PCMCIA 6036E + DAQmx (analog input) appears to only read in blocks of 512 data samples

[chassan]

Hello folks,

I'm asking this question before I any post code or software versions etc. to see if there is a simple answer.

I'm using a PCMCIA 6036E card to read one analog input channel (using DAQmx...i.e. create task, create channel, set timing (continuous samples), start task, read in a software timed while loop (as fast as possible)).  Regardless of how I set my sample rate, number of samples per channel (i.e. buffer size), or numer of samples to read, it looks like I can only get multiples of 512 samples.

The following lists some sample freq (Fs), the requested "number of samples to read", and the actual number of samples read:

Fs          numberOfSamplesRequested          numberOfSamplesActuallyRead

200Hz   20                                                 512

5000Hz 1000                                             1024

5000Hz 2000                                             2048

QUESTIONS:

1. Is this "normal" behavior for a PCMCIA 6036E?

2. If not, has anyone who may have seen this issue determined the cause and how to "fix" it?

Best Regards,

Chris

[TCPlomp · Accepted solution]

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chassan wrote:

Hello folks,

QUESTIONS:

1. Is this "normal" behavior for a PCMCIA 6036E?

Sort of.

2. If not, has anyone who may have seen this issue determined the cause and how to "fix" it?

Best Regards,

Chris

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On PCI systems DMA is used to transfer
the data, and daqmx get notified when the number of samples are
acquired. Now on PC-card that doesn't work, and the data is transfered
when the on-board buffer is full (after 2 kbyte of data) at 2 channels
10 Hz this can take a while.
There is a work around, there is a property where you can set the transfer mechanism (I don't have daqmx on this PC), or the polling mechanism of daqmx. Try these.

Found a KB document.

Ton

[chassan]

Hi TCPlomp,

Thanks for the great link; it explains exactly the situation that I'm experiencing!

I haven't tried the work-around yet, but at least I now understand why I'm seeing the behavior (i.e. it's not something in my code). 

Best Regards,

Chris