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Chapter 2
Analog Input Timing/Control
2.4.1.1 ADC Control
The basic function of the ADC control signals is to time ADC conversions and load the
resulting digital data into a latch. The primary output signals are CONVERT and SHIFTIN,
and the input signals are SOC and EOC. Figure 2-3 shows these signals in a basic data
acquisition sequence.
CONVERT
SOC
EOC
SHIFTIN
Figure 2-3. ADC Control
The SOC input informs the DAQ-STC when a conversion starts. Similarly, the EOC input
notifies the DAQ-STC when a conversion completes. The assertion of EOC leads to the
assertion of SHIFTIN, which loads the acquired data into its destination. An overrun
condition occurs when the conversion rate is too high for the A/D subsystem to maintain. If a
second CONVERT signal occurs before the current conversion is complete, the DAQ-STC
flags an overrun error and generates an interrupt if programmed to do so.
2.4.1.2 Data FIFO Control
Many DAQ products include data FIFOs to prevent loss of data at high speeds and to increase
bus bandwidth. The data FIFO control signals support such a FIFO. The SHIFTIN signal
loads the result of each conversion into the data FIFO. Three status flags indicate the amount
of data stored in the FIFO and are monitored by the DAQ-STC. The three status flags are
AIFFF, AIFHF, and AIFEF. When the FIFO flags indicate that the FIFO level has exceeded a
programmed threshold, the DAQ-STC notifies the host computer using either an interrupt
request, if the CPU performs data transfers, or the DMA request signal AIFREQ, if the DMA
controller performs data transfers. The host computer then transfers the input data stored in
the data FIFO to main memory.
2.4.1.3 Configuration FIFO and External Multiplexer Control
Typical DAQ products provide a means of setting channel, gain, polarity, and other
configuration information for the A/D subsystem. Recent products offer the ability to change
this information on a per conversion basis. The DAQ-STC fully supports a FIFO-based
configuration memory that can update on every CONVERT pulse. Typically, the
configuration FIFO contains one data word for each input channel included in the scan. At the
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DAQ-STC Technical Reference Manual