National Instruments 321645c-01

Chapter 2 Function Reference — AO_Change_Parameter

National Instruments Corporation 2-29 NI-DAQ FRM for PC Compatibles

For the VXI-AO-48XDC device the paramID of ND_OUTPUT_TYPE is used in conjunction

with the channel value to select the analog output channel to be affected. To select a voltage

channel, set the paramValue to ND_VOLTAGE. To select a current channel, set the

paramValue to ND_CURRENT.

FIFO Transfer Condition

You can specify the condition that causes more data to be transferred from the waveform

buffer into the analog output FIFO. NI-DAQ selects a default setting for you, in order to

achieve maximum performance. However, by changing this setting, you can force the FIFO

to remain as full as possible, or effectively disable, or reduce the size of the FIFO.

For example, to reduce the FIFO lag effect (the amount of time it takes data to come out of

the FIFO after being transferred into the FIFO), you can change the FIFO transfer condition

to FIFO empty. Notice that reducing the effective FIFO size can also reduce the maximum

sustainable update rate.

To change the FIFO transfer condition, set paramID to ND_DATA_TRANSFER_CONDITION,

and set paramValue to one of the values shown in Table 2-5:

Set channel to one of the channel numbers in your waveform group. For ex ample, if you ha ve

configured group 1 to contain channels 0 and 1, you can set channel to 0 or 1.

Note This option is valid only for PCI E Series devices.

Table 2-4. Voltage or Current Output Parameters

Device Type Per Channel

Selection Possible Legal Range for

paramValue Default Setting for

paramValue

PC-AO-2DC

DAQCard-AO-2DC

VXI-AO-48XDC

Yes ND_CURRENT_OUTPUT

and

ND_VOLTAGE_OUTPUT

Table 2-5. Parameter Values for FIFO Transfer Conditions

Transfer Condition NI-DAQ Constant

FIFO not full ND_FIFO_NOT_FULL

FIFO half-full or less ND_FIFO_HALF_FULL_OR_LESS

FIFO empty ND_FIFO_EMPTY

FIFO half-full or less until full

(DMA only) ND_FIFO_HALF_FULL_OR_LESS_UNTIL_FULL