ADLINK PCIe-7300A - Manual (Page 68)

58 Operation Theory
The operation flow is show as below:
Note: When the DMA function of digital output starts, the output
data will transfer to the output FIFO of cPCI/PCI/PCIe-7300A
when PCI bus is available. If the speed of translation from
the FIFO on board to the external device is higher than that
from system memory to the output FIFO or the PCI bus is
busy for a long time, the FIFO become empty and under-run
situation occurs after the next data being read from the out-
put FIFO. Users should check the under-run status to see
whether the under-run occurs or not. Some output data will
repeat when the output FIFO is under-run.
To avoid the under-run of output FIFO when digital output
starts and PCI bus is still busy, it is highly recommended to
set the starting mode to be WaitFIFO. The higher the timer
pacer rate is the larger amount of almost empty threshold
should be set to prevent the under-run situation.