ADLINK PXI-2006 - Manual (Page 76)

66 Operation Theory
EXTTIMEBASE
When the applications needs specific sampling frequency or
update rate that the card could not generate from its internal
TIMEBASE, the 40MHz clock, users could utilize the EXT-
TIMEBASE with internal counters to achieve the specific timing
intervals for both A/D and D/A operations. Note that once you
choose the TIMEBASE source, both A/D and D/A operations
will be affected because A/D and D/A operations share the
same TIMEBASE.
AFI[0]
Alternatively, users can also directly apply an external A/D con-
version signal to replace the internal ADCONV signal. This is
another way to achieve customized sampling frequencies. The
external ADCONV signal can only be inputted from the AFI[0].
As section 4.1 describes, the SI_counter triggers the genera-
tion of the A/D conversion signal, ADCONV, but when using the
AFI[0] to replace the internal ADCONV signal, then the
SI_counter and the internally generated SCAN_START will not
be effective. By controlling the ADCONV externally, users can
sample the data ac-cording to external events. In this mode,
the Trigger signal and trigger mode settings will are not avail-
able.
AFI[0] could also be used as SCAN_START signal for A/D
operations. Please refer to sections 4.1 and 4.6.1 for detailed
descriptions of the SCAN_START signal. When using external
signal (AFI[0]) to replace the internal SCAN_START signal, the
pulse width of the AFI[0] must be greater than two time of the
period of Timebase. This feature is suitable for the DAQ-2200/
PXI-2200 series, which can scan multiple channels data con-
trolled by an external event. Note that the AFI[0] is a multi-pur-
pose input, and it can only be utilized for one function at any
one time.
AFI[1]
Regarding the D/A operations, users could directly input the
external D/A update signal to replace the internal DAWR sig-
nal. This is another way to achieve customized D/A update
rates. The external DAWR signal can only be inputted from the