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Current Threshold Diagnostics

    Last Modified: May 15, 2018

    The C Series Functional Safety module uses five current regions to describe the state of the current input to the channel. You define these regions by setting four current thresholds in the Functional Safety Editor.

    Figure 1. Current Regions

    The module FPGA converts current inputs into digital signals to return the current region defined by the configured thresholds. The User Program only reads one current region for each analog configuration. If all the channels in a dual input or triple input configuration read the same current region, the module FPGA returns that current region. If one or more of the channel readings comes back in different current regions than the module FPGA implements a voting strategy to determine which current region to return to the User Program.

    C Series Functional Safety modules employ the De-Energize to Trip principle, so the voting strategy defines the Low low as the most safe current region. The second most safe current region is high high because high high could be a result of a fault such as short to AI Vsup or overcurrent.

    Refer to the following image for the evaluation of most safe current regions that the module FPGA uses to resolve voting situations.

    Figure 2. Current Region Voting Strategy

    To ensure the module FPGA returns a low low region for a channel, set the low low threshold high enough to filter out inaccuracy and noise.

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