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Fieldbus Technology
(Part 3 of 5)

By Dick Caro

E-Zine March 2010

Click here to review Part 1
Click here to review Part 2

HART began as an ISA SP50 proposal to enable backwards compatibility with analog field instrumentation. HART transmits its digital data superimposed on a 4-20 mA signal allowing the field instrument to continue to supply its primary data value in analog form with secondary data in digital format. However, due to the limitations of the analog current loop, the digital data rate at the time of design was limited to 1200 bps. The consensus has generally been to keep HART in its present form and to use PROFIBUS-PA or FOUNDATION™ Fieldbus if more digital capability is required. However, faster communication can be achieved by implementing the HART C8PSK specification that can operate at 9600 bps simultaneously with existing 1200 bps HART field instruments. As of this writing (mid-2007), a wireless version of HART is being developed but there have been no public information releases about it.

HART is the most widely used fieldbus and is anticipated to maintain this market position in the immediate future. The installed base of HART instruments is large since many suppliers of field instrumentation have discontinued producing pure analog instruments and supply only HART devices. In many cases, HART instruments have a lower overall production cost as compared to analog instruments, since one model can cover many common ranges of analog instruments. This enables suppliers and users to stock fewer devices in inventory.

The single most appealing factor in the popularity of HART instruments is the capability to change their range while in use, without being removed from the process. Further, the ability to change the range remotely from anywhere that the HART wiring is accessible, reduces the cost of instrument maintenance by reducing the need to have physical access to an instrument to make changes. Wireless HART extends this idea by making access to any wireless HART-enabled instrument accessible via the wireless link without needing to access wiring at all.

HART transmitters and control valve positioners have the ability to diagnose problems before they become serious. The HART processor can have direct access to various parts of the field instrument and can detect excessive internal temperatures, non-responsive components, and stickiness of control valves, before operational problems occur. However, this requires either a diligent field instrumentation team to check the HART digital data, or the ability to run diagnostics in controllers equipped to directly read HART data values and diagnostic results. Newer distributed control systems (DCS) often have this capability, but distributed control systems (DCS) without a direct HART interface (only 4-20mA) do not. Reliance on the field instrument maintenance team with a portable handheld HART reader has generally proven impractical to take advantage of the features offered by HART digital diagnostics.

HART devices are wired the same as 4-20 mA analog instrumentation, including the requirement for continuity of the wire pair from the field instrument to the controllerís terminations. Most new controllers can accept the digital content of HART on the same terminations that are used for the 4-20 mA analog primary variable measurement value.

HART field transmitters and control valve positioners are available from most of the same suppliers that sold analog equivalents. Typically, HART devices cost no more than the analog devices that they replaced.

Click here to read Part 4

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

ISSN 1538-5280

Spitzer and Boyes, LLC
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