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Discrepancy Faults for Analog Input Configurations

Version:
    Last Modified: May 15, 2018

    The module FPGA returns a Discrepancy Fault to the User Program in the following situations:

    • In a dual input (1oo2) configuration the two channels read currents that differ by more than the Discrepancy Current for a time greater than the Discrepancy Time.
    • In a triple input (2oo3) configuration all three channels read currents that differ from each other by more than the Discrepancy Current for a time greater than the Discrepancy Time.
    Figure 1. Discrepancy Fault Signal Diagram
    1. Measured currents exceed discrepancy parameter limits.
    2. Measured currents return to defined parameter limits.
    3. Discrepancy time expires. Since measured currents are within defined parameter limits, the module FPGA does not generate a discrepancy fault.
    4. Measured currents exceed discrepancy parameter limits.
    5. Discrepancy time expires. Since measured currents still exceed parameter limits, the module FPGA asserts a discrepancy fault and the fault latch timer starts.
    6. Measured currents return to defined parameter limits.
    7. Fault latch time expires. Since measured currents are within defined parameter limits, the module FPGA no longer asserts a discrepancy fault.
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    Tip  

    To ensure that the module detects and responds to discrepancy faults, set the discrepancy current parameter as low as the system will allow.

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    Note  

    You must set the discrepancy current greater than 0 mA.

    You can use discrepancy faults in two ways:

    • You check the Module failsafe box for AIn.DiscrepancyFault. If a discrepancy fault occurs, the module will go into Fail-safe Mode.
    • You can use AIn.DiscrepancyFault in the transition condition on the state machine diagram. If a discrepancy fault occurs, the User Program will transition to a user-configured state.

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