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Chapter 2 Function Reference — SCAN_Start
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National Instruments Corporation 2-313 NI-DAQ FRM for PC Compatibles
you cannot control the scan interval externally. In this case, NI-DAQ scans the
channels repeatedly as fast as you apply the external conversion pulses.
Note Simultaneous sampling devices ignore parameters for sampTimebase and
sampInterval. These devices sample all channels simultaneously. The acquisition
rate is controlled by scanTimebase and scanInterval; therefore, a scanInterval
value of 0 is not allowed.
Using This Function
SCAN_Start initializes the Mux-Gain Memory table to point to the start of the scan sequence
as specified by
SCAN_Setup. If you did not specify external sample-interval timing by the
DAQ_Config call, NI-DAQ sets the sample-interval counter to the specified sampInterval
and sampTimebase, sets the scan-interval counter to the specified scanInterval and
scanTimebase, and sets up the sample counter to count the number of samples acquired and
to stop the data acquisition process when the number of samples acquired equals count. If you
have specified external sample-interval timing, the data acquisition circuitry relies on pulses
received on the EXTCONV* input to initiate individual A/D conversions. In this case,
NI-DAQ scans the channels repeatedly as fast as you apply the external conversion pulses.
SCAN_Start initializes a background data acquisition process to handle storing of A/D
conversion samples into the buffer as NI-DAQ acquires them. When you use posttrigger mode
(with pretrigger mode disabled), the process stores up to count A/D conversion samples into
the buffer and ignores any subsequent conversions. NI-DAQ stores the acquired samples into
the buffer with the channel scan sequence data interleaved; that is, the first sample is the
conversion from the first channel, the second sample is the conversion from the second
channel, and so on.
You cannot make the second call to
SCAN_Start without terminating this background data
acquisition process. If a call to
DAQ_Check returns daqStopped = 1, the samples are available
and NI-DAQ terminates the process. In addition, a call to
DAQ_Clear terminates the
background data acquisition process. Notice that if a call to
DAQ_Check returns an error code
of overFlowError or overRunError, or daqStopped = 1, the process is automatically
terminated and there is no need to call
DAQ_Clear.
If you enable pretrigger mode,
SCAN_Start initiates a cyclical acquisition that continually
fills the buffer with data, wrapping around to the start of the buffer once NI-DAQ has written
to the entire buffer. When you apply the signal at the stop trigger input,
SCAN_Start
acquires an additional number of samples specified by the ptsAfterStoptrig parameter in
DAQ_StopTrigger_Config and then terminates. Be aware that a scan sequence always
completes. Therefore, NI-DAQ always obtains the most recent data point from the final
channel in the scan sequence. When you enable pretrigger mode, the length of the buffer,
which is greater than or equal to count, should be an integral multiple of numChans. If you
observed this rule, a sample from the first channel in the scan sequence always resides at
index = 0 in the buffer.