Two channels, and the assumption that holds them together.
A dual diverse electronic structure based on composite fail-safety with fail-safe comparison, with high diagnostic coverage obtained through frequent cross-checks, hardware-assisted diagnostics and software test routines.
The architecture consists of two channels in parallel, either of which can process the safety function. Each channel is required to have high effective diagnostic capability, for both random and systematic faults, realised through very frequent cross-checks, through hardware-assisted diagnostics in the GeminiX-Cores, and through software test routines.
A dangerous failure would therefore have to occur in both channels before the safety function fails. The assumption the whole architecture rests on is worth stating in public, because a serious evaluator will test it: no single random fault is dangerous, because a hazardous fault in one channel is detected and negated before a second fault occurs in the other. Only multiple faults, including those from common causes, can be dangerous.
Why diagnostic coverage is the hard part
Mutual comparison of results reveals single faults. It does not reveal the accumulation of latent errors, which requires continuously verifying every sub-system involved in the safety function, most of which are not easily verifiable and cannot be tested exhaustively. That is the reason the measures are implemented in FPGA rather than only in software: they become verifiable, reusable and largely independent of the hardware around them.
What the standards say about it.
And the vocabulary.
In conversation people say 2oo2. The normative wording is the one above, and we use it, glossing the shorthand for readers who search in those terms.
Beyond the architecture.
- Hardware diversity, insulation and independence as deliberate strategy
- Systematic and random failure management
- The safety lifecycle applied from hazard analysis to decommissioning
- Independence, and the role of independent assessment
- Functional safety management and competence management