The Benefits of HART Protocol Communication in Smart Instrumentation Systems


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

 

Introduction

HART® Field Communications Protocol is widely recognized as the industry standard for digitally enhanced 4-20 mA smart instrument communication. Use of the technology is growing rapidly, and today virtually all major global instrumentation suppliers offer products with HART communication.

The HART protocol provides a uniquely backward compatible solution for smart instrument communication as both 4- 20 mA analog and digital communication signals are transmitted simultaneously on the same wiring. HART provides many benefits promised by fieldbus, while retaining the compatibility and familiarity of existing 4-20 mA systems.

This paper provides a brief overview of the HART protocol and the benefits achievable with this important technology. Per instrument cost savings benefits of $300-500 in initial installation/commissioning and $100-200 per year in ongoing maintenance/operations are commonly reported.

 

Analog + Digital Communication

For many years, the field communication standard used by process automation equipment has been a milliampere (mA) analog current signal. In most applications the milliampere signal varies within a range of 4-20 mA in proportion to the process variable being represented. Virtually all installed plant instrumentation systems use this international standard to communicate process variable information.

 

Simultaneous Analog + Digital Communication


Figure 1 - HART uses Frequency Shift Keying to encode digital
information on top of the 4-20 mA analog signal

 

HART Field Communications Protocol extends the 4-20 mA standard to enhance communication with intelligent measurement and control instrumentation. A major step in the evolution of process control, the HART protocol is fostering significant innovation in process instrumentation capabilities. The enhanced communication characteristics of this important technology are reflected in the protocol name, HART which stands for “Highway Addressable Remote Transducer”.

The HART protocol enables two-way digital communication with smart instruments without disturbing the 4-20 mA analog signal. Both the 4-20 mA analog and HART digital communication signals can be transmitted simultaneously over the same wiring. Primary variable and control signal information is carried by the 4-20 mA (if desired), while additional measurements, process parameters, device configuration, calibration, and diagnostics information is accessible through the HART protocol over the same wires at the same time. Unlike other “open” digital communication technologies for process instrumentation, HART is compatible with existing systems.

 

The HART Technology

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 4-20 mA. Since the digital FSK signal is phase continuous, it does not interfere with the 4-20 mA signal. A logical “1” is represented by a frequency of 1200 Hz and a logical “0” is represented by a frequency of 2200 Hz as shown in Figures 1 and 2.

The HART FSK signaling enables two-way digital communication and makes it possible for additional information beyond just the normal process variable to be communicated to or from a smart field instrument. The HART protocol communicates at 1200 bits per second without interrupting the 4-20 mA signal and allows a host application (master) to get two or more digital updates per second from a field device.


Figure 2 - HART digital communication signal superimposed on the
4-20 mA analog signal

 

Flexible Application

HART is principally a master/slave protocol which means that a field (slave) device speaks only when spoken to by a master. Two masters (primary and secondary) can communicate with slave devices in a HART network. Secondary masters, such as handheld communicators, can be connected almost anywhere on the network and communicate with field devices without disturbing communication with the primary master. A primary master is typically a DCS, PLC, or computer based central control or monitoring system. A typical installation with two masters is shown in Figure 3.

The HART protocol can be used in various modes for communicating information to/from smart field instruments and central control or monitoring equipment. Digital master/slave communication simultaneous with the 4-20 mA analog signal is the most common. This mode, allows digital information from the slave device to be updated twice per second in the master. The 4-20 mA analog signal is continuous and can still carry the primary variable for control.



Figure 3 - The HART protocol allows two Master devices
to access information in Slave (field) devices

 

Innovative Application Example

The power of the HART protocol is evident in the control diagram of Figure 4. This innovative application uses the inherent feature of the HART protocol that both 4-20 mA analog and digital communication signals are transmitted simultaneously over the same wiring.

In this application, the HART-compatible transmitter has an internal PID control capability. The device is configured such that the 4-20 mA loop current is proportional to the control output of the PID algorithm executing in the device (not the measured variable as in most transmitter applications). Since the 4-20 mA loop current is regulated by the PID control output, it is used to drive the valve position directly.

The control loop executes entirely in the field between the transmitter (with PID) and the control valve. The control action is continuous as the traditional 4-20 mA analog signal drives the valve. HART digital communication links the operator with the control loop to change set point, and read the primary variable, or valve position output. Substantial savings are possible in applications where this innovative control architecture is appropriate.



Figure 4 - Some HART-bassed instruments include PID algorithms
for implementing cost effective control

 

Best Solution

The HART protocol provides users with the best solution and migration path for capturing the benefits of enhanced communication with smart instrumentation. No other communication technology can match the base of support or wide range of products that are available with HART today. The technology is easy to use and HART-compatible products are available from major instrumentation suppliers to address virtually all process measurement and control applications.

The emergence of fieldbus will not displace HART in either existing or new production facilities. HART provides users with many of the same benefits while retaining the compatibility and familiarity of existing 4-20 mA systems. HART allows the cost saving benefits of remote communication, flexible/accurate digital data transmission, field device diagnostics, and powerful multiparameter instruments to be captured without replacing entire systems.

Connection to current and future plant networks is assured by the digital communication capability and large installed base (more than 5,000,000 installations and growing rapidly).

Support of the HART Communication Foundation ensures that the technology will continue to evolve for serving the needs of smart instrumentation today and tomorrow.

Written by Ron Helson
HART Communication Foundation
9390 Research Blvd., Suite I-350
Austin, Texas 78759 USA
(Click here for more details)

For more information, visit the HART site at:
http://www.hartcomm.org