E-Zine February 2010
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HART was an interim development in the late 1980's stemming from the original SP50 work on Fieldbus for suppliers to retain the 4-20mA analog transmission and conduct a digital data exchange in the same two-wire loop. Technology at the time restricted the data rate to 1200 bps when the analog signal was present.
During the ANSI/ISA SP50 and IEC SC65C/WG6 standards committee deliberations, there were many candidates in favor of an all-digital fieldbus technology. It was agreed to adopt a totally new technology at the protocol layer in order to meet all of the requirements with a single set of rules. The protocol developed by the committee became the basis for FOUNDATION™ Fieldbus. Another group of suppliers, dissatisfied with this decision, proceeded to use a then existing protocol to develop the PROFIBUS specifications.
The final IEC standard (IEC 61158) includes both PROFIBUS in both PROFIBUS-DP and PROFIBUS-PA forms, FOUNDATION™ Fieldbus H1, FOUNDATION™ Fieldbus HSE, as well as ControlNet, Interbus, WorldFIP, P-Net, and SwiftNet. Since most of these are not used in commercial process control networks, only PROFIBUS and FOUNDATION™ Fieldbus forms are discussed in this report. HART is also used in process control, but is not part of this standard.
In this report, fieldbuses used for process control --- HART, PROFIBUS-PA, and FOUNDATION™ Fieldbus --- are discussed in order of increasing complexity and capability. Each fieldbus has its advantages and disadvantages, so the reader should evaluate the desired network capabilities and implementation issues given the requirements of each installation. In some applications, some plants use more than one fieldbus type to meet their needs.
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Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control