Solution:

The double buffer functions is for our PCL-1800 with dual-DMA hardware only. Please use single buffer for the other DAS cards. Actually, the sampling data might be lost while continuously measuring for a longer period with HIGH speed. That means the sample data will be lost in DMA transfer period, even with the cyclic mode. So, it's impossible to write back the data before previous samples are overwritten. Anyway, it's recommended to use the FIFO transfer method to transfer those sample data. You know the FIFO has a programmable index. After the card complete the number of index A/D conversions, the FIFO will generate an interrupt and transfer just the number of index data elements. The most important thing is that the sampling data will not be lost during this period!! Besides, those sampling data just written in memory or buffer which without channel index, so you've to identify them by yourself. Besides, those channels you want to inquire data must be continuously. For example, from channel 0 to channel 5, or from channel 3 to channel 6. Please notice that even you don't need to acquire data from channel 2, you still have to enable this channel and use the wire to short itself.