By Ed T. Ladd, Jr., HART Communication Foundation
This is the first in a series of articles designed to increase your basic knowledge of the HART Field Communications Protocol, to explain the benefits of using HART Communication to its full potential and to dispel the myths surrounding this powerful technology.
HART is an acronym for Highway Addressable Remote Transducer. HART is the largest single field communications protocol used in the major process and hybrid industries. HART-enabled products represent 70% of all smart devices installed in the world. For more than fifteen years, over 150 major manufacturers have supported the HART Protocol with hundreds of products and over 15 million installations worldwide.
A recent ARC Advisory Group survey confirms the continued user adoption and expansion of HART Communication for years to come. As the future automation landscape expands to include multiple communication protocols, HART will continue to hold a dominant position in the industry.
HART-enabled devices have two output signals traveling on a single pair of wires. This is significant because it allows users to receive the best of both worldsthe compatibility and speed of the primary measurement variable transmitted over the analog signal plus bi-directional communication of diagnostic and multivariable information transferred via a digital signal that is imposed on the analog loop. Using the two communication channels continuously allows users to increase performance and uptime, optimizing their assets and insuring that the highest safety integrity level is achieved.
Traditionally HART-enabled devices have been underutilized with the digital data only accessed during the system startup, annual calibration or when a technician needs to troubleshoot the system. This has propagated the myth that a handheld terminal is the only way to communicate with a HART device. With the HART-enabled control systems, multiplexers, HART modems and OPC servers available today, only the preconception that handheld configuration is the only way to use HART can keep you from utilizing the full potential of your plant assets.
To fully comprehend the Power of HART, you need to understand how it works. HART Communication technology has a very low power requirement typically working on less than 3.8 mA of current. HART utilizes a frequency shift keying (FSK) modem to communicate over the primary variable analog signal wires. By imposing the digital signal on top of the 4-20mA analog loop, HART allows for a simultaneous analog signal with a continuous digital communication signal that has no effect on the analog signal.
HART circuits or networks can be configured in one of three ways, through a conventional connection method (point-to-point), multi-drop (point to multiple points) or through a multiplexer (HART to RS-485). HART Communication is designed so that there must be minimum resistance in the current loop before communication will occur. The HART specification accommodates loop resistances of 230 to 1100 ohms, typically a 250-ohm resistor is recommended.
No matter the configuration employed, HART networks have three device type communication modes: Master, Slave and Burst. Masters or hosts are typically programmable logic controls, distributed control systems, PCs or handheld terminals. Up to two Masters can be connected to a HART network at once and the Masters can be primary or secondary masters. A handheld HART terminal is example a Secondary Master and Control System is an example of a Primary Master.
The Slave and Burst modes apply to field devices including transmitters, actuators and controllers. In the Slave mode field devices respond to requests for information from Masters on the network. When a field device is in the Burst mode, a field device will continually output information to the Master, up to four times a second.
The analog loop current (4-20mA) signal from HART devices is proportional to the primary control variable but for digital communication, they utilize a command structure to send and receive intelligent information. The HART Universal and Common Practice Commands are the basis for this structure and allow Masters to access device data such as process variables, configuration parameters and determine a devices status/health. Therefore, to communicate with a HART device a Master must know the HART Command structure to get the information.
In addition to the commands, many devices may require a Device Description (DD) to access device specific features. A DD is an electronic data file that describes specific features and functions of a device, including details of menus and graphic display features to be used by host applications (including handheld devices) to access all parameters and data in the corresponding device.
The need to know on Universal/Common Practice Commands and on Device Descriptions will appear here soon.
Upcoming HART Connection articles will provide information on applying HART technology, application notes to help you implement new HART strategies and what to expect from a HART-enabled device or host systems. For more information, please feel free to contact the HART Communication Foundation.