FIELD TRIALS TO PROVIDE EXPERIENCE FOR FIELD-BUS STANDARDIZATION

To gain actual field experience on digital communications between various sensing and control devices in the processing industry, the International Field-Bus Group will begin field trials in 1992.

Twenty-five companies will sponsor the multivendor field tests. They signed the "Field-Bus Field Trial Installation Unilateral Declaration" during April (see table).

The primary objectives of the trials are: To gain practical experience with digital communications between sensors, actuators, and control-room equipment for applications in the process industries, and to share information and knowledge gained from the trials with the various field-bus standards committees.

Increased user and vendor participation is also needed to further the development of a digital communications standard, according to Martin Zielinski, communications manager at Rosemount Inc., Eden Prairie, Minn.

Two levels of involvement in the field test project have been defined.

Members of the field-bus group that are actively involved with the project will supply field-bus communications equipment, provide field test site system designs, or provide the test sites to conduct the trials.

Other members will regularly promote the field-bus standard through the development, presentation, and publication of technical papers. The technical topics include: physical layer, data link layer, applications layer, user display interface, and user application algorithms.

Information concerning low-power instrumentation design to achieve intrinsically safe installations and power on the line will also be accomplished by the field trials. However, the group will not develop any products for sale in the marketplace as a result of the trials, according to Zielinski.

The preliminary schedule for the trials calls for specifications to be completed by August 1990, prototypes to be completed by September 1991, joint testing to be completed by March 1992, and the first field trial results to be presented at the Interkama/ISA trade show in 1992.

FIELD-BUS STANDARDIZATION

Field bus is a term which describes a totally digital communications protocol for all field measurement and control devices. Presently, there are analog as well as digital devices in use in process plants.

Because of the large number of device vendors, and because many of the devices accomplish the same control or measurement functions differently, there is a need to standardize the field-bus communications protocol (OGJ, Feb. 14, 1989, p. 42). There are already a number of initiatives throughout the world to develop a field-bus standard, according to James Cobb, project manager and chemical industry marketing manager for Rosemount Inc.

The most active organizations are the Instrument Society of America (ISA) Standards and Practices committee (SP50), the European Eureka Commission, the German Profibus effort, and the French FIP Club. These organizations, as well as other national standards committees, are also discussing field bus issues at the International Electrotechnical Commission (IEC), TC 65C, Working Group 6.

The ultimate objective is to gain agreement for a single, international field-bus standard.

It is common practice in Europe for national standards organizations to have restricted membership rules that allow the organizations to focus their efforts, according to Cobb. For instance, both the German and French field-bus standardization efforts have resulted in national standards that serve the individual constituencies of those countries.

The standards developed have been presented to ISA and IEC, but because the standards were developed in isolation, their acceptance beyond national borders has been limited, says Cobb.

ISA SP50 has open membership, including principals involved with Profibus, FIP, and other European and Japanese manufacturers. ISA also requires that the membership be balanced with both manufacturers and users.

Without a standardized field-bus communications protocol, multiple protocols would emerge, says Cobb. Users would be faced with the difficult task of determining which vendor offered the best combined control system elements.

Once that vendor were selected, changing to a different vendor to take advantage of new developments would be difficult, if not impossible, because each vendor's devices would likely use a different communications protocol.

In a large hydrocarbon processing complex, there could be multiple control systems, each with a different communications protocol. This would add to the maintenance cost of the systems, and it would not allow easy transfer of information, or integration of data, from one process unit to another.

A standardized field bus would allow the ability to utilize the full potential of microprocessor-based field instruments, along with the ability to apply the technology to a broad line of products, says Cobb.

This would allow inter changeability among the various devices.