Chapter 3
Connecting the Signals
©
National Instruments Corporation
3-23
DAQCard-1200 User Manual
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.
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.
Note
LabVIEW is scan clock oriented. When you specify the scan rate, you are
controlling the scan interval timing.
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*.
V
IH
V
IL
t
w
t
w
t
w
250 ns minimum
A/D Conversion Starts within
125 ns from this Point
EXTCONV*
