Chapter 2 Analog Input Timing/Control©National Instruments Corporation2-7DAQ-STC Technical Reference Manual2.4.1.1 ADC ControlThe basic function of the ADC control signals is to time ADC conversions and load theresulting 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 dataacquisition sequence.Figure 2-3. ADC ControlThe SOC input informs the DAQ-STC when a conversion starts. Similarly, the EOC inputnotifies the DAQ-STC when a conversion completes. The assertion of EOC leads to theassertion of SHIFTIN, which loads the acquired data into its destination. An overruncondition occurs when the conversion rate is too high for the A/D subsystem to maintain. If asecond CONVERT signal occurs before the current conversion is complete, the DAQ-STCflags an overrun error and generates an interrupt if programmed to do so.2.4.1.2 Data FIFO ControlMany DAQ products include data FIFOs to prevent loss of data at high speeds and to increasebus bandwidth. The data FIFO control signals support such a FIFO. The SHIFTIN signalloads the result of each conversion into the data FIFO. Three status flags indicate the amountof data stored in the FIFO and are monitored by the DAQ-STC. The three status flags areAIFFF, AIFHF, and AIFEF. When the FIFO flags indicate that the FIFO level has exceeded aprogrammed threshold, the DAQ-STC notifies the host computer using either an interruptrequest, if the CPU performs data transfers, or the DMA request signal AIFREQ, if the DMAcontroller performs data transfers. The host computer then transfers the input data stored inthe data FIFO to main memory.2.4.1.3 Configuration FIFO and External Multiplexer ControlTypical DAQ products provide a means of setting channel, gain, polarity, and otherconfiguration information for the A/D subsystem. Recent products offer the ability to changethis information on a per conversion basis. The DAQ-STC fully supports a FIFO-basedconfiguration memory that can update on every CONVERT pulse. Typically, theconfiguration FIFO contains one data word for each input channel included in the scan. At theCONVERTSOCEOCSHIFTIN
