This month we will explore the HART protocol. It’s been around for a long time and with the introduction of WirelessHART it is definitely here to stay!
HART Protocol has been around for a long time. Is it still a good choice?
HART is one of several different communications protocols used in plant automation. HART is the best overall solution for obtaining value-added device and diagnostic information in digital form while retaining compatibility with legacy 4-20mA automation architectures.
What is HART?
HART (“Highway Addressable Remote Transducer”) is a communication protocol designed for industrial process measurement and control applications. HART is an open standard and vendor independent. Because of this, it is the world’s most broadly supported protocol for the process industry, with thousands of HART based products available from different vendors. It’s called a hybrid protocol because it combines analogue and digital communication. It can communicate a single variable using a 4-20mA analogue signal while also communicating added information on a digital signal that is superimposed on the standard 4-20mA current loop. Using an analogue signal, information is sent only one way, either from the device to the host (inputs) or from the host to the device (outputs). Digital information can travel in both directions using HART. Traditional analogue devices communicate only a single process variable, and you typically have no easy way to tell if the information they’re sending is valid. With HART, you will get the process variable – but other types of information too. Information items are standard in every HART device such as:
• Device status and diagnostic alerts
• Process variables and units
• Loop current and percentage range
• Basic configuration parameters
• Manufacturer and device tag information
How does HART work?
HART digital communications uses a request/reply communications model. This means that in general, HART devices won’t transmit any information unless a request is sent from the host to the device. The exception to this is the burst mode where the HART device can send a single piece of information continuously without repeated host requests. A common use for burst mode is to send the process variable as a digital value to verify the analogue signal. Many control systems aren’t designed to accept HART information in digital form so it is common to see external multiplexers reading the digital signal. In this approach, the HART device is attached to both the control host and to the multiplexer. Although this increases the cost of the installation, reductions in maintenance cost generally pay back the investment in a very short time. Some hosts, such as Emerson’s DeltaV are able to capture and pass HART digital information to other applications (e.g., AMS Asset Management Suite) using a mechanism commonly called “HART pass through”. Using a system that supports HART pass through reduces the cost of acquiring and using the HART information by eliminating the need to install separate multiplexer systems.
WirelessHART is HART
WirelessHART devices are HART devices that communicate wirelessly, so if you know HART, you are already familiar with and have the tools to use WirelessHART devices. WirelessHART is approved as an international standard IEC62591 that is open and free to use with dozens of vendors supporting it, with Emerson leading the way with the Rosemount Wireless range of installations in process applications. WirelessHART can be used on existing wired instruments to collect the vast amount of information that was previously stranded in the instrument, and also provides for a cost-effective, simple and reliable way to deploy new points of measurement and control without the wiring costs.
“Does it work?” – check out this YouTube of actual working installations of WirelessHART instruments in extremely harsh environments: youtu.be/LNMmmjz5nCo