National Instruments DAQPad-1200 User Manual page 47

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IH
EXTCONV*
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Note LabVIEW is scan clock oriented. When you specify the scan rate, you are
controlling the scan interval timing.
© National Instruments Corporation
Instead of using counter A0 as the sample-interval counter, you can use
EXTCONV* to externally time conversions. Figure 3-13 shows the timing
requirements for the EXTCONV* input. A falling edge on the
EXTCONV* initiates an A/D conversion.
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A/D Conversion Starts within
125 ns from this Point
Figure 3-13. EXTCONV* Signal Timing
The external control signal EXTTRIG can either start a DAQ sequence or
terminate an ongoing DAQ sequence depending on the mode—posttrigger
or pretrigger. These modes are software-selectable.
In the posttrigger mode, EXTTRIG serves as an external trigger that
initiates a DAQ sequence. When you use counter A0 to time sample
intervals, a rising edge on EXTTRIG starts counter A0 and the DAQ
sequence. When you use EXTCONV* to time sample intervals, data
acquisition is enabled on a rising edge of EXTTRIG followed by a rising
edge on EXTCONV*. The first conversion occurs on the next falling edge
of EXTCONV*. Further transitions on the EXTTRIG line have no effect
until a new DAQ sequence is established.
Figure 3-14 shows a possible timed data acquisition sequence using
EXTCONV* and EXTTRIG. The rising edge of EXTCONV* that enables
external conversions must occur a minimum of 50 ns after the rising edge
of EXTTRIG. The first conversion occurs on the next falling edge of
EXTCONV*.
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3-23
Chapter 3 Connecting the Signals
t 250 ns minimum
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DAQCard-1200 User Manual