HART is one of several different communications protocols used in plant automation. Each has its strengths, but HART is the best overall solution for obtaining value-added device and diagnostic information in digital form while retaining compatibility with legacy 4-20 mA automation architectures.
HART (“Highway Addressable Remote Transducer”) is a communication protocol designed for industrial process measurement and control applications.
The protocol was developed by Rosemount Inc., built off the Bell 202 early communications standard in the mid-1980s as a proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART and in 1986 it was made an open protocol. Since then, the capabilities of the protocol have been enhanced by successive revisions to the specification.
It’s called a hybrid protocol because it combines analog and digital communication. It can communicate a single variable using a 4-20 ma analog signal, while also communicating added information on a digital signal. The digital information is carried by a low-level modulation superimposed on the standard 4-to-20 mA current loop. The digital signal does not affect the analog reading because it’s removed from the analog signal by standard filtering techniques.
The ability to carry this added digital information is the basis for HART’s key benefits. Traditional analog and discrete 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 still get the process variable — but other types of information, too. As many as 35-40 information items are standard in every HART device. Examples include
- Device Status & Diagnostic Alerts
- Process Variables & Units
- Loop Current & % Range
- Basic Configuration Parameters
- Manufacturer & Device Tag
With additional information like this, HART devices that are digitally polled by a host can tell you if they’re correctly configured and operating correctly. This eliminates the need for most routine checks — and helps you detect failure conditions before they cause a major process problem. Interoperability simply means that HART-compliant devices and host systems, regardless of vendor, can work together.
Some host systems use universal and common-practice commands to work with HART devices. Others go a step farther by also using Device Descriptions to understand all HART messages. Even hosts that aren’t designed to handle the digital information from a HART device can stillhave control interoperability through the 4-20 mA analog signal.
The HART communication protocol is based on the Bell 202 telephone communication standard and operates using the frequency shift keying (FSK) principle. The digital signal is made up of two frequencies—1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications .Because the average value of the FSK signal is always zero, the 4–20 mA analog signal is not affected. The digital communication signal has a response time of approximately 2–3 data updates per second without interrupting the analog signal. A minimum loop impedance of 230 W is required for communication
HART devices can operate in one of two network configurations—point-topoint or multidrop. POINT-TO-POINT In point-to-point mode, the traditional 4–20 mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART protocol (Figure 2). The 4–20 mA analog signal is not affected by the HART signal and can be used for control in the normal way.
The HART communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance, and diagnostic purposes.