1661900067521 Automationnetselection

Selecting an automation network

Oct. 28, 2004
With many projects requiring more than one network application, finding the network that best fits your needs is no easy task. This perspective will help get you started on the right path.


re you trying to make sense of all the different industrial automation networks on the market today? Whether you're a novice industrial network user or someone who simply needs to brush up on the technology, Automation Network Selection will help you better understand and select the "right" network for a given application. Author Dick Caro walks you through the various industrial networks (e.g., sensor, fieldbus, control, safety buses, and so on) and then gives you a perspective on the typical applications for industrial automation network technology. The emphasis is on the intended application for each network, rather than on the network protocol that is more typically discussed in most textbooks.

All process industries in general and water/wastewater in particular will be migrating to some form of fieldbus systems over the next half-decade. Caro’s book tells you why. The various fieldbus products are described and key features of fieldbus vs. current common buses and networks are identified and compared. The book is recommended for all technical persons from technicians to plant managers—anyone who may benefit by gaining a decision-making understanding of networking in the process industries.

An appropriate subtitle for this book could well be: All You Wanted to Know About Fieldbus But Were Afraid to Ask. It is written at a technician level; in a text format. The trend to integrating Ethernet to device levels is well covered, as are special needs of process networks such as intrinsic safety and powering devices from the network.

Author Dick Caro merits special mention. His process control engineering career parallels major advances in process control technology along with the emergence of digital methods for data handling & networking. He served on the ISA SP-50 and the IEC SC65/WG6 Fieldbus committees for over a decade; about half as the Chair. Leaders of these committees suffered through exasperating company and international pressures.

Why are there so many different industrial automation networks? Why can’t there be just one network? We hear these questions very often. Indeed, it could have been, but… Well that would be a long story too.

Here is the short version. In 1985, a bunch of us recognized the need for standardizing network communications for process control and we tried to prepare a standard in advance of the competition under ISA’s SP50 standards committee. We called our effort fieldbus since it was meant for field instruments. Almost immediately, we were joined by suppliers of programmable logic controllers (PLCs) who believed that they needed to standardize upon their remote I/O networks and we could share the same technology. We completed the physical layer protocol (wiring/cabling and signaling) in 1989 and the data link layer protocol in 1993. However, by then there were already numerous competitive commercial networks. The standards work was eventually completed in 1999 with the adoption of many of these commercial network architectures into the international fieldbus standard IEC 61158, which contains eight different protocols.

Market forces could not wait for a standard, and the standard could not incorporate all market forces into a single protocol. Not only that, but a standard cannot immediately, if ever, displace already implemented commercial products. Five of the network technologies included in IEC Fieldbus standard represent commercial products other than the two prepared by the Fieldbus Foundation based on the parts of IEC 61158 that were developed as the ISA S50.02 Fieldbus standard. One protocol has never been implemented.

I chaired both ISA SP50 and the IEC SC65C/WG6 Fieldbus committees for the end of their efforts. They were very turbulent times. Cullen Langford, a user from DuPont, chaired these committees during most of the time that they were creating the fieldbus protocol, while I was a contributing member of the user layer subcommittee. Being surrounded by some of the brightest people in the universe was a rare privilege and the experience of a lifetime. I didn’t enjoy many of the political moves being made by major manufacturers, but it did provide a challenge to my management style.

While politics was raging in the ISA/IEC committees, discrete automation identified by the use of PLCs was also developing rapidly. For a while, it appeared that one of the standards bodies, NEMA (National Electrical Manufacturer’s Association), was going to use the highspeed version (H2) of ISA’s fieldbus. However, NEMA concluded that H2 standards were too expensive and too complex for use in discrete manufacturing. As a result, most of the data communications buses developed by PLC manufacturers were incompatible with each other and did not conform to a standard set of specifications. Most of the manufacturers countered criticism about the “closed” nature of these protocols by forming “open” groups 1-2 to make the long-term evolution for each of these bus technologies independent from the originating manufacturer. These open bus associations contributed five of the additional protocols to the IEC eight-part fieldbus standard.

In retrospect, it is clear that no single bus technology can satisfy the demands for multiple applications in the manufacturing marketplace. This is a political, not a technological, statement. We always knew that the wiring, cabling, and connecting solutions embedded in the physical layer could not span all markets, but the committees used the technique known as the “meld of best features” method of standardization to combine the simple elegance of WorldFIP with the pragmatism of Profibus. In the process, we succeeded in developing a single protocol meeting the needs of both process control and factory automation, but with a complexity that caused it to not be accepted for discrete manufacturing. It was over these issues that the final approval of the fieldbus standard was delayed for seven years.

Also dating from the late 1980s were the developments of very simple buses for sensors. These, too, have been developed by independent manufacturers and typically share nothing in common with each other or with any of the higher level bus architectures. They were designed for low cost and low complexity. However, this did not prevent some of these bus structures from being promoted by their sponsors for higher level applications, thereby adding to the confusion.

This book gives you a perspective on the typical applications for industrial automation bus technology. The emphasis is upon the intended application for each bus, rather than the range of applications for each bus, which you would find in the supplier’s literature. With that goes a note of caution: Any bus can generally be used for any application; however, stretching a bus technology outside its intended area creates more problems than it solves. We will begin by discussing some bus applications and propose the bus technologies that should be used to provide the needed communications services. Often, several different bus solutions will be appropriate, and their differences in the application context will be discussed. Then we will discuss the bus technologies and requirements from an end user point of view. Although the bus technology sections will talk briefly about protocol used on the bus, the emphasis is on the wiring/cabling issues and the user interfaces. This book is generally free of mathematics except in those areas where the numbers have real application relevance.

Finally, I have always maintained my independence from all manufacturers to retain an unbiased viewpoint. This is an unbiased book, except for my fondness for the fine points of the fieldbus standard. I continue to be amazed at how little this formalized body of knowledge is used in industry, and I do reference some of these fine points.

Reviewed by Ken Ball

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