Application software safety lifecycle requirements

An example of an application software development lifecycle using an lEC 61508 series SIL 3 compliant PLC is given in Annex D. 

No reproduction or networking permitted without llcenee from IHS 

Where a new function or function biock is to be impiemented using elements of the lEC 61508 series compliant Limited Variability Language (for example, implementation of a common burner interlook sequence or pump interlock sequence) then  

NOTE Unless otherwise indicated, subclause numbers in this figure refer to lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 nEC61511-1 Modi. 

4 The following are considerations for the selection of methods, techniques and tools  

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ANSI/ISA-84.01-1996 requires that the application software be developed in accordance with the Safety Requirements Specification (SRS). ANSI/ISA-84.00.01-2004-1 also requires this, but discusses the development of the application software with relation to the safety lifecycle. Where hardware is prone to random failures, the software is more prone to systematic failures. The safety lifecycle is important, because it is the primary mechanism for reducing systematic failure. The inclusion of the lifecycle discussion in the software section does result in repetition of the design process described in ANSI/ISA-84.00.01-2004-1 Clause 11. This repetition is intended to highlight the importance of the lifecycle in the development, verification and validation of application software. ISA-TR84.00.04-1 Annex O provides a discussion of the evolution of application software development. 

We have gone through the complete chain in the safety lifecycle, from Safety Goals to AUTOSAR BSW requirements. Unlike application software, where the link to safety is more direct, safety requirements allocated on the platform software are challenging to identify. However, if successful it significantly reduces the cost for achieving complete evidence in a safety argumentation. 

Used in conjunction with ISA-TR84.00.04-2005 Part 1, the example set forth in this technical report is provided to illustrate howto apply ANSI/ISA-84.00.01-2004 Parts 1-3 (lEC 61511 Mod). It is intended to demonstrate one method to meet the requirements of the standards. The reader should be aware that ANSI/ISA-84.00.01-2004 Parts 1-3 (lEC 61511 Mod) is performance based, and that many approaches can be used to achieve compliance. Some of the methods applied in this example include what-if and HAZOP techniques for hazard and risk analysis, LOPA for allocation of safety functions to protection layers, fault tree analysis for SIL verification, and ladder logic to document the application software requirements. Other techniques and tools could be utilized at each of these steps in the safety lifecycle to meet the requirements of the standards. 

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