Counter value jumps on occasion (PXI 6255)

Question

I am using a PXI 8108 controller (Windows) with a PXI 6255 AI card. A pulse generator (1 KHz) is incrementing the hardware counter of that AI card in order to have an external reference clock, i. e. having a timestamp for each AI sample value.
I am reading 1000 AI values from several channels  (together with 1000 counter values) via DAQmx blocks with a higher frequency than the external clock's 1 KHz (e.g. AI Rate 5 KHz-> reading every counter value approx. 5 times). Every now an then I am losing counter values, e. g. counter value 6789 is followed by 6823.
Shouldn't be the PXI card's counter buffer independant of Window's CPU load? Getting 1000 values from the card buffer, the jumps are even located in the middle of the 1000 values. Maybe there's a problem with atomicity while reading from the buffer, so that I get 500 values from the buffer, (pause), and 500 more values from a later point in time. Is this possible?
I am sure, I can not blame the digital pulse generator (from Hameg Instruments) for dropping pulses.
Attachments:
counter-daq.png ‏20 KB

Answer 1

Hello,
If the first scan only places 4 samples in the AI FIFO (2 less than the number of channels requested), then I would expect this loop to run forever in aiex1.cpp (I added some debugging prints to make it easier to see the channels that should be getting returned).
       while ( m < numberOfChannels )
        if(!board->AI_Status_1.readAI_FIFO_Empty_St())
            value = board->AI_FIFO_Data.readRegister ();
            aiPolynomialScaler (&value, &scaled, &scale);
            printf ("Channel %d: %e,\n", m, scaled);
            m++;
    printf("\tEnd of Sample %d\n", n);
How is the example able to proceed past this loop?
I have checked the code for FIFO flushes and resets between aiStartOnDemand (board); and when the FIFO gets read.  I have also run the example with 6 channels, this time on Windows.
To proceed we would need these items:
1) We would need the exact output (with my added debug prints).
2) A complete dump of all registers accessed during the example.
Steven T.