FOUNDATION Fieldbus instrumentation Introduction and Design Philosophy
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 chapter is devoted to a discussion of FOUNDATION Fieldbus instrumentation, building on general concepts of digital data acquisition and communication previously explored in this book.
For brevity, “FOUNDATION Fieldbus” will be abbreviated as FF throughout the rest of this article series.
This particular 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.
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.
It is even possible to build a control system around a DCS using all digital field instruments, using a protocol such as Profibus PA to exchange process variable (PV) and manipulated variable (MV) signals to and from the DCS controllers:
Now, multivariable field instruments have the ability to quickly exchange their data with the DCS, along with maintenance-related information (calibration ranges, error messages, and alarms). Each “fieldbus” cable is a (potential) two-way path for digital information flow. Field wiring is reduced in cable length and connection count due to the use of coupling devices to connect multiple instruments to single “home run” network cables leading to the DCS. Still, however, all the automatic control algorithms are implemented in the DCS.
An FF system, by contrast, allows the embedding of all control algorithms within the field instruments rather than relying on the DCS controllers to execute automatic “decisions.” In fact, the DCS would not even be necessary if not for the need of operations personnel to monitor and alter control system status:
That being said, it is possible (and in fact common) for control algorithms to be placed in the DCS controllers in addition to algorithms executed by FF field devices.
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. The bulk of this chapter will focus on H1 rather than HSE.