What kind of error do you get?
Doing the math makes me wonder if you have a buffer size or buffer overflow issue. 720 000 points on each of 12 channels is 8.64E6 samples. Now consider 24 kHz * 12 channels * 28.1 s = 8.09E6 samples. 2^23 = 8.39E6. If the sample buffer is sized at a power of 2, 30 seconds might overflow the buffer while 28.1 might not.
You may be better off avoiding the DAQ Assistant and using the standard DAQmx functions for a situation like this. Set the acquisition for continuous samples outside the loop, read one or two seconds of data at a time in the loop, pass the data to a parallel loop via a queue, and then clear the task after leaving the loop. This is the basic idea of the Producer/Consumer Design Pattern.
Lynn
