Chapter 2
Theory of Operation
2-24
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National Instruments Corporation
The external strobe signal EXTSTROBE* is a general-purpose strobe
signal. Software can set the output of the EXTSTROBE* pin to either a
high or low state. EXTSTROBE* is not necessarily part of the digital I/O
circuitry but is included here because you can use it to latch digital output
from the AT E Series into an external device.
Timing I/O Circuitry
The DAQ-STC has two 24-bit general purpose counter/timers. The
counters are numbered 0 and 1 and are diagrammed as shown in
Figure 2-18.
Figure 2-18. DAQ-STC Counter Diagram
The counter counts the transitions, or edges, on the SOURCE line when the
GATE is enabled and generates timing signals at its OUT output pin. The
UPDOWN pin determines the direction of counting. Active polarities of
these pins are software selectable in the DAQ-STC. Notice that on the AT
E Series only the SOURCE, GATE, and OUT pins are brought out to the
I/O connector. The UPDOWN pin for counter 0 is internally connected to
DIO6 and to DIO7 for counter 1. To drive the UPDOWN pin from the
connector, you must tri-state the corresponding DIO line.
The SOURCE of these counters can be selected to be one of the 10 PFI lines
or the seven RTSI lines, or the internal 20 MHz or 100 kHz timebases, or
the TC of the other counter. With the last option, the two counters can be
concatenated. Similarly, the GATE can also be selected from a variety of
different sources. The sources for both of these signals are described in the
DAQ-STC Technical Reference Manual.
OUT
UPDOWN
GATE
SOURCE
Counter
Содержание AT-AI-16XE-10
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