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Architectural constraints or AC

Limitations that are imposed on the hardware selected to implement a safety-instrumented function, regardless of the performance calculated for a subsystem. Architectural constraints are specified (in IEC 61508-2-Table 2 and IEC 61511-Table 5) according to the required SIL of the subsystem, type of components used, and SFF of the subsystem’s components. Type A components are simple devices not incorporating microprocessors, and Type B devices are complex devices such as those incorporating microprocessors. See Fault Tolerance.


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