Hi Sven, That's interesting. You might have to do a little more investigating to determine why the counters seem to start prematurely (or why the analog in is not reporting the scan backlog correctly). Are you using the startscan (scan clock) signal as the source of your counters? This signal will pulse as many times as you sample.
One test you can perform is a finite acquisition with a specific number of samples. Your counter should count that exact number of samples. If the counter does in fact count the exact number of samples then we know your problem will be in monitoring the scan backlog of the analog input. Another thing that could contribute to the difference between the two backlog numbers is the fact that the analog input will store a certain number of samples in the onboard memory and then transfer them once the DMA FIFO is full. For the 16E-4 card, this FIFO size is 512 samples. This means that the card can accumulate up to 512 samples and then scan backlog would read 0. This is because the scan backlog monitors the samples on the PC buffer that you configured. The FIFO doesn't count in that scan backlog number. Only after samples have been transferred from the FIFO (MIO card memory) to the PC buffer (memory you declared in your program) will you see a correlation. Another way to easily monitor this is to use Interrupts as your transfer method instead of DMA. Interrupts will transfer each sample as it is acquired so your PC buffer will always be current (or off by no more than 1 sample). You can change this by using the Set DAQ Device Info function. Anyway, hope that clears things up. Have a good day. Ron
