitted by the HART Communicator

 

Without the loop resistance in place, the DC power supply would “short out” the communicator’s AC voltage signal just as effectively as it shorted out the transmitter’s AC current signal. The presence of a loop resistor in the circuit prevents the DC power supply from “loading” the AC voltage signal by the communicator. This AC voltage is seen in the diagram as being directly in parallel with the transmitter, where its internal HART modem receives the audio tones and processes the data packets.

Manufacturers’ instructions generally recommend HART communicator devices be connected directly in parallel with the HART field instrument, as shown in the previous schematic diagrams. However, it is also perfectly valid to connect the communicator device directly in parallel with the loop resistor like this:

 

Connecting the Communicator Device directly in parallel with the loop resistor

 

Connected directly in parallel with the loop resistor, the communicator is able to receive transmissions from the HART transmitter just fine, as the DC power source acts as a dead short to the AC current HART signal and passes it through to the transmitter.

This is nice to know, as it is often easier to achieve an alligator-clip connection across the leads of a resistor than it is to clip in parallel with the loop wires when at a terminal strip or at the controller end of the loop circuit.

HART technology has given a new lease on the venerable 4-20 mA analog instrumentation signal standard. It has allowed new features and capabilities to be added on to existing analog signal loops without having to upgrade wiring or change all instruments in the loop. Some of the features of HART are listed here:

 

Diagnostic data may be transmitted by the field device (self-test results, out-of-limit alarms, preventative maintenance alerts, etc.)

Field instruments may be re-ranged remotely through the use of HART communicators

Technicians may use HART communicators to force field instruments into different “manual” modes for diagnostic purposes (e.g. forcing a transmitter to output a fixed current so as to check calibration of other loop components, manually stroking a valve equipped with a HART-capable positioner)

Field instruments may be programmed with identification data (e.g. tag numbers corresponding to plant-wide instrument loop documentation)

 

HART multidrop mode

The HART standard also supports a mode of operation that is totally digital, and capable of supporting multiple HART instruments on the same pair of wires. This is known as multidrop mode.

Every HART instrument has an address number, which is typically set to a value of zero (0). A network address is a number used to distinguish one device from another on a broadcast network, so messages broadcast across the network may be directed to specific destinations. When a HART instrument operates in digital/analog hybrid mode, where it must have its own dedicated wire pair for communicating the 4-20 mA DC signal between it and an indicator or controller, there is no need for a digital address. An address becomes necessary only when multiple devices are connected to the same network wiring, and there arises a need to digitally distinguish one device from another on the same network.

This is a functionality the designers of HART intended from the beginning, although it is frequently unused in industry. Multiple HART instruments may be connected directly in parallel with one another along the same wire pair, and information exchanged between those instruments and a host system, if the HART address numbers are set to non-zero values (between 1 and 15):

 

Multiple HART devices on a single pair of wire

 

Setting an instrument’s HART address to a non-zero value is all that is necessary to engage multidrop mode. The address numbers themselves are irrelevant, as long as they fall within the range of 1 to 15 and are unique to that network.

The major disadvantage of using HART instruments in multidrop mode is its slow speed. Due to HART’s slow data rate (1200 bits per second), it may take several seconds to access a particular instrument’s data on a multidropped network. For some applications such as temperature measurement, this slow response time may be acceptable. For inherently faster processes such as liquid flow control, it would not be nearly fast enough to provide up-to-date information for the control system to act upon.

 

HART multi-variable transmitters

Some “smart” instruments have the ability to report multiple process variables. A good example of this is Coriolis-effect flowmeters, which by their very nature simultaneously measure the density, flow rate, and temperature of the fluid passing through them. A single pair of wires can only convey one 4-20 mA analog signal, but that same pair of wires may convey multiple digital signals encoded in the HART protocol. Digital signal transmission is required to realize the full capability of such “multi-variable” transmitters.

If the host system receiving the transmitter’s signal(s) is HART-ready, it may digitally poll the transmitters for all variables. If, however, the host system does not “talk” using the HART protocol, some other means must be found to “decode” the wealth of digital data coming from the multivariable transmitter. One such device is Rosemount’s model 333 HART “Tri-Loop” demultiplexer shown in the following photograph:

 

Rosemount Model 333 HART

 

This device polls the multi-variable transmitter and converts up to three HART variables into independent 4-20 mA analog output signals, which any suitable analog indicator or controller device may receive.

It should be noted that the same caveat applicable to multidrop HART systems (i.e. slow speed) applies to HART polling of multi-variable transmitters. HART is a relatively slow digital bus standard, and as such it should never be considered for applications demanding quick response. In applications where speed is not a concern, however, it is a very practical solution for acquiring multiple channels of data over a single pair of wires.

 

1The HART standard specifies “master” devices in a HART network transmit AC voltage signals, while “slave” devices transmit AC current signals.

 

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Comments (3)Add Comment
0
Instrument and Control Engineer
written by Gajendrasingh Rajput, November 27, 2012
Excellent article to understand the importance of working of HART transmitter with 250ohm resistor.
0
...
written by Ramasamy, December 12, 2012
Very useful informative article about HART transmitter with 250 ohm resister
0
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written by Futing Wang, July 09, 2015
So excellent!

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