Chapter 2
Theory of Operation
©
National Instruments Corporation
2-11
read and then written to calibration DACs that adjust input offset, output
offset, and gain errors associated with the analog input section. When the
board leaves the factory, the upper one-fourth of the EEPROM is protected
and cannot be overwritten. The lower three-fourths is unprotected and the
top fourth of that can be used to store alternate calibration constants for the
different conditions under which you use the board. The entire lower half
of the EEPROM is used for storing Plug and Play information and should
never be written to.
Data Acquisition Timing Circuitry
This section describes the different methods of acquiring A/D data from a
single channel or multiple channels.
From this section through the end of this manual, you are assumed to have
a working knowledge of the DAQ-STC features. These features are
explained in the DAQ-STC Technical Reference Manual. If you have not
read the functional description of each DAQ-STC module, you must do so
before completing this register-level programmer manual.
Single-Read Timing
To acquire data from the ADC, initiate a single conversion and read the
resulting value from the ADC FIFO buffer after the conversion is complete.
You can generate a single conversion in three different ways—apply an
active low pulse to the CONVERT* pin of the I/O connector, generate a
falling edge on the sample-interval counter of the DAQ-STC, or strobe the
appropriate bit in a register in the AT E Series register set. Any one of these
operations will generate the timing shown in Figure 2-8.
Figure 2-8. ADC Timing
When SHIFTIN* shifts the ADC value into the ADC FIFO buffer, the
AI_FIFO_Empty_St bit in the status register is cleared, which indicates
that valid data is available to be read. Single conversion timing of this type
is appropriate for reading channel data on an ad hoc basis. However, if you
need a sequence of conversions, the time interval between successive
CONVERT*
ADC_BUSY*
SHIFTIN*
Содержание AT-AI-16XE-10
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