User`s guide

60 • Operation Theory
4.6.1 DAQ timing signals
The user-controllable DAQ timing-signals contains: (Please refer to 4.1.4.1
for the internal timing signal definition)
1. TIMEBASE, providing TIMEBASE for all DAQ operations, which
could be from internal 40MHz oscillator, EXTTIMEBASE from I/O
connector or the SSI_TIMEBASE. Note that the frequency range of
the EXTTIMEBASE is 1MHz to 40MHz, and the EXTTIMEBASE
should be TTL-compatible.
2. AD_TRIG, the trigger signal for the A/D operation, which could come
from external digital trigger, analog trigger, internal software trigger
and SSI_AD_TRIG. Refer to Section 4.5 for detailed description.
3. SCAN_START, the signal to start a scan, which would bring the fol-
lowing ADCONV signals for AD conversion, and could come from the
internal SI_counter, AFI[0] and SSI_AD_START. This signal is syn-
chronous to the TIMEBASE. Note that the AFI[0] should be
TTL-compatible and the minimum pulse width should be the pulse
width of the TIMEBASE to guarantee correct functionalities.
4. ADCONV, the conversion signal to initiate a single conversion, which
could be derived from internal counter, AFI[0] or SSI_ADCONV.
Note that this signal is edge-sensitive. When using AFI[0] as the
external ADCONV source, each rising edge of AFI[0] would bring an
effective conversion signal. Also note that the AFI[0] signal should be
TTL-compatible and the minimum pulse width is 20ns.
5. DA_TRIG, the trigger signal for the D/A operation, which could be
derived from external digital trigger, analog trigger, internal software
trigger and SSI_AD_TRIG. Refer to Section 4.5 for detailed de-
scription.
6. DAWR, the update signal to initiate a single D/A conversion, which
could be derived from internal counter, AFI[1] or SSI_DAWR. Note
that this signal is edge-sensitive. When using AFI[1] as the external
DAWR source, each rising edge of AFI[1] would bring an effective
update signal. Also note that the AFI[1] signal should be
TTL-compatible and the minimum pulse width is 20ns.
4.6.2 Auxiliary Function Inputs (AFI)
Users could use the AFI in applications that take advantage of external
circuitry to directly control the DAQ/PXI-22XX cards. The AFI includes 2
categories of timing signals: one group is the dedicated input, and the other
is the multi-function input. Table 15 illustrates this categorization.