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Chapter 2 Function Reference — AO_Change_Parameter
NI-DAQ FRM for PC Compatibles 2-30
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National Instruments Corporation
When using PCI E Series devices with DMA (default data transfer condition), the device has
an effective FIFO size 32 samples larger than the FIFO size specified for the board. This is
due to a 32-sample FIFO on the miniMITE, the onboard DMA controller used for DMA
transfers.
FIFO Transfer Count
The FIFO transfer count specifies the number of samples to be transferred from the waveform
buffer into the analog output FIFO when FIFO requests are generated. This option is for use
in conjunction with the FIFO transfer condition, as described above.
AO_Change_Parameter should be called once to set the FIFO transfer condition, and can
optionally be called again to specify the FIFO transfer count. If you do not specify the FIFO
transfer count, NI-DAQ chooses an appropriate value for you.
The value of FIFO transfer count is used during interrupt-driven waveform generation but is
ignored during DMA-driven waveform generation. When you use DMA, DMA requests are
generated as long as the transfer condition is true.
Table 2-6 contains the default values that are used if you do not specify FIFO transfer count,
in addition to the valid values that can be set.
For example, if you choose the FIFO empty transfer condition and set the transfer count to
10, each time the board is interrupted with a FIFO empty interrupt, NI-DAQ transfers 10
samples from the user buffer into the analog output FIFO. Although this does not improve the
maximum sustainable update rate, it reduces the number of interrupts, and reduces the FIFO
lag effect to a maximum of 10 samples.
If you choose the FIFO half full or less transfer condition and set the transfer count to 100
on a board with a 2048-sample FIFO, the FIFO fills with a maximum of 1124 samples (half
the FIFO plus 100 samples). Each time the number of samples in the FIFO falls to less than
1024, another interrupt is generated, at which time 100 samples are transferred from the
waveform buffer to the FIFO.
Table 2-6.
Default Values for FIFO Transfer Condition
Transfer Condition Default Transfer Count Valid Input Values
FIFO not full 1 1
FIFO half-full or less half-FIFO size 1—half-FIFO size
FIFO empty 1 1—FIFO size
FIFO half-full or less until
full (DMA only)
FIFO transfer count cannot be
specified for this transfer
condition
N/A