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Discrepancy Diagnostics for Digital Inputs

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    Last Modified: September 6, 2017

    You can configure discrepancy diagnostics on consecutive even and odd digital input channels (DIn and DIn+1). Select the dual input configuration on the even-numbered channel (DIn) in the I/O Configuration table. The discrepancy diagnostic monitors both channels. If a discrepancy occurs, the User Program returns a fault on DIn.DiscrepancyFault.

    If you configure the channels as equivalent inputs, both channels must be in the same state after the discrepancy time elapses. If you configure the channels as complementary inputs, the two channels must be in opposite states before the discrepancy time elapses.

    The User Program returns discrepancy faults on the even-numbered channel (DIn). You can use discrepancy faults in two ways:

    • You can check the Fail-safe box for DIn.DiscrepancyFault. If a discrepancy fault occurs, the module will go into Fail-safe Mode.
    • You can use DIn.DiscrepancyFault in transition logic on the State Machine Diagram. If a discrepancy fault occurs, the User Program will transition to a user-configured state.

    Safe State

    C Series Functional Safety modules are designed for use in De-Energize to Trip functions. A de-energized (low) state represents the safe state for a given channel. Refer to the following table for channel safe states and user program input values based on configuration and true value.

    Table 1. Channel Safe States for Discrepancy Configurations
    Discrepancy Fault Configuration True Value DI Even (DIn) Safe State DI Odd (DIn+1) Safe State User Program Input Value (DIn)
    Equivalent Active high Off Off False
    Active low Off Off True
    Complementary Active high Off On False
    Active low Off On True

    Equivalent Inputs with Active High True Value

    If you select active high on equivalent inputs, low is the safe state for both the even-numbered channel (DIn) and the odd-numbered channel (DIn+1). If both inputs go high, the User Program returns a fault on DIn.DiscrepancyFault.

    Figure 1. Equivalent Inputs with Active High True Value

    Equivalent Inputs with Active Low True Value

    If you select active low on equivalent inputs, high is the safe state for both the even-numbered channel (DIn) and the odd-numbered channel (DIn+1). If either input goes low, the User Program returns a fault on DIn.DiscrepancyFault.

    Figure 2. Equivalent Inputs with Active Low True Value

    Complementary Inputs with Active High True Value

    If you select active high on complementary inputs, low is the safe state for the even-numbered channel (DIn) and high is the safe state for the odd-numbered channel (DIn+1). If both inputs go into an active state, the User Program returns a fault on DIn.DiscrepancyFault.

    Figure 3. Complementary Inputs with Active High True Value

    Complementary Inputs with Active Low True Value

    If you select active low on complementary inputs, high is the safe state for the even-numbered channel (DIn) and the low is the safe state for the odd-numbered channel (DIn+1). If either input goes into an active state, the User Program returns a fault on DIn.DiscrepancyFault.

    Figure 4. Complementary Inputs with Active Low True Value
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    Note  

    If one input channel toggles twice while the other input channel remains the same, the User Program returns a fault on DIn.DiscrepancyFault. This diagnostic is monitored post-debounce filter.


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