HART Technology Explained
HART is a bi-directional communication protocol that provides data access between intelligent field instruments and host systems. A host can be any software application from technician's hand-held device or laptop to a plant's process control, asset management, safety or other system using any control platform. Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Instrumentation grade wiring and standard termination practices assure reliable communication.
HART provides two simultaneous communication channels, one analog, the other digital: A 4-20mA signal communicates the primary measured value (PV) as an analog value of current using the wiring that provides power to the instrument. The host system then converts the current value to a physical value according to parameters defined by HART Software. For example, 7 mA = 80 degrees F.
Digital device information is communicated by encoding a digital signal, generally using a technique known as Frequency Shift Keying on the same 4-20mA wiring used for analog communications. The digital signal contains information from the device including PV, device status, diagnostics, and additional measured or calculated values, etc.
Together, the two communication channels provide a complete field communications solution that is easy to design, simple to use, low cost and extremely reliable.
How HART Works
“HART” is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes use of the Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument.
The HART Protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a smart field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal. The HART Protocol provides two simultaneous communication channels: the 4-20mA analog signal and a digital signal. The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA current loop - the fastest and most reliable industry standard. Additional device information is communicated using a digital signal that is superimposed on the analog signal.
The digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc. Together, the two communication channels provide a low-cost and very robust complete field communication solution that is easy to use and configure.
Frequency Shift Keying (FSK)
The HART digital communication is based on Bell 202 and uses a Frequency Shift Keying (FSK) technique to superimpose two-way digital communication on to the 4-20 mA current loop signal. The HART Signal communicates at 1200 bits per seconds – typically capable of providing 2 - 3 Primary Variable updates per second.
Primary and Secondary Hosts
The HART Protocol permits several hosts on the network.
Request-Reply Protocol
HART technology is a request-reply protocol, which means that a smart field device only speaks when spoken to by a host. The HART Protocol can be used for communicating information to/from smart field instruments and central control or monitoring systems.
Two Types of Configuration
The HART Protocol conducts all digital communication with field devices in either point-to-point or multidrop network configurations.
Two Types of Configuration
The HART Protocol conducts all digital communication with field devices in either point-to-point or multidrop network configurations.
Two Types of Messaging
The are two types of messaging used in HART communication - Request-Reply and Burst Mode (optional). In Request-Reply, a host sends a command and the device replies.
Two Types of Messaging
The are two types of messaging used in HART communication - Request-Reply and Burst Mode (optional). In Burst mode, a host sends a command and the device continues to send data to the host.
To learn more about how HART works, take our new self-paced online course: HART Protocol - Technology Overview.
HART COMMANDS
The HART Protocol is a request-response communication protocol which means that during normal operation, each communication is initiated by a request (or command) from the host communication device. The host is generally a distributed control, PLC, or PC-based asset management system for example. The device is typically a field measurement device such as pressure, level, temperature, flow or other transmitters.
In order to make certain any HART-enabled device from any supplier can communicate properly and respond to a command with the correct information, the set and types of commands are defined in the HART Specifications and implemented in all HART registered devices.
Users need not worry about these commands because they are included in the functions of the host. The specific capabilities of a device (device specific commands) are available to the host when the host is given the instructions included in the Device Description (DD) of a specific device.
An important point is that defined device status indications are included with each communication response to the host. The host then interprets these status indicators and may provide basic device diagnostic information.
The HART Command Set provides uniform and consistent communication for all field devices. Host applications may implement any of the necessary commands for a particular application. The command set includes three classes:
Universal
All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units).
Common Practice
Common Practice commands provide functions implemented by many, but not necessarily all, HART Communication devices.
Device Specific
Device Specific commands represent functions that are unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on Device Specific commands is available from device manufacturers.
A Partial List of HART Commands:
| Universal Commands | Common Practice Commands | Device Specific Commands | ||||
|
|
|
To learn more about HART Commands, take our new self-paced online course (free of charge for a limited time): HART Protocol - Technology Overview.
HART Protocol Layers
HART is based on the basic Open System Interconnection (OSI) reference model:
Of the 7 layers HART uses 3: Physical Layer, Data Link Layer and Application Layers.
To learn more about HART Protocol layers, take our new self-paced online course (free of charge for a limited time): HART Protocol - Technology Overview.
Additional Resources
Check out these resources, including brochures, white papers and books.