Building a better fieldbus

Building a better fieldbus

by Greg Paula, Associate Editor

Fieldbuses such as DeviceNet are allowing intelligence to be distributed throughout a control system, offering users a variety of benefits including reduced wiring, easier configuration, and enhanced diagnostics

Devices such as sensors, valves, and actuators for newer industrial control systems are increasingly being designed and manufactured with built-in intelligence. To take advantage of all the benefits of having intelligence distributed throughout a control system, however, it is necessary to use a communications network, commonly known as a fieldbus.

A DeviceNet-compliant transducer gauges vacuum pressures from 1 X 10^-2to 1 X 10^-10 torr by measuring the ionization potential of the gas

One relatively new fieldbus is DeviceNet, developed in 1994 by Allen-Bradley (now known as Rockwell Automation Inc.) in Mayfield Heights, Ohio, and turned over to an independent organization, the Open DeviceNet Vendor Association (ODVA) in Coral Springs, Fla. Since DeviceNet is an open standard, its protocol and specifications are available without a license to any automation supplier to build compliant components. The number of devices that comply with the DeviceNet standard has tripled since 1995.

One big advantage is reduced wiring. Total installation cost is typically up to 40 percent lower than that of a traditional configuration. The other major cost savings from DeviceNet come from productivity increases. The intelligent devices can help locate problems quickly and can also predict failure. The system can be PC-based and devices can be swapped and added while the power is on. Rockwell claims that DeviceNet's life-cycle costs are 30 to 40 percent less than those of traditional systems.

Devices that comply with the DeviceNet protocol use the Controller Area Network (CAN) chip, developed by Bosch Automation Products in Santa Clara, Calif., for the auto industry. A CAN chip goes into each node on the network and enables the device to initiate and respond to messages on the wire. DeviceNet uses a producer/consumer model (rather than source/destination) to send and receive information, which can dramatically reduce network traffic and the load on node resources.

Interest in DeviceNet among chip manufacturers has recently increased dramatically. MKS Instruments Inc. in Andover, Mass., has shipped DeviceNet-compliant transducers and received positive feedback.

The above was adapted from an article by Greg Paula, Associate Editor. The full text may be found in the June 1997 issue of Mechanical Engineering magazine. © 1997 ASME International.© To obtain a copy of this issue, click here.