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Determining Maximum Scan Rate for Multiple SCXI Modules

Hardware: SCXI

Problem:
How do I determine the maximum scan rate with the modules and DAQ board I have? Is it based on the module with the slowest scan rate? Or is there a simple equation I can use?

Solution:
The maximum scan rate depends on several factors.
  1. The speed of the DAQ board.
    In the case of the PCI-MIO-16XE-50, 20kS/s is the maximum sampling rate. Now, if you were working with one of our faster boards, say the PCI-MIO-16E-1, which has a maximum rate of 1.25MS/s, you look at your module's speed.
  2. The speed of the slowest module in the SCXI chassis.
    So, a module that can go 333kS/s will be only able to go 100kS/s if a 100kS/s module is in the same chassis.
  3. The number of channels in the scan list.
    For example, a 100kS/s module with a single channel will read 100kS/s off of that single channel. If you add a channel, each channel will be able to go 50kS/s or 100kS/s divided by 2 channels.
  4. The resolution of the DAQ board.
    The settling time requirements of your SCXI modules and the resolution of the DAQ board go hand in hand. Many SCXI modules have the following settling time requirements:

    Accuracy (percent of full range) 0.012% 0.006% 0.0015%
    Equivalent bits of resolution 13 bits 14 bits 16 bits
    Settling time / Scan Interval 3 us 10 us 20 us
    Equivalent Max Sample Rate 333kS/sec 100kS/sec 50kS/sec

    Note: You should always consult your module's specifications for relevant settling time requirements.

    If the resolution of the DAQ board is 16 bits, then your maximum sample rate will be limited to 50kS/s, which is the fastest rate at which SCXI can settle analog signals to 16 bits of accuracy. For example, if you are using the 6052E (16 bit 333kS/s) to control your SCXI chassis, then you must keep your sample rate at 50kS/s or below in order to maintain the full 16 bits of accuracy. However, there are no errors that prevent you from going faster than 50kS/s. NI-DAQmx will not produce an error until the sample rate of 333kS/s is exceeded, which is the maximum sample rate for 12 bit accuracy. Using Traditional NI-DAQ, the driver will not give you an error or warning until the maximum sampling rate of the DAQ device is exceeded.

  5. Simultaneous Sample and Hold Timing Requirements
    If you are using a SCXI module that supports Simultaneous Sample and Hold (such as the SCXI-1140, SCXI-1520, SCXI-153x, or SCXI-1540), then there are additional track and hold timing requirements that must be accounted for when computing the maximum sample rate. Refer to the knowledgebase "What Is the Maximum Achievable Scan Rate of the Simultaneous Sampling SCXI modules?" which is linked below for more details.

    To get more specific you will need the specifications on the modules you are working with to add in factors like settling time. Some modules have certain formulas you can use to determine the achievable scan rates. See the KnowledgeBase entries below.


Related Links:
Product Manuals: Getting Started with SCXI
Knowledge Base 2K1BIKWD: Calculating the Maximum Achievable Scan Rate of Simultaneous Sampling SCXI Modules
KnowledgeBase 2K2EACWD: What Parameters Do I Need to Calculate the Maximum Scanning Rate of a Simultaneous Sampling SCXI Module?

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Report Date: 03/29/2001
Last Updated: 01/14/2010
Document ID: 27SDGMCM

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