If you study or work in automation, then you’ve heard of and probably even used the HART communication protocol or WirelessHART. They have become integral parts of the industry and this article will tell you all you need to know about them, including how they work, how to use them, and why they are so popular.

If you would like to know more about HART communicators, check out our guide.

What is the HART communication protocol?

HART stands for Highway Addressable Remote Transducer. This hybrid open protocol uses a mix of analog and digital signals to communicate.

Image of Hart communication
Image courtesy of fieldcommgroup.org

HART communication occurs in analog current loops. This means that the same pair of wires carry an analog signal as well as the HART communication protocol. Most systems use the analog signal to control processes and HART to set up field devices.

A company called Rosemount developed HART as a proprietary protocol. In 1986, it became an open protocol and today remains globally popular for configuration and control.

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RIA15 Loop-powered indicator for 4...20 mA or HART®
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How does the HART communication protocol work?

The HART communication protocol uses the Bell 202 Frequency Shift Keying (FSK) standard. That means that the protocol can superimpose digital signals at a low level on top of the 4-20 mA output.

The analog signal sends the primary process variable in most cases, and the HART protocol sends field device data such as status, diagnostics, and configuration.

hart communication protocol diagram
Image courtesy of vdi.de

The HART signal shifts frequency, using 1200 Hz to represent binary one and 2200 Hz for binary zero. This graphic will show you more.

Communicating through an analog or HART protocol also makes it possible to display data for many types of field devices. For example, RIA15 can display data for many types of field devices and communicate through analog or HART protocol.

Now, let’s talk about the types of HART you can have:

Point-to-point communication

Common in many industries, point-to-point has the field device or final control element connected to the control system using analog to transmit process data or commands.

In this scenario, you’ll use the HART communication protocol to set up these devices or check their diagnostics. To do that, you can use a handheld or multiplexer to collect the data to send to a device management system. You’ll also configure your devices using the polling address of zero to allow analog modulation.

The HART communication protocol works on the master-slave concept, meaning field devices only send data when a master asks. However, the network allows primary and secondary masters. When you have a device management system accessing field devices remotely, you can use your handheld as a secondary master.

Finally, you can have burst communication in a point-to-point installation. Here a slave continuously sends standard HART messages. You can read more about that in one of our other articles.

Multi-drop communication

You won’t see this type much, but you can find a few interesting applications using WirelessHART and local controllers to collect data from many devices in a single network.

Here the devices connected in parallel in a single trunk, and all of them use polling addresses other than zero. This setup creates digital-only communication, with the analog signal fixed at four milliamps.

Here are the polling addresses, depending on the version of your field devices:

  • Versions 3, 4, and 5: 1 to 15
  • Version 6: 1 to 63
  • Version 7: 0 to 63
Image for Hart multidrop
Image courtesy of fieldcommgroup.org

Intelligent device management (IDM)

HART devices are smart devices, which means you can get tons of information about your field devices using the HART communication protocol. If you set your devices to read only the analog signal, then you won’t receive all the data they can send. You must set them up to send digital data too.

Fortunately, a HART system offers intelligent device management (IDM). As a matter of fact, a HART network can make the device setup easy with its configuration tool. Of course, your control system will need to use the analog signal to control your loops.

WirelessHART

WirelessHART was the first standard wireless communication dedicated to field devices. HART communication evolved over the years. As digitalization grew, wireless became a popular solution, making wirelessHART a natural step in the progress of data transmission.

You can buy adapters to turn standard HART field devices into wirelessHART devices. You can also find wireless devices with built-in antennas. These solutions have interesting pros and cons, but we’ll have to leave that topic to the wirelessHART article.

To know about such products, you can get in touch with our engineers!

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