In the industrial world, instruments typically communicate with one another over two wires by conveying a 4-20 mA analog instrument signal over them. To improve communication, engineers introduced a technological advance, which they called HART or Highway Addressable Remote Transmitter. HART is a hybrid communication standard that allows instruments to digitally communicate over the same 4-20 mA DC signal wires.
HART offers two channels of communication for instruments. One variable channel for communicating using the analog value of a 4-20 mA DC signal. The other variable channel offers digital communication using pulses of current representing binary bit values of 0 and 1, utilizing several variables. With the digital current pulses superimposing upon the analog DC current signals, the same two wires can carry both analog and digital data simultaneously.
A typical standard loop-powered, two-wire process transmitter has a DC power supply and a 250-ohm resistor for creating the 1 to 5 V signal from the 4-20 mA current signal that any voltage-sensing recorder, controller, or indicator can read easily.
In such circuits, the primary function of the transmitter is to regulate the current to a value representing the process variable it is measuring. The process variable may represent flow, temperature, pressure, etc. that the circuit measures using a current range of 4-20 mA. The DC voltage source provides the necessary power for operating the transmitter. In industrial instrumentation, such loop-powered instruments are common, as they allow conveying of both power and data on the same pair of wires.
With process transmitters now involving microprocessors, the instrument technicians can configure parameters digitally within the transmitter. These include damping values, range values, and more. They can query the transmitter for self-diagnostic alarms. However, to make full use of the digital functionality, it is necessary for the process transmitter to be able to communicate digital data to and fro over the same two wires that convey the 4-20 mA analog signal.
Another way of accessing the digital data within the transmitter would be to connect a communicator to a data port located on the transmitter. However, this would be inconvenient considering the location of these transmitters, mainly in inaccessible places in the industry, making it difficult to carry a personal computer or other communication devices to the transmitter.
The communication protocol, HART, solved the issue by adding digital data in the form of AC signals using audio-frequency tones, along the same two wires that carried the 4-20 mA DC current signal. A smart transmitter, with a modem built into it, translates these digital binary bits into AC signals, and vice-versa.
This allowed instrument technicians to communicate with the new microprocessor-equipped transmitters. They can do this simply by connecting a HART modem at any point on the two-wire cable, even at the far end, where it terminates at the hardware of the control system.
The ability to transmit and receive digital data over the same wire pair carrying the analog signal and DC power opens an entirely new range of possibilities. Now, technicians can send and receive status reports, self-diagnostic information, multiple process variables, alarms, and more in addition to the original signal representing the primary process variable.