Handling of software systematic errors

When PE devices are being evaluated for use in an SIF, the integrity of the software in the PE device should be considered. Unfortunately, software failures are difficult to predict. The mathematical models used to predict hardware failures are not easily applied to software, because software failures are not random they are systematic. Software errors are also difficult to identify because of the overwhelming number of combinations of events that could lead to a fail-dangerous or fail-safe event. Unlike hardware 

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There are a number of approaches to address systematic software errors. These approaches often consider the type of software being used by the device. This section presents an approach for both FPL and LVL devices, in accordance with ANSI/ISA-84.00.01-2004, the use of FVL devices or the development of FVL application software should follow lEC 61508-3 to address systematic errors. 

FPL devices have very limited functionality. Hence, the implementation of an FPL device involves highly repetitive, clearly defined tasks, which minimize the potential for undetected systematic errors. An approach to minimize systematic errors within FPL devices, such as PE-based transmitters, should take into account all of the following  

LVL devices use application software written to perform a wide range of functions. PE logic solvers (e.g., PLCs) are commonly used LVL devices. Errors in the application software can be a major contributor to incorrect results. Systematic errors in LVL devices are typically managed as follows  

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