Introduction
FOUNDATION Fieldbus is a standard for digital field instrumentation enabling field instruments to not only communicate with each other digitally, but also to execute all continuous control algorithms (such as PID, ratio control, cascade control, feedforward control, etc.) traditionally implemented in dedicated control devices. In essence, FOUNDATION Fieldbus extends the general concept of a distributed control system (DCS) all the way to the field devices themselves. In this way, FOUNDATION Fieldbus sets itself apart as more than just another digital communication “bus” for industry – it truly represents a new way to implement measurement and control systems.
This industrial network standard was first proposed as a concept in 1984, and officially standardized by the Fieldbus Foundation (the organization overseeing all FF standards and validation) in 1996. To date, adoption of FF has been somewhat slow, mostly limited to new construction projects. One of the “selling points” of FF is decreased installation time, which makes it a more attractive technology for brand-new installations than for retrofit projects.
How FOUNDATION Fieldbus works
To understand just how different FF is from other digital instrument systems, consider a typical layout for a distributed control system (DCS), where all the calculations and logical “decisions” are made in dedicated controllers, usually taking the form of a multi-card “rack” with processor(s), analog input cards, analog output cards, and other types of I/O (input/output) cards:
Information is communicated in analog form between the DCS controllers and the field instruments. If equipped with the proper types of I/O cards, the DCS may even communicate digitally with some of the field instruments using HART protocol. This allows multivariable instruments to communicate multiple variables to and from the DCS controllers (albeit slowly) over a single wire pair
Information is communicated in analog form between the DCS controllers and the field instruments. If equipped with the proper types of I/O cards, the DCS may even communicate digitally with some of the field instruments using HART protocol. This allows multivariable instruments to communicate multiple variables to and from the DCS controllers (albeit slowly) over a single wire pair
When the FF standard was being designed, two different network levels were planned: a “low speed” network for the connection of field instruments to each other to form network segments, and a “high speed” network for use as a plant-wide “backbone” for conveying large amounts of process data over longer distances. The low-speed (field) network was designated H1, while the high-speed (plant) network was designated H2. Later in the FF standard development process, it was realized that existing Ethernet technology would address all the basic requirements of a high-speed “backbone,” and so it was decided to abandon work on the H2 standard, settling on an extension of 100 Mbps Ethernet called HSE (“High Speed Ethernet”) as the backbone FF network instead.
A minimal FF H1 segment consists of a DC power supply, a “power conditioner,” exactly two terminator resistors1(one at each extreme end of the cable), a shielded and twisted-pair cable, and of course at least two FF instruments to communicate with each other. The cable connecting each instrument to the nearest junction is called a spur (or sometimes a stub or a drop), while the cable connecting all junctions to the main power source (where a host DCS would typically be located) is called a trunk (or sometimes a home run for the section leading directly to a host system):
