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Although Pilz claims that conventional fieldbus networks are not suitable for safety systems, the market appears to be deciding otherwise. Single networks that can carry both control and safety components have a huge advantage in cost and simplicity. For example, Profibus, AS-i, DeviceNet Safety, and Safety Internet are all based on fieldbus systems that do not require an extra level of complexity, and all are predicted to outsell Safety Bus P. Pilz is likely to remain a strong safety market force, given the reputation of its safety relays.
Foundation Fieldbus--According to Dave Glanzer, director of technology development, Foundation Fieldbus'(FF)upcoming safety network is based on IEC 61508 for equipment design and IEC 61511 for end user application. Glanzer says ISA SP84 work is similar to IEC 61511, but ISA does not have a standard equivalent to IEC 61508 for equipment design.
"Foundation Fieldbus' Safety Instrumented Systems (SIS) project is an End User Advisory Council initiative that was approved by the board of directors in October 2002," explains Glanzer. The draft preliminary technical specifications have been completed and planning is underway for laboratory validation testing during 2004. Glanzer claims safety certified devices with FF SIS technology should be available in 2005."
Bud Adler, who sits on the ISA committee, explains the problem: "Foundation Fieldbus has a strong following of those companies that support its use in safety-related applications. However, it's not that a digital network cannot be made safe enough for safety-related applications; it is the fact that there is not an approved measure of that safety availability in accordance with the safety standard."
Emerson Process Management, a big FF supporter, is probably agonizing over the delays. Emerson offers a full line of SIL and SIS equipment, and recently announced its DeltaV SIS, with redundant processors, HART networks, logic solvers, and digital communications. It has everything it needs, but Emerson can't call it a Safety Network because it doesn't fall into any of the categories listed above. "We do not conform to any of the listed safety networks," says Gary Law, DeltaV product manager.
Interbus--Phoenix Contact (www.phoenixcon.com) is the chief proponent of Interbus. It is almost a single-vendor system, but we have been assured that several German companies also make products. "An Interbus safety network is clearing significant milestones on its way to the market," says David Skelton, director of automation systems at Phoenix Contact. "Interbus safety, which is based upon the Interbus protocol standards, received Tů"V approval in November for its system specification. Products will be submitted for Tů"V approval in 2004 and should arrive in the market late next year."
Skelton claims the Interbus safety system will provide safety functions up to Category 4 according to EN 954, and SIL 3 according to IEC 61508. "Depending on the application, the user can use either a one-cable solution with integrated safety or a two-cable solution, where one bus cable is used for standard signals and the other for safety signals," he says.
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