ADLINK PXI-2208 - Manual (Page 59)

Operation Theory 47
Middle-Trigger Acquisition
Use middle-trigger acquisition in applications where you want
to collect data before and after a trigger event. The number of
scans (M) stored before the trigger is specified in M_counter,
while the number of scans (N) after the trigger is specified in
PSC_counter.
Like pre-trigger mode, the number of stored data could be less
than the specified amount of data (M+N), if an external trigger
occurs before M scans of data are converted. The M_enable bit
in middle-trigger mode takes the same effect as in pre-trigger
mode. If M_enable is set to 1, the trigger signal will be ignored
until the first M scans of data are converted, and it assures the
user with (M+N) scans of data under middle-trigger mode.
However, if M_enable is set to 0, the trigger signal will be
accepted at any time. Figure 4-8 shows the acquisition timing
with M_enable=1.
Figure 4-8: Middle-Trigger with M_enable = 1