Table Of Contents

Digital Filtering Considerations for C Series Devices

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    Last Modified: November 16, 2017

    For C Series devices, you can filter digital I/O lines and digital input signals.

    Timing and Triggering Filters for Digital I/O Lines

    You can configure digital filters on the device by choosing three fixed values (112.5 nS, 6.4 µS, 2.56 mS) or a custom filter value. The custom filter value must be the same for all lines across the device. For example, if you choose a filter value of 2 µS for PFI 0, any other filterable line on the device can only choose from the three fixed values and the 2 µs value selected for the custom filter. For each digital line or input terminal, there are four attributes/properties associated with these digital filters: Digital Filter Enable, Digital Filter Minimum Pulse Width, Digital Filter Timebase Source, and Digital Filter Timebase Rate.

    When you set the Digital Filter Enable to true, you must also configure the Digital Filter Minimum Pulse Width attribute/property. When you select a filter value with the Digital Filter Minimum Pulse Width attribute/property, the device uses an internal 32-bit utility counter to generate the desired filter value. If you would like to generate the filter clock using your own external signal, you can use the Digital Filter Timebase Source and Digital Filter Timebase Rate attributes/properties. You must configure both to use an external signal as the source for the digital filter. The Digital Filter Minimum Pulse Width attribute/property represents the minimum value that is guaranteed to be passed into the device. The maximum pulse width guaranteed to be blocked by the device is one filter clock tick smaller than the minimum pulse width guaranteed to pass the filter.

    The following table lists the attributes/properties for terminals that can be digitally filtered.

    Type Attribute/Property
    Channel Frequency Input Terminal
    Period Input Terminal
    Count Edges Input Terminal
    Count Edges Count Direction
    Position A Input Terminal
    Position B Input Terminal
    Position Z Input Terminal
    Pulse Input Terminal (Time, Ticks, and Frequency)
    Pulse Width Input Terminal
    Two-Edge First Input Terminal
    Two-Edge Second Input Terminal
    Semi-Period Input Terminal
    Counter Input Timebase Source (External Only)
    Counter Output Timebase Source (External Only)
    Timing Sample Clock Source
    Triggering Arm Start Digital Edge Source
    Pause Analog Level Source
    Pause Analog Window Source
    Pause Digital Level Source
    Reference Analog Edge Source
    Reference Analog Window Source
    Reference Digital Edge Source
    Start Analog Edge Source
    Start Analog Window Source
    Start Digital Edge Source

    Filters for Digital Input Signals

    Filters are also available on digital input lines, such as cDAQ1Mod1/port0/line0, but the filters do not support the fixed values mentioned previously or external timebase sources. The minimum filter pulse width for digital input lines is 50 nS and can be set in increments of 25 nS. All digital input lines on a module must use the same minimum filter pulse width. The maximum pulse width guaranteed to be rejected by the filter is half the pulse width guaranteed to pass the filter.


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