Lighthouse PLCs, Inc. : Automation Forum
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Hopefully the information presented here will help provide solutions to some of today's automation challenges. Some issues are probably old, some are probably new. Many are products of the economy, the resultant reduction in manpower, exacerbated by aging equipment crippled by decreases in maintenance budgets. The ULTIMATE goal here is that we all become more successful in our careers and chosen professions.
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For sake of breviety, let me just say "NO". None of the Large-H Processors (or any of the Hitachi PLC product families for that matter) have "redundancy".
I take pride in being pretty knowledgeable with regards to the Hitachi PLC families. I am the first to admit however, that I am not a SME (Subject Matter Expert) on PLC Redundancy. This one topic could occupy a whole Forum by itself!
Most Engineers/Technicians agree that Redundancy is an essential capability only for the most critical processes and systems. Generally, the cost is just too prohibitive for the majority of applications.
For those unfamiliar with "redundancy", this (briefly) is how it works. Should a problem arise in a system with "reduncnacy" the primary CPU automatically switches to a second CPU within one program scan, enabling continuous operation. The faulty CPU can be changed out while operation continues. Continuous operation is often enhanced by 'hot swappable' I/O and specialty modules.
Whenever this topic comes up, invariably someone starts talking about "Safety PLCs" (or sometimes "Life Safety PLCs"). Strictly speaking, (Life) Safety Systems do not allow even "Redundant" PLCs. NFPA79 Section 9.6.3 states "Category 0..shall only have hard-wired electro-mechanical components and shall not depend on electronic components (hardware or software) or the transmission of commands over a communications network." However some Government Regulatory Agencies with Local Jusrisdition have recently allowed Redundant PLCs in some Category 1 installations.
I welcome comments from others on this topic!
