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Chapter 2

Function Reference — DAQ_DB_Transfer

NI-DAQ FRM for PC Compatibles

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Transfers half of the data from the buffer being used for double-buffered data acquisition to 
another buffer, which is passed to the function, and waits until the data to be transferred is 
available before returning. You can execute 

DAQ_DB_Transfer

 repeatedly to return 

sequential half buffers of the data.

halfBuffer is an integer array. The size of the array must be at least half the size of the circular 
buffer being used for double-buffered data acquisition. 

ptsTfr is the number of points transferred to halfBuffer. This value is always equal to half 
the number of samples specified in 

DAQ_Start

, 

SCAN_Start

, or 

Lab_ISCAN_Start

 unless 

the acquisition has not yet begun, or the acquisition stopped while in pretrigger mode. In the 
former case, until NI-DAQ applies an external start trigger, ptsTfr is 0. In the latter case 
(pretrigger mode), the acquisition can stop at any point in the circular buffer after acquiring 
the specified number of samples after the board receives NI-DAQ applies a pulse at 
STOPTRIG for the MIO-16 stop trigger input. Refer to EXTTRIG* of the non-E Series MIO 
devices, or to EXTTRIG of Lab and 1200 Series devices. If you are using all E Series devices, 
see the 

Select_Signal

 function for information about the external timing signals. Thus, 

i16

assigned by configuration utility

[i16]

integer array to which the data is to be transferred

u32

number of points transferred

i16

indicates the completion of a pretrigger mode 
acquisition