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Differences Between RTSI/PXI Lines And PFI Lines In HSDIO Devices

Hardware: Digital I/O (DIO)>>High-Speed>>PXI-6551, Digital I/O (DIO)>>High-Speed>>PXI-6552, Digital I/O (DIO)>>High-Speed>>PCI-6551, Digital I/O (DIO)>>High-Speed>>PCI-6552, Digital I/O (DIO)>>High-Speed>>PXI-6541, Digital I/O (DIO)>>High-Speed>>PXI-6542, Digital I/O (DIO)>>High-Speed>>PCI-6541, Digital I/O (DIO)>>High-Speed>>PCI-6542, Digital I/O (DIO)>>High-Speed>>PCI-6561, Digital I/O (DIO)>>High-Speed>>PCI-6562, Digital I/O (DIO)>>High-Speed>>PXI-6561, Digital I/O (DIO)>>High-Speed>>PXI-6562

Problem:
I am seeing unexpected behavior when using the RTSI/PXI lines of my HSDIO devices. What am I doing wrong?

Solution:
This behavior may be caused by a fundamental difference in the HSDIO cards RTSI/PXI and PFI lines. The difference is:
  • Triggers acquired or events generated from PFI lines are synchronous with the data. This means these signals will have the expected data/clock phase relationship.
  • Triggers acquired or events generated from RTSI/PXI lines are not synchronous with the data. This means these signals may not have the expected data/clock phase relationship.
Ensuring the triggers and events are synchronous with the clock/data is important in most digital applications. This helps ensure timing requirements are met and triggers/events are seen on the correct clock edge. Furthermore, if a trigger or event is not synchronous with the clock/data, the behavior may change when running at different frequencies.

In summary, if you need triggers and events synchronous with the clock, use the HSDIO cards PFI lines. The block diagrams in the help file (linked below) show how the RTSI connector does not go to the same circuitry as the PFI lines (for example, NI Digital Waveform Generator/Analyzer Help»Devices»NI 655x»Hardware Architecture»Block Diagram).

Related Links:
Product Manuals: NI Digital Waveform Generator/Analyzer Help (September 2007)
KnowledgeBase 3UCA272W: Synchronization with a High Speed Digital Input/Output (HSDIO) Board
KnowledgeBase 44KF4H17: Triggering An Acquisition Session of a HSDIO Device

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Report Date: 10/15/2007
Last Updated: 06/29/2009
Document ID: 4EED91JX

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