May 1999 Supplement  Feature Article

Foundation fieldbus helps meet regulatory and profitability challenges

The technology simplifies field-level communications and provides new functionality

D. A. Glanzer, Fieldbus Foundation, Austin, Texas

oday, fieldbus — a general term for digital communications protocols that allow devices in the field, such as transmitters, analyzers, etc., to communicate bidirectionally with control systems — is providing oil and gas producers and processors with a valuable tool to meet business challenges. For example, fieldbus’ multidrop capabilities (the ability to attach numerous field devices to a single fieldbus connection) and the self-diagnostic capabilities of fieldbus instruments simplify network topologies and aid in maintenance tasks. Those are big advantages in an industry dependent on remote, unmanned offshore platforms or vast onshore oil fields in difficult-to-reach areas.

Because fieldbus networks and instrumentation can provide unprecedented richness of data and higher degrees of accuracy, they are improving the industry’s ability to "tune" its processes to comply with and report on environmental regulatory compliance. Measurement accuracy and improved repeatability also contribute to the industry’s ability to accurately verify product volume in custody transfer.

But despite these capabilities, lack of a single international standard for fieldbus has held back many suppliers and end users from full implementation of this technology.

Developing a single standard. The controls and instrumentation industry, as well as keenly-interested end users, expended a great deal of time and energy from the mid-1980s to the mid-1990s striving to define a single, global fieldbus. A number of companies had developed their own digital and hybrid analog / digital field protocols, but most were proprietary — meaning a control system provider’s systems could communicate digitally only with its own instruments or those from a small handful of select companies. In this proprietary environment, the user lacked the ability to "mix and match" instruments and control systems. Since no single vendor supplied all of the instrumentation a company might need, users clamored for a single, agreed-upon fieldbus protocol.

Building upon the work of the joint IEC / ISA fieldbus committee, as well as the efforts of two earlier industry consortiums, WorldFIP and the Interoperable Systems Project (ISP), major controls suppliers and end users joined together in 1994 to form the not-for-profit Fieldbus Foundation. Headquartered in Austin, Texas, this 125-member organization includes virtually all leading systems and instrumentation providers throughout the world.

Working in a cooperative environment, Fieldbus Foundation members developed a technically-advanced fieldbus protocol known as Foundation fieldbus. With its ability to support intrinsic safety applications, Foundation fieldbus has a number of features and qualities that make it an ideal control network solution for the oil and gas industry. It is a genuinely open protocol, controlled by no single member company, and is in compliance with the pending international fieldbus standard, IEC 61158.

Foundation fieldbus has a number of important advantages over some other bus networks and the existing 4-20 mA analog standard for field communications.

Simplified wiring and installation. Analog communication requires each field device be connected to the controller by a twisted pair of copper wires, resulting in complex wiring schemes, many points of potential failure and a need for considerable wiring maintenance. Foundation fieldbus, on the other hand, uses a multi-drop bus configuration, where multiple instruments are wired to a bus, requiring a single connection to the control system.

The result is a simpler wiring scheme, a need for fewer marshalling cabinets and intrinsic safety barriers, simplified wiring diagrams and an overall reduction in maintenance problems. Simplified wiring schemes afforded by Foundation fieldbus also reduce labor during system installation and commissioning.

Easier configuration and calibration. Unlike analog field systems, Foundation fieldbus’ communications are bidirectional. This means instruments can be configured directly from the control system, in addition to the instruments sending back process data.

Enhanced device diagnostics. Foundation fieldbus systems have the ability to carry self-diagnostic data from field devices back to the control system, immediately alerting operators to potential trouble points in the field. This capability also lends itself to improved predictive maintenance capabilities.

Expanded process view. Using fieldbus communications, Foundation devices can deliver data on multiple variables, reducing the number of instruments in the field and increasing the volume of valuable data, giving operators a detailed "picture" of the state of their processes. The high-resolution and distortion-free digital signals of fieldbus enable improved control capabilities that can increase product yields.

Increased control flexibility. Devices on a Foundation fieldbus are able to communicate with each other on a peer-to-peer basis. This level of flexibility enables end users to locate process control where it best meets their needs — in the host system, in the field devices, or in both locations.

Understanding the technology. The Foundation protocol is based on the seven-layer OSI communications model, which is the standard template for network communications. However, Foundation fieldbus also features an eighth layer — a User Layer — containing tools that deliver an unsurpassed level of device interoperability (Fig. 1).

Interoperability allows a fieldbus instrument to be substituted with a device from another supplier without loss of functionality or degree of integration with the control system or host. In other words, interoperability allows the user to choose the right devices for a measurement and control application independent of the host supplier or providers of other devices on the network.

The Foundation fieldbus User Layer currently contains 21 standard function blocks that encapsulate common control functions, such as analog input / output and PID control. These include:

The Fieldbus Foundation is continuing to develop additional function blocks to address industry needs.

Suppliers embed these blocks into fieldbus devices to achieve desired functionality (Fig. 2). For example, a simple temperature transmitter could contain an analog input (AI) function block, while a control valve may contain a PID block as well as an analog output (AO) block. Thus, a complete control loop could be built using only a simple transmitter and control valve. Foundation fieldbus’ robust, deterministic communications ensures that function block execution occurs at precisely defined intervals and in the proper sequence.

In addition, Foundation fieldbus delivers true "plug-and-play" functionality. Device Descriptions (DDs), which are part of the User Layer, are device-specific drivers that allow the network to immediately recognize the new device and its full range of capabilities (Fig. 3). Through its system management capabilities, the network automatically assigns network addresses when a new device is installed on the system and prevents duplication of addresses. The network also ensures that the clocks of all devices are synchronized, and tag search capabilities eliminate the need to replicate the system database containing device information.

The Fieldbus Foundation provides both 31.25 kbps H1, and 100 Mbps high-speed Ethernet (HSE) networks. The H1 fieldbus is intended primarily for process control applications. Typical devices on an H1 network include transmitters and actuators. The HSE fieldbus provides a backbone for integrating H1 networks. It supports complex logic functions, such as those performed by programmable logic controllers, or for data-intensive process devices, such as analyzers and gateways to other networks.

A new level of distributed control. Many of the fieldbus benefits described earlier — such as reduced wiring cost and interoperability — are well-established and are delivering benefits to companies that already have adopted Foundation fieldbus. An area in which Foundation fieldbus may have its greatest impact, however, is in allowing users to achieve a new level of distributed control.

Because Foundation fieldbus is a peer-to-peer network, built on the publisher-subscriber model, instruments on a fieldbus segment can communicate with each other without intervention or mediation of a supervisory host. This enables users to program control routines directly into the field devices. The result, in many cases, will be greater responsiveness as well as faster and more accurate control that will allow companies to run their processes with tighter tolerances.

The ability to distribute control between the host system and the field network is one of the many key differentiators between Foundation fieldbus and some other "fieldbus" solutions.

For example, bus architectures that lack DD technology have limited interoperability between compliant instrumentation. Such instruments conform to device "profiles" that include a basic set of functions but are generally rigid and not extensible. Adding to the capabilities of a particular device on the bus often requires custom programming.

In addition, these older, less-capable buses are typically built on a master-slave architecture, meaning that field devices do not communicate with each other peer-to-peer. For these reasons, most other fieldbus systems can only support rudimentary field-level data acquisition functions, and do not allow end users to take full advantage of the capabilities of modern "smart" fieldbus instrumentation.

Registered products available. Aside from the unique attributes of its technology, the Fieldbus Foundation is the only industry organization with a formal process for registering suppliers’ fieldbus devices. As of this writing, there are approximately 30 registered devices from a growing list of controls and instrumentation manufacturers worldwide. Users purchasing fieldbus products with the official Foundation registration mark can be certain these devices provide all of the capabilities of Fieldbus Foundation’s technology.

The Fieldbus Foundation proceeds with the registration process only after devices from multiple suppliers, containing communications "stacks" from different developers, have passed interoperability tests. These interoperability tests are conducted by the Foundation at its independent laboratory in Austin, Texas.

However, passing interoperability tests alone are not sufficient for Foundation registration; products must meet all of the requirements of the device registration process to be officially registered. As specified by the registration process, suppliers must provide evidence that their Device Under Test (DUT) fieldbus physical interface conforms to the Foundation fieldbus Physical Layer Conformance Test Specification, and that the DUT contains an unmodified, Foundation-registered communications stack.

The registration process also requires the DUT to pass an interoperability test of its standard function blocks, resource blocks and system / network management functions. This test is conducted using an Interoperability Test System (ITS) developed by the respected Fraunhofer Institute in Karlsruhe, Germany. The ITS’ test engine executes test library functions (macros) and more than 300 individual test cases which exercise the DUT implementation. A test function block in the ITS tests function block input and output parameters in the DUT User Layer that are configured and linked to other blocks. Additionally, the ITS verifies that Object Dictionary (OD) extension fields for standard function blocks are correct.

A DD interoperability test for the DUT verifies that its DD and OD extension fields match the specifications for the standard function blocks and standard resource block, and checks the DD and OD extension fields for standard transducer blocks, supplier-defined transducer blocks and any other supplier-defined parameters.

Once all of the registration requirements have been met, the Fieldbus Foundation issues a Device Registration Agreement for each registered device. If a registered device’s User Layer or communication stack is later modified (any electronic hardware or software change), it must be retested and reregistered through the device registration process. FB

The author

David A. Glanzer is director of technology development for the Fieldbus Foundation in Austin, Texas. He has a BS degree in electrical engineering from the South Dakota School of Mines and Technology, and is a member of the American Society of Quality(ASQ), the Industrial Computing Society (ICS) and a senior member of ISA (The international society for measurement and control).

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