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System structures
The nomenclature for system structures describes the possible degradation behaviour after fault detection and fault localisation for the central processing units. Variations are possible due to different implementation possibilities. This table shall give a first guideline.
In the I/O area different degradation mechanisms are possible and implemented on the different systems.
| Fault-free system | Degradation 1 | Degradation 2 | Degradation 3 | System structure |
|---|---|---|---|---|
| 2oo4 | 1oo2 | Shutdown | Safety related and fault tolerant | |
| 1oo3 | Shutdown | Safety related | ||
| 2oo3 | 1oo2 | 1oo1* | Shutdown | Safety related and fault tolerant *with time restriction |
| 2oo3 | 1oo2 | Shutdown | Safety related and fault tolerant | |
| 1oo2D | 1oo1D | Shutdown | Safety related and fault tolerant
with time restriction |
|
| 1oo2 | Shutdown | Safety related | ||
| 2oo2 | 1oo1 | Shutdown | Safety related and fault tolerant | |
| 1oo1 | Shutdown | Safety related |
Part 4 of the IEC 61508 gives
the defintion of
MooN : M out of N channel architecture (for example 1oo2 is 1 out of
2 architecture, where either of the two channels can perform the safety function)
MooND : M out of N channel
architecture with diagnostic
last change : 14. May 2003
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