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Voting in Triple Input (2oo3) Safety Architectures

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    Last Modified: May 15, 2018

    In a 2oo3 architecture, the module FPGA employs a two step voting process before returning a current region to the User Program.

    Figure 1. Triple Input (2oo3) Voting
    1. The C Series Functional Safety module reads the current on three input channels.
    2. The module FPGA assigns a current region to each channel based on the configured thresholds.
    3. The module FPGA employs a pessimistic voting strategy to compare the current regions between each pair of channels: 0 and 1, 1 and 2, 0 and 2.
    4. The voting strategy selects the most safe current region of each channel pair to produce three reference regions.
    5. The module FPGA employs an optimistic voting strategy to compare the three reference regions. The voting strategy selects the least safe reference region of the three.
    6. The User Program reads the single current region selected and returned by the module FPGA.
    Figure 2. Triple Input (2oo3) Signal Diagram

    The following table shows the current region returned to the User Program for each unique combination of inputs.

    Table 1. Triple Input (2oo3) Voting
    Current Region Combinations Region Returned to the User Program
    High high High high High high High high
    High high High high High High high
    High high High high Normal High high
    High high High high Low High high
    High high High high Low low High high
    High high High High High
    High high High Normal High
    High high High Low Low
    High high High Low low High high
    High high Normal Normal Normal
    High high Normal Low Low
    High high Normal Low low High high
    High high Low Low Low
    High high Low Low low High high
    High high Low low Low High high
    High high Low low Low low Low low
    High High High High
    High High Normal High
    High High Low High
    High High Low low High
    High Normal Normal Normal
    High Normal Low High
    High Normal Low low High
    High Low Low Low
    High Low Low low Low
    High Low low Low low Low low
    Normal Normal Normal Normal
    Normal Normal Low Normal
    Normal Normal Low low Normal
    Normal Low Low Low
    Normal Low Low low Low
    Normal Low low Low low Low low
    Low Low Low Low
    Low Low Low low Low
    Low Low low Low low Low low
    Low low Low low Low low Low low

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