Digital Filtering Considerations for X Series and NI 661x Devices

NI-DAQmx 18.6 Help


Edition Date: January 2019
Part Number: 370466AH-01
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For X Series and NI 661x devices, you can filter digital I/O lines and input signals.

You can configure digital filters on the device by choosing three fixed values (90 nS, 5.12 µS, 2.56 mS). 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. This value represents the minimum value that is guaranteed to be passed into the device. The maximum pulse guaranteed to be blocked by the device varies by digital line. Refer to your device documentation for details. When you select a custom filter value with the Minimum Pulse Width attribute/property, NI-DAQmx 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.

When using digital filtering on port 0, you can enable bus mode using the Enable Bus Mode attribute/property. When enabled, the device observes changes to multiple lines as a single change if these conditions are met:

  • The skew between them is less than the minimum pulse width of the filter.
  • The lines remain stable for an additional minimum pulse width of the filter.

When using any other signal on the device that supports filtering (excluding port 0), you can select a custom filter value in addition to the three fixed values mentioned previously. 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.

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

Type Attribute/Property
Channel Digital Input 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
Pulse Width Input Terminal
Two-Edge First Input Terminal
Two-Edge Second Input Terminal
Semi-Period Input Terminal
Counter Input Timebase Source
Counter Output Timebase Source
Timing Sample Clock Source
AI Convert Clock Source
Triggering Arm Start Trigger Source
Pause Analog Level Source
Pause Analog Window Source
Pause Trigger Source
Reference Analog Edge Source
Reference Analog Window Source
Reference Digital Edge Source
Start Analog Edge Source
Start Analog Window Source
Start Trigger Source

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