In a PCI-4451 what is the relationship between the input and output sampling and output rates?

[JackMetcalfe]

I understand the input and output channels share a common 26.2144mHz clock. The max input sampling rate divides this by 128 for 204,800Hz. The max output rate divides this by 512 for 51,200Hz.

The output channels are updated simultaneously and the input channels are sampled simultaneously. Because the input channels are bound together they must be sampled at the same rate. The same is true of the output channels.

Is it possible to output on both channels at 51,200Hz while sampling on both channels at 204,800Hz? It seems like it should be.

Why is the actual output rate reported from a call to the NI-DAQ API WFM_Set_Clock() when asking for 44kHz con
sistently 60kHz? Per the specifications 60kHz is beyond the range of the board.

Similarly DAQ_Set_Clock() reports an actual output rate of up to 240,000Hz. Per the specifications this is beyond the 204,800Hz max of the board.

Thanks,
Jack Metcalfe

[Andy_Hinde]

Jack,
There is a relationship between possible update rate (output) and scan rate (input) combinations. If you are doing both input and output at the same time, then the update and scan rates can differ only by a power of two, due to the fact that the update and scan clocks are derived from the same base clock.
For example, 204.8 kHz scan rate and 51.2 kHz update rate is a valid combination which will work, since they differ by a factor of 4 (204.8k / 51.2k = 4) and 4 is a power of 2.
I am not sure why the NI-DAQ API is reporting the output rate to be 60 kHz when a 44 kHz rate is entered. I also don't know why the board is reporting a 240 kHz input rate. I tried reproducing the same behavior but was unsuccessful.
You might try the same examples I used, available at
:

ftp.ni.com\incoming\JackM.zip

They don't so the input and output simultaneously, however. If you try these out and are still experiencing difficulties, I would suggest contacting the National Instruments Applications Engineering department and obtaining some direct technical support. You can either call (512)795-8248 or send an email by going to:

http://sine.ni.com/apps/we/niae_asc.main

and following the instructions there.