ADLINK PXI-2208 - Manual (Page 55)

Operation Theory 43
Pre-Trigger Acquisition
Use pre-trigger acquisition in applications where you want to
collect data before a trigger event. The A/D starts to sample
when you execute the specified function calls to begin the pre-
trigger operation, and it stops when the trigger event occurs.
Users must program the value M in M_counter (16 bits) to
specify the amount of the stored scans before the trigger event.
If an external trigger occurs, the program only stores the last M
scans of data converted before the trigger event, as illustrated
in Figure 4-4, where M_counter = M =3, PSC_counter = 0. The
post scan count is 0 because there is no sampling after the trig-
ger event in pre-trigger acquisition. The total stored amount of
data = Number of enabled channels * M_counter.
Figure 4-4: Pre-trigger (Trigger occurs after M scans)
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