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The History of Fieldbus | ||
In the 1940s, process instrumentation relied upon pressure signals of 3-15 psi for the monitoring of control devices. In the 1960s, the 4-20 mA analogue signal standard was introduced for instrumentation. Despite this standard, various signal levels were used to suit many instruments which were not designed to the standards specification. The development of digital processors in the 1970s sparked the use of computers to monitor and control a system of instruments from a central point. The specific nature of the tasks to be controlled called for instruments and control methods to be custom designed. In the 1980s smart sensors began to be developed and implemented in a digital control, microprocessor environment. This prompted the need to integrate the various types of digital instrumentation into field networks to optimise system performance. While the "if it works then use it" mentality progressed, it became obvious that a fieldbus standard was required to formalise the control of smart instruments. Ongoing Standard The decision to provide an international standard saw the Instrument Society of America (ISA), the International Electrotechnical Commission (IEC), Profibus (German national standard) and FIP (French national standard), form the IEC/ISA SP50 Fieldbus committee. The standard to be developed must integrate the enormous range of control instruments, provide them with interfaces to operate various devices simultaneously, and set a communication protocol to support them all. This daunting task was perceived by many to be moving too slow, a problem compounded by companies world wide pushing to have their own product ideas standardised. With the diversity in products and methods of implementation, there was no one direct solution for the standard to be set to. In 1992, two groups, each consisting many major companies world wide, emerged to lead the market in a fieldbus solution. The ISP (Interoperable Systems Project) and WorldFIP (Factory Instrumentation Protocol) both share differing views on the implementation of fieldbus, but they claim they will alter their products to conform to the ISA's SP50 standard when it is formalised. The SP50 committee decided to concentrate on four layers for the fieldbus solution...1. Physical Layer: This defines the media that communication occurs over and could be viewed as the 4-20mA standard replacement. 2. Data Link Layer: This monitors the communications taking place among the various devices and detects errors. 3. Application Layer: This formats the data into messages which all devices connected to the network can understand and provides the services for process control, supplying them to the user layer. 4. User Layer: This connects the individual plant areas and provides an environment for applications. It is implemented using high level control functions. Of these layers, the ISA S50.02 Part 2 Physical Layer was approved in September 1992. The Data Link Layer is expected to be drafted into an IEC standard by mid 1996. The Application and User Layers are in committee with balloting due to be resolved early and late this year respectively. The System and Network Management is expected to be completed by mid 1996. (Dick Johnson, December 1994) In September of 1994, WorldFIP and ISP, joined forces to become Fieldbus Foundation (FF), in an effort to speed up the process of completing the fieldbus standard. Fieldbus Testing For several years now companies world wide have been engaged in the testing of the evolving fieldbus standard through implementation in small areas of already operational plants. The aims of these companies is undoubtedly to test the suitability of fieldbus in their operating environments. This real life testing is the best way to examine the reliability of a fieldbus system, and to determine whether fieldbus will live up to the process industry's high expectations. In early 1993, all eyes were turned towards the BP Research site in Sunbury, where the first tests on a fieldbus system were coming to conclusion. Large numbers of instruments were set up in a flow test rig to perform various tests in line with the currently evolving fieldbus standard. This first phase test examined the physical layer set down by the SP50 committee with the intent of verifying its operation. Successful tests confirmed physical layer layed out and demonstrated that the cable limits suggested could be safely exceeded by 50%. The second phase of fieldbus testing is taking place in Copyright 2008 - France BtoB from Wikipédia
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