Programming instructions

Chapter 20 DSA Devices
© National Instruments Corporation 20-3 DAQ Hardware Overview Guide
Pretrigger mode collects data both before and after the device receives a
trigger in posttrigger mode, either through software or by applying
a hardware signal. The device collects samples and fills the user-specified
buffer without stopping until the device receives the stop trigger signal.
Refer to the stop trigger discussion in the data acquisition timing section
later in this chapter for details. The device then collects a specified number
of samples and stops the acquisition. The buffer is treated as a circular
buffer—when the entire buffer has been written to, data is stored at the
beginning again, overwriting the old data. When data acquisition stops, the
buffer has samples from before and after the stop trigger occurred. The
number of samples saved depends on the length of the user-specified buffer
and on the number of samples specified to be acquired after receipt of the
trigger.
Double-buffered mode, like pretrigger mode, also fills the user-specified
buffer continuously. You can call
DAQ_DB_Transfer to transfer old data
into a second buffer before it is overwritten by new data.
DAQ_DB_Transfer transfers data out of one half of the buffer while the
other half is filled with new data.
Data Acquisition Timing
A direct digital synthesis (DDS) clock generation circuit generates an
extremely stable clock signal that can drive the converters at scan rates up
to 204.8 kHz in increments of 200 µHz. The antialiasing filter in the
converter adjusts to the scan rate to reject frequency components from
frequencies above the scan rate. Because the converters require the stability
of the DDS clock, you cannot supply an external clock. Clock signals can
be synchronized between multiple PCI-445X and NI 455X devices over the
RTSIbus.
You can configure a data acquisition operation to use the following trigger
signals:
•A start trigger is a signal that initiates a data acquisition sequence. You
can supply this signal externally through a selected I/O connector pin,
through a RTSIbus trigger line, or by software.
•A stop trigger is a signal used for pretriggered data acquisition to
notify your device to stop acquiring data after a specified number of
scans. Data acquisition operation is continuously performed until the
device receives this signal. You can supply this signal externally
through a selected I/O connector pin, through a RTSIbus trigger line,
or by software.