Physically separate and diverse SIS logic solver

ANSI/ISA-84.00.01-2004-1 does not require physical separation. However, many owners/operators do use physical separation and diverse logic solvers. It is paramount to remember the primary purpose of many SIS. The SIS is designed to restore the plant to a safe state when the process moves out of the normal operating envelope for a number of causes, including a failure or erroneous operation of the control functions. The need for the SIS to act as a layer of protection in the event of control function failure has resulted in many standards and practices recommending that SIF and control functions be implemented in physically separate and diverse equipment. 

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This physical, as well as functional, separation of SIS and BPCS has served the industry well for several reasons  

Refer to ISA-TR84.00.04-1, Annex K, for information regarding fault tolerance, including a treatment of 

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