User`s guide

32 • Operation Theory
4.1.4.3 Trigger Modes
DAQ/PXI-22XX provides 3 trigger sources (internal software, external
analog and digital trigger sources). You must select one of them as the
source of the trigger event. A trigger event occurs when the specified
condition is detected on the selected trigger source (For example, a rising
edge on the external digital trigger input).
There are 4 trigger modes (pre-trigger, post-trigger, middle-trigger, and
delay-trigger) working with the 4 trigger sources to initiate different scan
data acquisition timing when a trigger event occurs. They are described as
follows. For information of trigger sources, please refer to section 4.5.
Pre-Trigger Acquisition
Use pre-trigger acquisition in applications where you want to collect data
before a trigger event. The A/D starts when you execute the specified
function calls to begin the operation, and it stops when the external trigger
event occurs. Users must program the value M in M_counter (16bit) to
specify the amount of stored scanned data before the trigger event. If an
external trigger occurs after M scans of data are converted, the program
only stores the last M scans of data, as illustrated in fig 12, where
M_counter = M =3, NumChan_counter =4, PSC_counter = 0. The total
stored amount of data = NumChan_counter *M_counter =12.
Acquisition_in_progress
Scan_start
AD_conversion
Scan_in_progress
(SSHOUT)(pin8 on CN2)
(M_counter = M = 3, NumChan_counter=4, PSC_counter=0)
Aquired data
Acquired & stored data
(M scans)
Operation start
Trigger
Figure 12: Pre-trigger (trigger occurs after M scans)