Looking at a standard loop-powered (2-wire) process transmitter circuit, we see the transmitter, a DC power supply (voltage source), and usually a 250 ohm resistor to create a 1 to 5 volt signal readable by any voltage-sensing indicator, controller, or recorder:
The transmitter’s primary function in this circuit is to regulate current to a value representative of the measured process variable (e.g. pressure, temperature, flow, etc.) using a range of 4 to 20 mA, while the DC voltage source provides power for the transmitter to operate. Loop-powered instruments are very common in industrial instrumentation because they allow both power and (analog) data to be conveyed on the same pair of wires.
HART communication extends this efficiency one more step: in addition to DC power and analog signal current being carried by a common wire pair, digital data may also be carried to and from the loop-powered transmitter in the form of electrical pulses superimposed on the cable.
With the incorporation of digital data on the same wire pair as the DC power and analog signal, a wide range of new possibilities suddenly emerge. Now, the field-mounted transmitter has the ability to communicate self-diagnostic information, status reports, alarms, and even multiple process variables in addition to the original analog signal representing the (main) process variable. We may even communicate information to the transmitter using the same digital protocol, using this digital data channel to set measurement ranges, activate special features (e.g. square-root characterization, damping, etc.), from a remote location.
With HART, the traditional series-connected circuit configuration of transmitter, DC power supply, and resistor remains unchanged. A HART-enabled transmitter, though, is equipped with a built-in digital microcontroller managing its functions, and this miniature computer is able to send and receive digital data as AC signals (current pulses in sending mode, voltage pulses in receiving mode) superimposed on the same two wires carrying the 4 to 20 mA analog signal and DC power. Any other computer device equipped with a HART modem, the necessary configuration software, and an appropriate device description for that particular instrument may communicate with the HART transmitter if connected in parallel with the transmitter’s loop power terminals:
This external computer – through the use of HART data transmission – now has the ability to monitor details of the transmitter’s operation, configure the transmitter, update its measurement
ranges, and perform a host of additional functions.
The connection between the HART modem and the HART-enabled transmitter need not be made directly at the transmitter’s terminals. Any set of points in the circuit electrically parallel to the transmitter’s terminals are permissible as connection points for the HART modem. This flexibility is a great advantage in loop circuits spanning long distances, enabling the technician to connect their HART configuration equipment at the most physically convenient location:
A convenient alternative to a personal computer and HART modem is a special hand-held device called a HART communicator. Two different models of HART communicator are shown in the
following photographs, a Rosemount model 268 on the left and an Emerson model 375 on the right:
HART communicators are battery-powered, portable devices built specifically for configuring HART-enabled field instruments. Like personal computers, they need to be updated with DD files to be able to communicate with the latest models of HART-enabled field instruments.
It should be mentioned that HART is a legacy technology.
HART protocol is still the most popular form of digital field instrument communication in industrial use. However, more modern digital standards such as Profibus and FOUNDATION Fieldbus deliver all the benefits of HART technology and more. It seems that HART will remain in wide industrial use for many years to come, but it is really just the beginning of digital field instrument technology and by no means represents the state of the art.