Feiwen,
I looked at your VI, which uses a single-record acquisition and restarts the acquisition every time through the loop.
Based on the description of your application, I think what you really want to do is configure a multi-record acquisition. Each record will be triggered by a rising edge on your trigger channel. For each record, you will acquire the specified number of points, then the hardware will re-arm itself for a new record and wait for the next trigger. You won't have to restart each record in your software loop- the hardware will deterministically re-arm itself.
Here is some pseudocode for what your code will look like:
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niScope init session
niScope Configure Vertical
niScope Configure Trigger Edge
niScope Configure Horizontal Timing (set the number of records to be a large number)
Open output file
niScope Initiate Acquisition
Loop
check how many records have completed since last loop iteration
fetch the records that have been acquired since the last loop iteration
write the records to the output file
End loop
niScope Abort
niScope Close
Close output file
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Here are some more things to worry about...
Since you are doing a continuous acquisition, the number of records you acquire may be very large. They may not all fit into the device's onboard memory. You can work around this by setting the "Enable Records > Memory" attribute to be true. This allows the memory to work like a circular buffer- when it gets to the end, it will start storing more records at the beginning. But remember, you have to fetch the old records out before they get overwritten.
Finally, there is a limit to how quickly you can re-trigger the device. Another way of saying this is that the records can't be too close together in time. If the trigger wave is oscillating too quickly, the device may miss a trigger. For the 5922, the trigger re-arm time is (144 * sample clock period). So the triggers cannot occur faster than this or you will miss records.
One more thing: there is an example that demonstrates some of what I have described. Please look at "niScope EX Multi Record Fetch More Than Available Memory.vi" which is installed by the driver.
Hope this helps,
Patrick
