Adoption of a standard digital communication interface for measurement and control devices promises many benefits, especially the ability to link various types of field sensing and control devices, and reduced wiring and maintenance costs.

As microprocessor-based intelligent devices increasingly penetrate the markets for both process and discrete sensors, numerous other benefits are expected to emerge. For example, a communication bus with smart pressure transmitters and smart valve positioners will enable users in the process industries to perform closed-loop control and other monitoring functions.

What is fieldbus?

Fieldbus is a digital communication standard used to link multiple measurement and control devices. A fieldbus configuration typically consists of a twisted pair of wires to which devices, such as sensors, can be easily added or removed. Other options range from point-to-point connections between two devices to more complex configurations.

The fieldbus standard was originally targeted as a digital replacement for the 4-20 mA standard, which provides analog transmission of process variables.

The one-way transmission capability of a 4-20 mA analog system often creates an information bottleneck when used with intelligent sensors and a digital control system. This bottleneck was the primary catalyst for developing a fieldbus system with bidirectional capabilities. But the realm of possibilities involving microprocessor- based field devices linked by a digital system has greatly expanded the potential of such a standard. For example, it now includes a high-speed link for control devices such as contactors, drives, and PLCs.

Fieldbus is a network similar to control or business information networks in process plants. But, these networks reside in different layers in the plant automation hierarchy and have different characteristics. Fieldbus is similar to a control network because they both transmit data. Also, both fieldbus and control networks support a small number of devices, typically five or six, due to power constraints. However, major differences lie in the areas of message size, sophistication of the protocol (see box), interface cost, and network speed. Because fieldbus links measurement and control devices, it also differs from local area networks (LAN) which are designed to network computers.

Fieldbus developments

Many communications protocols are vying for international recognition, Figure 1. The major ones include IEC/SP50, which represents the efforts of ISA and IEC; FIP, the French national standard; and Profibus, the German national standard.

Until recently, the ISA was the focal point for North American fieldbus development, whereas FIP and Profibus were the focus in Europe. Now, groups such as the Interoperable Systems Project and WorldFIP have tossed their hats into the fray. Interbus S recently became a German national standard because of its increasing use in Europe. Allen-Bradley adopted another German standard, called CAN, in their new drives because of its low cost and simplicity.

The fieldbus standard uses the framework of the ISO 7 layer model (which defines the structure of communication networks) in which each layer performs a specific function. Fieldbus however, embodies only layers 1, 2, and 7, called the physical, data link, and application layers, respectively. ISA’s SP50 design also includes layer 8, called the user layer. The physical layer connects devices, the datalink layer detects errors, the application layer formats data, and the user layer translates data into useful information such as PID blocks, Figure 2. In the user layer, control system variables are represented as standard function blocks.

Fieldbus bears little resemblance to the 4-20 mA standard it was intended to replace. Beyond the use of digital technology, major differences between these standards include the number of devices that each system can support and the number of communication layers in each. Software is a key component in the fieldbus standard, whereas hardware was the focus in the 4-20 mA standard. These characteristics contribute to the complexity and higher cost of fieldbus, Table 1.

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