These networks include Profibus, DeviceNet, Foundation Fieldbus (FF) and HART, among others.
On the whole, having a choice in network technology is a good thing. However, it forces automation system and device vendors to support multiple competing networks with iterations of communication drivers. And manufacturing users, for a variety of reasons, are often stuck with supporting multiple protocols, including the inherent training, configuration software, documentation and programming tools that entails.
A group of automation suppliers is proposing a different solution—one that is simple in concept yet requires extensive collaboration among vendors to make it work.
The Field Device Tool (FDT) specification standardizes the interface between intelligent field devices and host systems, regardless of device vendor or fieldbus protocol.
Device manufacturers include an FDT-compliant piece of software in their devices, called a Device Type Manager (DTM). This is used by an FDT-compliant frame application in the host to communicate with devices across different protocols. An oft-cited analogy is that of printer drivers, which allow multiple computer applications to access different printers with little user effort.
History in the making
FDT was originally conceived by a group of European companies, led by automation conglomerate ABB, in the late 1990s. After development, the specification was turned over to the Profibus Nutzerorganisation (PNO), with overview by the German-based ZVEI, a standards body equivalent to the IEEE in the U.S. ABB, and other vendors, have a number of customer applications running FDT-enabled devices at sites in Europe and Asia, including a BASF chemical plant in Germany.
The FDT specification made its U.S. debut at the ISA Expo 2002, when nine member companies of the FDT Group—ABB, Endress + Hauser, Invensys Foxboro, ifak system, M&M Software, Metso Automation, Smar, Softing, and Vega—demonstrated FDT interoperability for Profibus and HART protocols.
At ISA Expo 2003, the group announced formation of the FDT Joint Interest Group (JIG) as well as the launch of a Web site (www.fdt-jig.org), the release of test and specification software and the joint cooperation with the OPC Foundation for “plug-and-play” interoperability. New members joined the JIG, which, in addition to the nine companies previously mentioned, now includes Omron, Pepperl &Fuchs, Rockwell Automation, Sick and Siemens, among others. Nick Zucchero, engineering manager for the Invensys Fieldbus Program Group, and the JIG steering committee member from Invensys, says that much of the momentum for FDT has occurred in the last year.
How it works
According to Zucchero, user benefits of FDT can be summarized as, “End users can choose the best-fit fieldbus device products and integrate them into a single configuration platform. Any field device from any device manufacturer, connected by any fieldbus communications protocol, can be commissioned and managed in one engineering environment. This reduces total cost of ownership, lowers maintenance costs and frees engineering resources.”
Automation suppliers benefit by minimizing the number of configurators that they need to maintain for their devices. Device suppliers can focus on their key competitive differentiators.
There are three major components to the FDT technology, explains Zucchero. The first is the communications DTM, which is an executable piece of code created by the control systems supplier to make the bus protocol transparent to the device. On the field side, the communications DTM knows how to speak HART, FF or Profibus, for example. On the host side, it knows how to speak “DTM interface.”
The second piece is the device DTM, or what Zucchero refers to as “a DD (Device Description) on steroids.” It characterizes the field device for its parameters, visualization, and other sophisticated functions, such as linearization or valve signature calculations.
The third component is the FDT frame application, which sits in the workstation and communicates with devices. It knows how to invoke the functionality contained in the device DTMs.
As long as all of these pieces are FDT-compliant, they will interoperate across multiple manufacturers’ devices and fieldbus protocols. Users do not have to bulldoze their existing fieldbus-connected devices, says Zucchero, because the FDT software is layered on top of the application.
Jane Gerold, firstname.lastname@example.org