ADLINK PXI-2208 - Manual (Page 50)

38 Operation Theory
Programmable Scan Acquisition Mode
Scan Timing and Procedure
It is recommended that you use this mode 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 four counters which need to be
specified:
X SI_counter (24-bit): Specify the Scan Interval = SI_counter /
Timebase
X
SI2_counter (16-bit): Specify the data Sampling Interval =
SI2_counter/Timebase
X
PSC_counter (24-bit): Specify Post Scan Counts after a
trigger event
X NumChan_counter (9-bit): Specify the number of samples
per scan
The acquisition timing and the meanings of the 2 counters are
illustrated in Figure 4-3.
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 CN2 connector. 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 while the
minimum should be 1 MHz. Refer to section 4.6 for information
on user-controllable timing signals.