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A Fieldbus is an industrial network system for real-time distributed control. A complex Automated industrial system — say a manufacturing assembly line — usually needs an organized hierarchy of controller systems to function. In this hierarchy there is usually a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of Programmable Logic Controller s (PLC) via a non time critical communications system (e.g. Ethernet ). At the bottom of the control chain is the fieldbus which links the PLCs to the components which actually do the work such as Sensor s, Actuator s, Electric Motors , console lights, Switch es, Valve s and Contactor s. WHAT IS FIELDBUS? Fieldbus is an industrial network system for real-time distributed control. It is one of the ways to connect Instruments in a plant design. Fieldbus works on a network type structure which typically allows daisy-chain, star, ring, branch, and tree style network topologies. The easiest way to think about Fieldbus is with comparison to previous communication schemes between computers. Previously computers were connected using RS-232 (serial connections) by which only two devices could communicate, this is the equivalent of currently used 4-20 mA communication scheme which requires that each device has its own communication point at the controller level, while the Fieldbus is the equivalent of the current LAN type connections, which require only one communication at the controller level and allow multiple (100's) of analog and digital points to be connected at the same time. This reduces both the length of the cable required, and the number of cables required. Furthermore, since devices that communicate through Fieldbus require a Microprocessor , multiple points are typically provided by the same device. In addition some Fieldbus devices now support control schemes, such as PID control on the device side instead of forcing the controller to do the processing. HISTORY Although fieldbus technology has been around since 1988, the development of the international standard took many years. In 2000, a set of interested parties converged to create the IEC fieldbus standard, IEC 61158 with eight (8) different protocol sets called "Types" as follows: 1) FOUNDATION Fieldbus H1 2) ControlNet 3) PROFIBUS 4) P-Net 5) FOUNDATION Fieldbus HSE (High Speed Ethernet) 6) Interbus 7) SwiftNet (a protocol developed for Boeing, since withdrawn) 8) WorldFIP This form of "standard" was first developed for the European Common Market, concentrates less on commonality, and achieves its primary purpose — elimination of restraint of trade between nations. Issues of commonality are now left to the international consortiums that support each of the fieldbus standard types. Almost as soon as this "8-headed monster" was approved, the IEC standards development work ceased and the committee was dissolved. A new IEC committee SC65C/MT-9 was formed to resolve the conflicts in form and substance within the more than 4000 pages of IEC 61158. The work on the above protocol types is substantially complete. New protocols, like for safety fieldbusses or realtime ethernet based fieldbusses are being accepted into the definition of the international fieldbus standard only in form of a typical 5 year maintenance cycle. Recent additions or planned additions to IEC 61158 include but are not limited to: 10) PROFINET IO 12) EtherCAT Both FOUNDATION Fieldbus and Profibus technologies are now commonly implemented within the process control field, both for new developments and major refits. In 2006, China saw the largest FF systems installations at NanHai and SECCO, each with around 15000 fieldbus devices connected. IEC 61158 SPECIFICATION There were many competing technologies for fieldbus and the original hope for one single unified communications mechanism has not been realised. This should not be unexpected since fieldbus technology is required to be implemented differently in different applications; automotive fieldbus is functionally different to process plant control. The final edition of IEC standard IEC61158 allows 8 technologies. IEC 61158 consists of the following parts, under the general title Digital data communications for measurement and control – Fieldbus for use in industrial control systems:
STANDARDS There are a wide variety of concurring fieldbus standards. Some of the most widely used ones include:
See List Of Automation Protocols for more examples. ADVANTAGES OF FIELDBUS The fieldbus has a multitude of advantages from which end users can benefit. At first glance, a major advantage of fieldbus implementation is the Capital Expenditure (CAPEX) savings associated with cable elimination; multiple devices share wire-pairs in order to communicate over the bus network and savings are also available through speedier commissioning. Users have now found that ongoing maintenance and process control system performance are also very significantly enhanced through adopting fieldbus systems, which results in Operations Expense savings (OPEX). DISADVANTAGES OF FIELDBUS There are disadvantages to using fieldbus compared to the 4-20 MA Analog Signal Standard (or to 4-20 mA with HART ):
CURRENT DEVELOPMENTS During recent years a number of Ethernet based industrial communication system have been established, most of them with extensions for real-time communication. These have the potential to replace the traditional field buses in the long term. Currently the issue stopping most Ethernet fieldbus implementations is the availability of device power. Most industrial measurement & control devices need to be powered from the bus and Power-Over-Ethernet (PoE) does not deliver enough. Here is a partial list of the new Ethernet based industrial communication systems:
FIELDBUS SAFETY Fieldbus can be used for systems which must meet safety relevant standards like IEC 61508 or EN 954-1 . Depending of the actual protocol, fieldbus can provide measures like counters, CRC's, echo, timeout, unique sender and receiver ID's or cross check. Both FOUNDATION Fieldbus and Profibus ( PROFIsafe ) have varieties of their communications protocol which are compatible with safety systems. MARKET In process control systems, the market is dominated by FOUNDATION Fieldbus and PROFIBUS . Both technologies use the same physical layer (2-wire manchester-encoded current modulation at 31.25 kHz) but are not interchangeable. As a general guide, applications which are controlled and monitored by PLCs (programmable logic controllers) tend towards PROFIBUS, and applications which are controlled and monitored by a DCS (digital/distributed control system) tend towards FOUNDATION Fieldbus. PROFIBUS technology is made available through Profibus Internatonal with headquarters in Karlsruhe, Germany. FOUNDATION Fieldbus technology is owned and distributed by the Fieldbus Foundation of Austin, Texas. REFERENCES
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