08-16-2017, 10:14 PM
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What is HART?
(Highway Addressable Remote Transducer)
The majority of smart field devices installed worldwide today are HART-enabled. But some new in the automation field may need a refresher on this powerful technology.
The HART (Highway Addressable Remote Transducer) Protocol is the global standard for sending and receiving digital information across analog wires between smart devices and control or monitoring system.
More specifically, 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.
( The HART Communications Protocol (Highway Addressable Remote Transducer Protocol) is an early implementation of Fieldbus, a digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4-20 mA analog instrumentation wiring, sharing the pair of wires used by the older system. According to Emerson[1], due to the huge installed base of 4-20 mA systems throughout the world, the HART Protocol was one of the most popular industrial protocols today. HART protocol made a good transition protocol for users who were comfortable using the legacy 4-20 mA signals, but wanted to implement a "smart" protocol. Industries seem to be using Foundation fieldbus (also by Rosemount) more and more as users become familiar with current technology.
The protocol was developed by Rosemount Inc., built off the Bell 202 early communications standard, in the mid-1980s as proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART. In 1986, it was made an open protocol. Since then, the capabilities of the protocol have been enhanced by successive revisions to the specification .)
For many years, the field communication standard for process automation equipment has been a nalog current signal. The milliamp current signal varies within a range of 4-2OmA in proportion to the process
There are several reasons to have a host communicate with smart devices. These include:
Device Configuration or re-configuration
Device Diagnostics
Device Troubleshooting
Reading the additional measurement values provided by the device
Device Health and Status
Much more: There are many benefits of using HART technology, and more users are reporting benefits in their projects on a continual basis
The most important performance features of the HART protocol include:
-- proven in practice, simple design, easy to maintain and operate
4 compatible with conventional analog instrumentation
4 simultaneous analog and digital communication
4 option of point-to-point or multidrop operation
4 flexible data access via up to two master devices
4 supports multivariable field devices
4 sufficient response time of approx. 500 ms
4 open de-facto standard freely available to any manufacturer or user
How HART Works
HART is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA.
Figure 1. Frequency Shift Keying (FSK)
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.
HART technology is a master/slave protocol, which means that a smart field (slave) device only speaks when spoken to by a master. The HART Protocol can be used in various modes such as point-to-point or multidrop for communicating information to/from smart field instruments and central control or monitoring systems.
HART Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices.
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.
Figure 2. Two Communication Channels
The HART Protocol provides for up to two masters (primary and secondary). This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system.
Figure 3. Primary and Secondary Masters
The HART Protocol permits all digital communication with field devices in either point-to-point or multidrop network configurations:
Figure 4. Point-to-Point Configuration
Multidrop Configuration
There is also an optional "burst" communication mode where a single slave device can continuously broadcast a standard HART reply message. Higher update rates are possible with this optional burst communication mode and use is normally restricted to point-to-point configuration.
Figure 5. Multidrop Configuration
The Benefits of HART Protocol Communication
Digital Capability
Access to all instrument parameters & diagnostics
Supports multivariable instruments
On-line device status
Analog Compatibility
Simultaneous analog & digital communication
Compatible with existing 4-20 mA equipment & wiring Interoperability
Fully open de facto standard
Common Command and data structure
Enhanced by Device Description Language
Availability
Field proven technology with more than 1,400,000installations
Large and growing selection of products
Used by more smart instruments than any other in the industry
OR
HART technology can help you:
Leverage the capabilities of a full set of intelligent device data for operational improvements.
Gain early warnings to variances in device, product or process performance.
Speed the troubleshooting time between the identification and resolution of problems.
Continuously validate the integrity of loops and control/automation system strategies.
Increase asset productivity and system availability.
Increase Plant Availability
Integrate devices and systems for detection of previously undetectable problems.
Detect device and/or process connection problems real time.
Minimize the impact of deviations by gaining new, early warnings.
Avoid the high cost of unscheduled shutdowns or process disruptions.
Reduce Maintenance Costs
Quickly verify and validate control loop and device configuration.
Use remote diagnostics to reduce unnecessary field checks.
Capture performance trend data for predictive maintenance diagnostics.
Reduce spares inventory and device management costs.
Improve regulatory compliance
Enable automated record keeping of compliance data.
Facilitates automated safety shutdown testing.
Raise SIL/safety integrity level with advanced diagnostics.
Take advantage of intelligent multivariable devices for more thorough, accurate reporting.