National Instruments Isolated Analog Input Device NI PXI-4224 - Manual (Page 60)

Appendix B Timing Signal Information
NI PXI-4224 User Manual B-4 ni.com
The device also uses AI START TRIG to initiate pretriggered operations.
In pretriggered applications, AI START TRIG is generated by a software
trigger unless a PFI pin is selected as the source of AI START TRIG. Refer
to the AI REF TRIG Signal section for a complete description of the use of
AI START TRIG and AI REF TRIG in a pretriggered operation.
AI REF TRIG Signal
The AI REF TRIG signal can be input through PFI 0, PXI_Trig<0..5>, or
PXI_Star. Refer to Figure B-2 for the relationship of AI REF TRIG to the
sequence.
As an input, AI REF TRIG is configured in edge-detection mode. You can
configure the polarity selection for either rising or falling edge. The
selected edge of AI REF TRIG initiates the posttriggered phase of a
pretriggered sequence. In pretriggered mode, the AI START TRIG signal
initiates the acquisition. The scan counter (SC) indicates the minimum
number of scans before AI REF TRIG is recognized. After the SC
decrements to zero, it is loaded with the number of posttrigger scans to
acquire while the acquisition continues. The device ignores AI REF TRIG
if it is asserted prior to the SC decrementing to zero. After the selected edge
of AI REF TRIG is received, the device acquires a fixed number of scans
and the acquisition stops. In pretriggered mode, the device acquires data
both before and after receiving AI REF TRIG.
As an output, AI REF TRIG reflects the posttrigger in a pretriggered
sequence, even if the acquisition is externally triggered by another PFI.
AI REF TRIG is not used in posttriggered DAQ. The output is an active
high pulse with a pulse width of 50 to 100 ns. This output is set to
high-impedance at startup.
Figures B-5 and B-6 show the input and output timing requirements
for AI REF TRIG.
Figure B-5. AI REF TRIG Input Signal Timing
Rising-Edge
Polarity
Falling-Edge
Polarity
t
w
= 10 ns minimum
t
w