ADLINK PCI-9846 - Manual (Page 50)

40 Operation Theory
3.5.2 Pre-trigger Acquisition
Use pre-trigger acquisition to collect data before the trigger event.
The acquisition starts once specified function calls are executed to
begin the pre-trigger operation, and it stops when the trigger event
occurs.
If the trigger event occurs after the specified amount of data has
been acquired, the system only stores the data before the trigger
event with specified amount, as illustrated in Figure 3-12.
Figure 3-12: Pre-trigger Mode Operation
The trigger event occurs after the specified amount of data has
been acquired. However, if the trigger event occurs before the
specified amount of data has been acquired, the acquisition
engine will ignore the trigger signal until the specified amount of
data has been acquired. Refer to Figure 3-13 for an example.
Figure 3-13: Pre-trigger Mode Operation
Time
Operation start
Acquisition start
Trigger
Data
Trigger Event Occurs
Acquisition stop
Begin to transfer data to
system
N samples
These data will be
discarded.
Only acquired N
samples will be
transfer back to
system.
Time
Operation start
Acquisition start
Trigger
Data
Trigger Event Occurs
Acquisition stop
Begin to transfer data to system
N samples
X samples have been acquired
before trigger occurs, where
X<N
Trigger signals that occur before
the specified amount of data has
been acquired will be ignored.