User manual

Chapter 3 Software Overview
NI-DAQ User Manual for PC Compatibles 3-52 ni.com
Three functions are affected by the FIFO lag effect—
WFM_Chan_Control
,
WFM_Check
, and double-buffered waveform generation.
•
WFM_Chan_Control
—When you execute operation=PAUSE for a
group 1 channel, the effective pause does not occur until the FIFO has
finished writing all of the data remaining in the FIFO for the specified
channel. The same is true for the RESUME operation on a group 1
channel; NI-DAQ cannot place data for the specified channel into the
FIFO until the FIFO is empty.
•
WFM_Check
—The values returned in pointsDone and itersDone
indicate the number of points that NI-DAQ has written to the FIFO for
the specified channel. A time lag occurs from the point when NI-DAQ
writes the data to the FIFO when NI-DAQ outputs the data to the DAC.
• When you use double-buffered waveform generation with group 1,
make sure the total number of points for all of the group 1 channels
(specified in the count parameter in
WFM_Load
) is at least twice the
size of the FIFO. Refer to your device user manual for information on
the analog output FIFO size.
• For 61XX devices with onboard memory, data is transferred
to the memory in blocks of 32 bytes. Therefore, when you use
double-buffered waveform generation that does not end in a 32-byte
sample boundary, the last few points will not be output.
With PCI E Series, 622X devices, and 671X devices in NI-DAQ 5.1 and
later, you can reduce or even eliminate the FIFO lag effect by specifying the
FIFO condition NI-DAQ uses to determine when to put more data into the
FIFO. Refer to the
AO_Change_Parameter
function in NI-DAQ Function
Reference Online Help file for details.
Externally Triggering Your Waveform Generation Operation
You can initiate a waveform generation operation from an external
trigger signal in much the same manner as for analog input. See the
Select_Signal
function in the NI-DAQ Function Reference Online
Help file.
Digital I/O Function Group
The Digital I/O function group contains three sets of functions—the Digital
I/O (
DIG
) functions, the Group Digital I/O (
DIG_Block
,
DIG_Grp
,and
DIG_SCAN
) functions, and the double-buffered Digital I/O (
DIG_DB
)
functions. Refer to the NI-DAQ Functions Listed by Hardware Product