ADLINK PXI-2006 - Manual (Page 45)

Operation Theory 35
This situation can be avoided by setting M_enable. 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 M scans of data under
pre-trigger mode, as illustrated in Figure 4-7. However, if
M_enable is set to 0, the trigger signal will be accepted any time,
as illustrated in Figure 13. Note that the total amount of stored
data will always be equal to the number in the M_counter because
the data acquisition won’t stop until a scan is completed.
Figure 4-6: Pre-trigger with M_enable = 0 (Trigger occurs before M scans)
Figure 4-7: Pre-trigger with M_enable = 1