October 2007

Columns

HP Automation Safety: Still not using a safety PLC?

The concept of using a special-purpose programmable logic control (PLC)/controller designed for fail-safe operation started over 25 years ago. The early programmable machines were designed applying th..

Goble, W., exida LLC

The concept of using a special-purpose programmable logic control (PLC)/controller designed for fail-safe operation started over 25 years ago. The early programmable machines were designed applying three sets of PLC hardware, and two out of three (2oo3) voting circuits on the outputs would provide fault tolerance to a single failure. It was quickly realized that these machines would provide higher levels of functional safety than conventional PLC designs. When the marketing people figured that out, the concept of the "safety PLC" was born. As the technology advanced, additional designs beyond 2oo3 became available in dual arrangements. At present, there are flexible hybrid designs where some

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