Chapter 2 Theory of OperationPCI E Series RLPM2-24 ©National Instruments Corporationto retransmit the same FIFO data to the DACs over and over. When theFIFO is empty, the retransmit signal is asserted, which restores that FIFOdata to its original state.When both DACs are used in waveform generation, the data in the FIFO isinterleaved; in other words, every alternate sample belongs to one DAC.Digital I/O CircuitryThe PCI E Series boards have eight digital I/O lines. Each of the eightdigital lines can be individually programmed to be input or output, if usedin parallel. For serial data transfer, DIO4 is used as the serial data in pin,and DIO0 is used as the serial data out pin.You can use handshaking with the EXTSTROBE* pin to do either parallelor serial data transfer. Refer to the DAQ-STC Technical Reference Manualfor more details on the DIO features.The external strobe signal EXTSTROBE* is a general-purpose strobesignal. Software can set the output of the EXTSTROBE* pin to either ahigh or low state. EXTSTROBE* is not necessarily part of the digital I/Ocircuitry, but is included here because you can use it to latch digital outputfrom the PCI E Series into an external device.Timing I/O CircuitryThe DAQ-STC has two 24-bit general purpose counter/timers. Thecounters are numbered 0 and 1 and are diagrammed as shown inFigure 2-18.Figure 2-18. DAQ-STC Counter DiagramOUTUPDOWNGATESOURCECounter
