Highway Addressable Remote Transducer
HART is a digital signal that rides on a standard 4 mA ... 20 mA process control loop. In the field of process automation, the 4 mA ... 20 mA loop is very steady. You may hear of it being referred to as “quasi-static,” as it doesn't change much. Field devices like mass flow, temperature, pressure transmitters, or valve positioners use this 4 mA ... 20 mA signal. HART information is extra information that you get back from your field instruments.
Historically, HART communication uses the BELL 202 telephone communication standard, which telephone land lines still use today. This standard was introduced in the early 1980s and uses Frequency Shift Keying (FSK) technology. FSK simply means that the information is keyed or coded into the frequency, which is how the data communicates back and forth. HART layers its digital communication signals on top of the 4 mA ... 20 mA control signal.
A drawback based on today's standards is the speed of HART communication, which is rather slow. HART is limited to 1200 bits/second and ranges from 1200 Hz to 2200 Hz. Information (1, 0) is represented by different frequencies. The HART signal creates 0s and 1s. A logic 1 is represented by 1200 Hz. A logic 0 is represented by 2200 Hz. On the plus side, HART communication doesn't interrupt the 4 mA ... 20 mA signal, and it allows a host application (master) to get up to three digital updates per second from a field device.
Back in the day when this technology was introduced, 1200 baud was screaming fast, but now it is painfully slow and just doesn't measure up to our expectations. As an example, a LAN (Local Area Network), in common use today, perks along at 100 Mbits/second. That's 1000 times faster than the BELL 202!
If the communication speeds are so slow, why do we continue to use HART communication for field instruments and devices? Well, the 4 mA ... 20 mA control signals with FSK are reliable and have been in use for decades. There is a large installed base of devices. By our estimates, there are at least 30 million HART-compatible field devices in use worldwide.
Also, the HART information is easily extracted without interfering with the 4 mA … 20 mA signal used by the host system. Host systems are most commonly a distributed control system (DCS), programmable logic controller (PLC), asset management system, safety system, or a handheld device. HART enables two-way field communication to take place and makes it possible for additional information beyond the normal process variable to be communicated to or from a smart field instrument.