What is HART?

A simple explanation of HART – Highway Addressable Remote Transducer technology.

Many instruments that incorporate 4-20mA analog signals are utilizing HART protocol, and most PLC I/O enclosures that Interstates builds have HART analog I/O compatibility. But what actually is HART? And why is it chosen more often than not?

HART is an acronym for Highway Addressable Remote Transducer. It involves a digital signal superimposed onto a 4-20mA analog signal in order to encrypt instrument information beyond the standard analog signal. What this means is that the standard 4-20mA signal oscillates up and down from the average 4-20mA value by a small amount.

The actual analog signal from the device is represented by the blue curve. The average of the signal is represented by the red curve, and this is the standard 4-20mA analog signal measured by the PLC. A HART-compatible I/O card interprets the oscillation as a series of 0s and 1s, which is the basis for communication in the digital world. The system works like this: A PLC will transmit a message to the device in oscillations as 0s and 1s, and the device will receive, interpret, and transmit back a response corresponding to the message that was received.

This is known as a master/slave relationship and is useful for functionality beyond the standard 4-20mA measurement, such as:

  • Configuration of the instrument remotely
  • Receiving instrument status
  • Receiving instrument diagnostics
  • Performing instrument troubleshooting
  • Gathering additional measurements beyond the analog signal

In our industry, these benefits lead to decreased downtime and improved record keeping and trending. Also, imagine the reduction in maintenance costs by not requiring a resource to be physically onsite to troubleshoot. It adds up to savings in travel costs, site costs, and time, because the programmer is able to immediately request information from the device at the comfort of his/her own workstation. Another advantage of HART is that a maintenance technician can use a HART-compatible handheld at the control panel to obtain device status/diagnostics instead of having to physically go to the device, which may be on top of a tank in the middle of winter. As you can see, HART technology speeds up maintenance and troubleshooting issues, as well as provides the added benefit of having more data available in order to optimize your plant.

By Shane Heier