Gates VOTING

As already noted, all gate types can eventually be broken down into a combination of AND and OR gates. For example, the voting gate shown in Figure 15.10 can be broken down to the stmcture shown in Figure 15.11. 

The mathematics for a Two-Out-Of-Three (2oo3) voting Gate is shown in Eq. (15.8). 

Gate 5 represents the loo2 vote of the pressure switch and the temperature switch. The simplified approximation for gate 5, PFD is ... 

The majority of fault tree computer programs support a number of other Boolean operators, such as Exclusive OR (XOR), Voting and Priority AND gates however, this is outside of the scope of this chapter (for more information, see the references in paragraph 6). 

Figure 5.7 shows another example, this time of two out of three redundancy, where a voted gate is used. 

Data is processed in parallel via the three controllers. Comparison checks are performed between each controller. Each processor has its own input and output register. The output register in each device is compared in an AND gate. An output will only be enabled when all three agree. This is the case for all bit and word functions. In other words, the PSS acts as a 3 out of 3 (3oo3) voting system. ... 

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