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Beyond FISCO – split architecture solution

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Modern digital control systems have greatly improved process management and industrial automation, allowing far more exacting process control.

However, when it came to the hazardous environments often encountered in many chemical, pharmaceutical, plastics and petroleum plants communication protocols such as fieldbus initially fell short when intrinsically safe (IS) techniques were required.

In response, the Entity Concept came along. However, it only allowed 80mA through the IS barrier, enough to drive just four FOUNDATION fieldbus or PROFIBUS PA devices -- at an average draw of 20mA per PLC, sensor, or transmitter.

Taking the next step, FISCO was developed in the late 1990s. It succeeded in making more current available in hazardous locations -- a full 115mA in worst-case (hydrogen) areas, enough to comfortably drive five devices. Still, I & C engineers needed more PLC and sensor devices to improve process management at their plants.

The split architecture solution

The capacity barrier of FISCO has now been recently surpassed by a novel split architecture design that employs a field-mounted device coupler and an associated power supply with a safe-area interface.

Engineers at MooreHawke -- a division of Moore Industries -International, interface solution experts based in North Hills, California -- developed this new technology that meets the total resistance requirement via a split resistance approach: a small resistor is used in the I.S. interface and a larger resistor is placed in the field device coupler.

The small (trunk) resistor "sees" a large current (sum of all devices), but only generates a small voltage drop. The larger (spur) resistor "sees" a small current (single device) and also only generates a small voltage drop.

Subsequently approved by the FM (US) and SIRA (ATEX) certifying organisations, this design enables intrinsically safe segments that support up to 350mA -- enough to power 16 DCS devices, like a PLC or sensor, while still being intrinsically safe for hydrogen at the individual spur connection.

This power -- along with having access to a full 1900 metres per trunk length and 120 metres per spur -- allows systems designers and plant management personnel to apply the operational advantages of FOUNDATION fieldbus <http://www.miinet.com/moorehawke> or PROFIBUS PA to all hazardous areas of a plant, inside or out, almost as if they were looking at a non-hazardous installation.

In terms of reliability, the split-architecture power supply steps around the complexity associated with traditional FISCO circuits by the use of a conventional wire-wound resistor which, in IS terms, is deemed to be infallible.

To further augment the overall systems reliability, the MooreHawke design also incorporates full ac/dc power conversion, simple linear power supply, and full galvanic isolation, with built in redundant supplies. In this case, fewer components translate into greater reliability, so MTTFs rise.

Considering all of the above, I & C engineers can now enjoy the best of all worlds: 1) the DCS efficiencies of FOUNDATION fieldbus and PROFIBUS PA; 2) the long cable lengths, reduced documentation, and unrestricted device benefits of the Entity Concept; and 3) vastly increased power (350 mA) that goes beyond FISCO to provide intrinsically-safe segment capacity and process management control, virtually indistinguishable from non-hazardous fieldbus implementations.

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