NI-PCI 6221 'sampling rate'

[tan0207]

Our system has NI-pci 6221 card, SCXI-1000 chassis, 2 x  SCXI-1102, 1 SCXI-1100, 2 x TC-2095, 1 BNC-2095 and sensors. Software WinXP, VI Logger Lite 2.01.

The problems is I can not set the scan rate slower than 214.748s (rate 4mHz or lower), could you tell me what is the cause of this problem.

Regards,

Mr Tan Nguyen

[Matthew_W]

 Hello Mr. Nyugen,
 
 The scan rate can be set as slow as you can provide a clock by using a External source for the sample clock. This is done in the Timing configuration, set the Sample Clock Source to be external        then choose a PFI line to input a external clock into. What error specifically are you seeing from your system?
 Here is a document that explains how to calculate the maximum sampling rate achievable with a SCXI system, this may prove useful in understanding how your system functions:
   http://digital.ni.com/public.nsf/websearch/410A70C25A4D12B486256A1E0070BDAE?OpenDocument

Thank you and best regards.

 MatthewW
 Applications Engineer
 National Instruments

 

 

[tan0207]

Hi Matthew,

The problem is the number 214.748s relate to the Rate(Hz): 4mHz or any number lower.

If I set Rate(Hz): 5mHz I got sampling rate 200s, but if smaller Rate(Hz) was set, it always show sampling rate 214.748s, where is that number come from?

Regards,

Mr Tan Nguyen

[Matthew_W]

Hello Mr. Nguyen,

 The actual scan rate is determined by the NI-DAQmx driver. The values are being coerced to the closest legal value allowed by NI-DAQmx.

 Is the 214.748s value being reported from a property node in Labview?

 Here is another knowledgebase article that deals with calculating the actual scan rate of a single data acquisition board, this article discusses why the NI-DAQmx driver will coerce the sample rate
depending on what your timebase is.

 Try selecting the 100Khz sample clock, this should allow a slower scan rate than the 80Mhz timebase. Here is instructions on selecting the 100khz timebase as a sample clock source.

Best regards,

MatthewW
Applications Engineer
National Instruments