Overview of Fieldbus Technologies 

The trend in many communication networks is changing from analog toward all-digital communication. This trend is also occurring in process control networks. Digital communication provides a means for increasing the amount of data communicated on a single cable between devices such as transmitters, valve positioners, controllers and computer servers. Analog signals can carry one to a few hundred different variables on a single cable while thousands and even millions of pieces of data can be communicated along just one digital network cable. This makes it possible to extract more information from each field device than is realistically possible with analog signals.

The evolution of digital communication and today’s network protocols has been a long and bumpy road. It began with many vendor specific and proprietary protocols. Devices could only work with products from the same vendor. In 1985, industry experts began work on a vendor-independent fieldbus standard that could be supported by multiple vendors. Many delays, bus wars, organizations formed, split and merged to eventually develop the IEC 61158 Fieldbus Standard employed in process control networks.

Process control networks are used in factory and process automation systems. There are distinct differences with unique characteristics and varying requirements between tasks performed for applications in these very different industry sectors. Therefore, industrial fieldbuses are optimized for different applications.

Factory automation involves fast-moving machinery and very quick action by discrete I/O devices. Automotive, bottling, packaging and other assembly-line manufacturing are predominantly controlled using discrete logic and sensors. Traditionally, these tasks have been handled by PLCs. Discrete I/O networks are generally characterized by low overhead and small data packets necessary to achieve the fast response times. AS-Interface (AS-I), DeviceNet, ControlNet and PROFIBUS DP are common in these manufacturing processes.

Process automation requires continuous regulatory control. Oil and gas processing, pulp & paper, power generation and chemical processes utilize scalar measurement devices and continuous modulated control. Process-related networks include FOUNDATION fieldbus™, PROFIBUS PA and HART. (Note: most HART installations today do not use the digital communication capability that is superimposed on a conventional 4-20 mA signal.) These fieldbus protocols are designed for remote configuration and continuous processing with bus-powered field instruments.

For several years now, many instrument and network equipment manufacturers have been delivering products based on these open standards and specifications. Although we expect the technologies will continue to evolve, they have all been field-proven and the right fieldbus for your specific application is ready today.