Справочник Пользователя для 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

.

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

Table 2-4.  Voltage or Current Output Parameters

PC-AO-2DC
DAQCard-AO-2DC
VXI-AO-48XDC

Yes

ND_CURRENT_OUTPUT

 

and 

ND_VOLTAGE_OUTPUT

ND_VOLTAGE_OUTPUT

Table 2-5.  Parameter Values for FIFO Transfer Conditions

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