Fieldbus Wars, Ten Years Later

This year marks the tenth general assemblies of both the Profibus Trade Organization (the North American marketing arm of Profibus International, www.profibus.com) and the Open Devicenet Vendors Association (ODVA, www.odva.org).

Indications point to a resumption of the “fieldbus wars” that generated much trade press ink in the late 1990s and early 21st century. Each organization has recently begun communication initiatives touting their unique benefits.

Visionary engineers in the late 1980s looked at the costs, troubleshooting headaches and maintenance expense associated with parallel wiring of field devices to input/output (I/O) points of controllers. There had to be a better method of installing and maintaining control wiring. So, they set to work developing industrial networks called fieldbuses. Instead of individual wires connecting each field device—switches, motor starters, sensors and the like—to a corresponding I/O point, engineers could design systems in which all devices communicated on one cable.

The original idea was to create one unified standard industrial network, used by every supplier. This would be heaven for users, who would be able to buy whichever control system they wished and simply plug it in. Unfortunately, some companies began to think that a competitor was gaining an “unfair” advantage with the fieldbus it promoted and several began to develop their own. The long and short of the situation is that the International Electrotechnical Commission (IEC) decided to legitimize many fieldbuses as “standard.” Each standard is owned by an independent organization that controls development of its own standard. Some of these networks are designed primarily for discrete devices, while others readily handle the analog signals commonly found in process manufacturing.

ARC Advisory Group, a manufacturing analyst company located in Dedham, Mass. (www.arcweb.com), released a white paper in May 2004 by analyst David Humphrey, who studied the characteristics of Profibus and the industries where it would hold the most promise. Regarding fieldbuses in general, Humphreys notes, “While the use of a fieldbus can reduce installation and engineering costs, the greatest benefits are realized in the areas of maintenance and operations, long after a plant has been commissioned.”

Analyzing various industry segments, Humphrey says, “In typical hybrid industries, fieldbuses can contribute significantly to the achievement of operational excellence.” Hybrid production processes are made up of both process and discrete applications. Humphrey’s conclusion is that Profibus, while a widely used network for discrete industries, offers also a common communication layer to its implementation of a process industry-oriented network. Therefore it offers a value proposition to the hybrid industries.

On the other hand, Katherine Voss, ODVA executive director, also points to a common communications protocol that enhances intercommunication among different networking applications. This protocol is called “common industrial protocol (CIP)” and is used by DeviceNet, ControlNet, EtherNet/IP, CIP Safety and CIP Sync.

DeviceNet is a fieldbus suited for discrete manufacturing. Implementing CIP as part of the communication protocol, data from machine level can be passed seamlessly to the cell controller level or business level computers over EtherNet/IP—an implementation of Ethernet using CIP.

CIP is based on producer-consumer technology. In this implementation, messages identify the type of data contained in the frame rather than the destination for which the data are intended. Thus, data contained in a single frame may be consumed by multiple devices, and data intended for multiple devices and contained within a single frame need be produced only once.

Ethernet, the standard business network, is extending to the factory floor as a transport for the real-time information required by enterprise applications. For example, Gary Workman, staff development engineer at General Motors Corp., says, “To meet our needs, we wanted an Ethernet implementation that is open, readily available, capable of real-time data delivery and uses standard infrastructure devices. The EtherNet/IP networking solution clearly meets all these criteria.”

This short report cannot delve deeply into each fieldbus. However, it should serve notice that readers should expect renewed competition among the various fieldbuses.

Gary Mintchell, gmintchell@automationworld.com

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