ADLINK DAQe-2006 - Manual (Page 47)

Operation Theory 35
Programmable Scan Acquisition Mode
Scan Timing and Procedure
It's recommended that this mode be used if your applications
need a fixed and precise A/D sampling rate. You can accu-
rately program the period between conversions of individual
channels. There are at least two counters which need to be
specified:
SI_counter (24 bit):Specify the Scan Interval =
SI_counter / TIMEBASE
PSC_counter (24 bit):Specify Post Scan Counts,
i.e. the total sample count after a trigger
event,
The acquisition timing and the meanings of the 2 counters are
illustrated in Figure 4-3. The SCAN_START signal is derived
from the SI_counter, which will lead to the A/D conversion sig-
nal generation. Note that the DAQ-/DAQe-/PXI-2016/2010/
2006/2005 card is a simultaneous sampling A/D card, so the
scan interval equals the sampling interval.
Example: Post-trigger acquisition
Set
SI_counter = 160
PSC_counter = 30
TIMEBASE = Internal clock source
Then
Scan Interval = 160/40M s = 4 us
Total acquisition time = 30 X 4 us = 120 us
TIMEBASE Clock Source
In scan acquisition mode, all the A/D conversions start on the
output of counters, which use TIMEBASE as the clock source.
By software you can specify the TIMEBASE to be either an
internal clock source (onboard 40 MHz clock) or an external
clock input (EXTTIMEBASE) on J5 connector (68-pin VHDCI).
The external TIMEBASE is useful when you want to acquire
data at rates not available with the internal A/D sample clock.
The external clock source should generate TTL-compatible
continuous clocks and with a maximum frequency of 40 MHz