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.
With the advent of microprocessor-based process transmitters, it became possible for instrument technicians to digitally configure parameters inside the transmitter (e.g. range values, damping values) and also query the transmitter for self-diagnostic alarms. In order to make full use of this digital functionality, though, there had to be some way to communicate digital data to and from the process transmitter over the same two wires used to convey the 4-20 mA analog signal. Otherwise, the only way to access this rich array of digital data inside the transmitter would be to connect a communicator device to some data port located on the transmitter itself, which is inconvenient due to the nature of how these transmitters are used in industry (located in dirty places, often hard to access while carrying a personal computer or other communication device).
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| HART Transmitter (Siemens) |
Thus the HART communication protocol was born to address this need. HART communicates digital data along the loop conductors in the form of AC signals (audio-frequency tones) superimposed on the 4-20 mA DC current signal. A modem built into the smart transmitter translates these AC signals into binary bits, and vice-versa. Now, instrument technicians could “talk” with the new microprocessor-based transmitters simply by connecting a HART communications device at any point along the two-wire cable, even at the far end where the cable terminates at the control system hardware (panel-mounted controller, PLC, DCS, etc.).
Being able to communicate digital data over the same wire pair as the DC power and analog signal opens a whole new range of possibilities. Now, the field-mounted transmitter can communicate self-diagnostic information, status reports, alarms, and even multiple process variables to the control system in addition to the original analog signal representing the (main) process variable. With digital communication, the only data limitation is speed (data rate), not quantity. The control system may even communicate information to the transmitter using the same digital protocol, using this digital data channel to switch between different measurement range sets, activating special features (e.g. square-root characterization, damping, etc.), automatically and remotely.
Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.
