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.
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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The International Electrotechnical Commission (IEC) has the primary responsibility for defining an international fieldbus standard. ISA SP50 and IEC committees meet jointly to accelerate development of the standard. The organization of both ISA and IEC committees parallels that of the protocol structure — each subcommittee is charged with developing a particular layer.
In 1990, the International Fieldbus Consortium (IFC) was formed to implement and test the IEC/SP50 fieldbus protocol. They obtained proposals for implementing and testing the protocol from different companies and consortiums. Five test sites have been identified in locations ranging from the U.S. to Europe and Asia.
FIP and WorldFIP
Factory Instrumentation Protocol (FIP) is a French national standard for networking field devices. Started in 1986, this effort also has a significant Italian influence. All three layers of FIP protocol have been defined and accepted by UTE, the French National Standards organization. Recently, several FIP projects were implemented in France and Italy.
FIP Club was created in 1987 to promote and represent FIP, encourage development of compatible products, certify products, and run the FIP Technical Center. The center, which provides training and technical assistance, has developed software for implementing and testing interoperability among FIP products. Hardware products supporting FIP include the FullFIP chip from Cegelec and the FIPIU chip from Telemecanique. FullFIP is a complete implementation of FIP protocol, whereas FIPIU supports only layers 1 and 2. FIP Club members plan to develop FIP-compatible products ranging from board-level devices, sensors, and actuators, to complete control systems.
The WorldFIP effort — supported by Honeywell, Allen-Bradley, Elsag Bailey, and Square D — encompasses the FIP Club, which has only European members. Though the WorldFIP organization was formed to hasten adoption of a standard fieldbus protocol, the scope of this system will extend beyond process automation into areas such as factory and building automation. Measurement and control products supporting WorldFIP are slated for introduction in 1994.
Profibus and ISP
Profibus, or Process Fieldbus, is a German national standard for networking field devices through the use of currently available technologies. All three layers (physical, data link, and application) have been accepted as a standard of DIN, the German national standards association. Over 100 companies have developed Profibus products, and Siemens has developed a chipset and software that supports its protocol.
Profibus was sponsored by the Federal Ministry for Research and Technology (BMFT) until 1989. Then the Profibus User Group was formed to further develop the concept and test compatible products.
The Interoperable Systems Project (ISP), championed by Rosemount, Fisher Controls, Siemens, and Yokogawa, draws heavily on Profibus technology. The ISP protocol is designed to merge the Profibus data link layer with the intrinsically safe SP50 physical layer. ISP is targeting product introductions for 1994.
Available for more than four years from Phoenix Contact, the Interbus S sensor/actuator bus was recently accepted as a German national standard. Over 200 companies worldwide support this system and nearly 10,000 systems have been installed. Because of promotion by the Drivecom User Group in Germany, many European companies have incorporated the Interbus S protocol in their drives and sensors. Key members of the Drivecom User Group include ATB Flender, Lenze, Loher, Mannesmann Demag, SEW Eurodrive, and Control Techniques.
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Originally designed for automobiles, the Controller Area Network (CAN) is used in applications ranging from industrial machinery, medical products, elevator control, and robot control, to building automation. Reasons for its growing popularity include low cost, high immunity to noise, and simplicity.
The CAN in Industrial Automation (CiA) association was recently formed to promote industrial use of this system. Low-cost chips implementing the basic protocol have been available for several years. CAN controllers with extended capabilities were recently announced by Intel, NEC, and Siemens.
CAN is already used in Mercedes Benz S cars and is slated for future BMW models. As a result, this system has a large installed base — estimated at over 5 million devices. Some of the companies supporting CAN are Allen-Bradley, Force Computers, Honeywell Control Components, Industrial Computers, Philips Medical Systems, and PEP Modular Computers.
While the supply-side battle surrounding fieldbus standardization rages on, the benefits inherent in adopting a standard communications protocol at the sensor level continue to emerge. As mentioned earlier, users see the ability to link many types of field devices and reduced wiring costs as the primary benefits. Reduced wiring is a major contributor to lowering the installed cost of measurement and control devices in both process and discrete manufacturing plants. For example, linking multiple proximity sensors or limit switches over a single wire is less expensive than the current point-to-point wiring schemes typically used.
Meanwhile, users and suppliers alike view fieldbus as an enabling technology for achieving numerous other business objectives. Thus, a universal digital link for communicating between microprocessor- based field devices will allow control to be truly distributed among numerous devices.
Accurate and plentiful process information is vitally important to precise control of today’s complex processes. Factors such as strict regulatory requirements and quality certification mandate an increase in the number of process variables that are controlled and monitored.
In this scenario, fieldbus eliminates the communication bottleneck inherent in using one-way analog transmission for interfacing between digital devices. Valuable information on the process and control devices that is stored in microprocessor- based field devices is accessible from engineering workstations. With bidirectional communication capabilities, not only will engineers be able to access field monitoring and control devices, but these devices will be able to accomplish tasks such as broadcasting messages over the process network and feeding other controllers with on-going information regarding process status, Figure 3.
In some industries, plant maintenance is a major part of the total operating cost. In electrical power generation, for example, up to 60% of non-fuel related costs can be attributed to maintenance. Utilities can therefore use fieldbus to communicate with field devices from the control room, gathering diagnostic and predictive maintenance information.
Communication protocols are the accepted rules for exchanging messages between different devices. Protocols dictate message structure and format, how messages are transmitted, and how errors are handled. Devices adhering to a common protocol are able to communicate directly with one another; devices that do not support a common protocol typically require a protocol converter.
Andy Chatha is the president of Automation Research Corp., Medfield, Mass.