Chapter 2
Theory of Operation
2-12
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National Instruments Corporation
conversions is not constant because it relies on the software to generate the
conversions. For finely timed conversions that require triggering and
gating, you must program the boards to automatically generate timed
signals that initiate and gate conversions. This is known as a data
acquisition sequence.
Data Acquisition Sequence Timing
The following counters are used for a data acquisition sequence:
•
Scan interval (SI)
24 bits
•
Sample interval (SI2)
16 bits
•
Divide by (DIV)
16 bits
•
Scan counter (SC)
24 bits
This section presents a concise summary of only the most important
features of your board. For a complete description of all the analog input
modes and features of the AT E Series, refer to the DAQ-STC Technical
Reference Manual.
The most basic timing signal in the analog input model is the CONVERT*
signal. A group of precisely timed CONVERT* pulses is a SCAN. The
sequence of channels selected in each conversion in a SCAN is
programmed in the configuration memory prior to starting the operation.
The SI2 counter is a 16-bit counter in the DAQ-STC. This counter
determines the interval between CONVERT* pulses. It can be programmed
for a maximum interval of 3.3 ms and a minimum interval of 50 ns. If
alternate slow timebases are used, the maximum interval is 0.65 s. Each
time the counter reaches terminal count (TC), a CONVERT* pulse is
generated. Alternatively, CONVERT* pulses could be given externally.
A SCAN sequence is started by the START pulse, which is generated by the
terminal count of the SI counter. This counter is a 24-bit counter that
determines the time between the start of each SCAN. The minimum
duration is 50 ns and the maximum duration is 0.8 s when the internal
20 MHz timebase is used. If the internal 100 kHz timebase is used, the
maximum is 167 s. The START pulse triggers the SI2 counter to generate
CONVERT* pulses. With each conversion, the configuration memory
advances by one and selects the next set of analog input conditions—
channel number, gain, polarity, etc. A STOP pulse ends the SCAN
sequence. This STOP could be generated in two ways—either by using the
LASTCHANNEL bit in the configuration memory or by programming the
16-bit DIV counter to count the number of conversions per SCAN and
using the terminal count of the DIV counter as a STOP pulse.
Содержание AT-MIO E Series
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