ADLINK PCI-7300A - Manual (Page 48)

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• Operation Theory
4.10.4 Continuous Digital Input
If the digital input operation still active after the competition of the previous
DMA transfer and do not clear the data in the input FIFO when the next DMA
starts, the cPCI/PCI-7300A can achieve the continuous digital input function in
a high-speed sampling rate. In this case, the input FIFO buffers the input data
and waits for the next DMA to move the queued data to the system memory.
To avoid the overrun of input FIFO causes the data lost of the continuous
digital input, the latency time of the next DMA should be smaller than the time
to overrun the input FIFO. There are some rules of thumb should be mentioned
here:
1. The lower the sampling frequency is, the longer the time to overrun the input
FIFO is. That means the fewer overrun situations will occur.
2. To reduce the latency time between two DMA transfers, please disable
unnecessary PCI bus mastering devices, and remove the unnecessary
processes in your application programs.
3. When high-speed sampling frequency is applied, the larger block size will
improve the efficiency of DMA transferring, and probability of overrun in the
DMA process will be reduced.
4. To apply the high-speed continuous digital input, it is recommended to
execute your application programs in the non-multitask operation system to
reduce the latency time between two DMA transfers.