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Fieldbus System Architecture
(Part 1 of 4) - HART

By Dick Caro

E-Zine March 2006

A fieldbus is a communications network used in industrial manufacturing to collect measurement and sensor data, and to distribute outputs to final control elements. Fieldbus is not restricted to process control or any variety of factory automation. In general, a fieldbus is designed to operate in the industrial environment. It also delivers power to the field devices when required.

HART was created in the late 1980's to demonstrate that it was possible to carry digital data on the same wired network as used for 4-20 mA DC data transmissions. Compromises made to carry both the analog current signal and digital data on the same wire resulted in a relatively slow digital data rate (1200 bps). However, HART specification C8PSK will allow higher speed digital data transmission (9600 bps) in a manner compatible with older HART transmitters.

HART process control field instrumentation sales exceed the sales of equipment for the other fieldbuses. Few analog only field transmitters are being sold by the major instrument suppliers. This occurred because HART enables the instrument range to be set using software at any time after the instrument is manufactured using a commonly available HART handheld communicator or computer-based configuration software.

Instrument suppliers have been offering HART instruments with wide-range transducers to exploit this advantage. This allows the manufacturer to make fewer models to meet the range needs of their customers. For example, analog pressure transmitters may have been manufactured with maximum full scale ranges of 50, 100, 200, and 1000 psig. A single HART transmitter with a maximum range of 1000 psig can meet these needs. Prior to shipment, the range values are set with a HART terminal to meet the specifications of the user. As a result, the manufacturer needs to make only one model for these ranges. This production volume of the model decreases inventory and reduces manufacturing cost.

Due to the wider rangeability of HART instruments, both the supplier and end user need to stock fewer models of field instruments to meet their range needs. This lowers their inventory cost and makes spare parts more universal. These inventory cost reductions have caused many users to specify only HART transmitters, even when the digital data capability of HART is not used.

HART instruments have the capability to scale their analog output values anywhere within the hardware limits of the transmitter. The universal parameters of the transmitter configuration can be modified using a HART handheld communicator or another configuration tool such as application software for a personal computer. HART configuration depends on the form of configuration tools used and the Device Descriptions supported by the supplier of the device. Device Descriptions that are available in the configuration tool can be used to more fully configure the instrument and display internal data. As a result, HART devices are able to provide multiple measurements and diagnostic capabilities.

Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control

Click here to go to Part 2
Click here to go to Part 3
Click here to go to Part 4

ISSN 1538-5280

Spitzer and Boyes, LLC
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