Everything you need to know about HART communication
If you study automation, then you’ve heard of and probably even used HART communication, an integral part of this industry. However, some people came here from other fields, so maybe they don’t know it as well.
And based on my past visits with customers, even some automation folks don’t know it as well as they could. So let’s dive into HART, reviewing its history and specs to find out why this protocol still prevails today.
What is HART communication?
First, let’s explain the acronym. HART stands for Highway Addressable Remote Transducer. This hybrid open protocol uses a mix of analog and digital signals to communicate.
HART communication occurs in analog current loops. This means that the same pair of wires carry an analog signal as well as the HART protocol. Most systems use the analog signal to control processes and the 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.
How does HART communication work?
This protocol developed from Bell 202 frequency-shift keying (FSK), which imposes the digital signal over the analog signal.
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.
The HART signal shifts frequency, using 1200 Hz to represent binary one and 2200 Hz for binary zero. This graphic will show you more.
Now let’s talk about the types of HART you can have.
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 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 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.
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 connect 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
Intelligent device management (IDM)
HART devices are smart devices, which means you can get tons of information about your field devices using the HART 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 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.