Chapter 6
Digital I/O
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National Instruments Corporation
6-3
Digital Waveform Triggering
M Series devices do not have an independent DI or DO Start Trigger for
digital waveform acquisition or generation. To trigger a DI or DO
operation, first select a signal to be the source of DI Sample Clock or DO
Sample Clock. Then, generate a trigger that initiates pulses on the source
signal. The method for generating this trigger depends on which signal is
the source of DI Sample Clock or DO Sample Clock.
For example, consider the case where you are using AI Sample Clock as the
source of DI Sample Clock. To initiate pulses on AI Sample Clock (and
therefore on DI Sample Clock), you use AI Start Trigger to trigger the start
of an AI operation. The AI Start Trigger causes the M Series device to
begin generating AI Sample clock pulses, which in turn generates DI
Sample clock pulses, as shown in Figure 6-2.
Figure 6-2.
Digital Waveform Triggering
Similarly, if you are using AO Sample Clock as the source of DI Sample
Clock, then AO Start Trigger initiates both AO and DI operations.
If you are using a Counter output as the source of DI Sample Clock, the
counter’s start trigger, enables the counter which drives DI Sample Clock.
If you are using an external signal (such as PFI
x
) as the source for DI
Sample Clock or DO Sample Clock, you must trigger that external signal.
Digital Waveform Acquisition
You can acquire digital waveforms on the Port 0 DIO lines. The DI
waveform acquisition FIFO stores the digital samples. M Series devices
have a DMA controller dedicated to moving data from the DI waveform
acquisition FIFO to system memory. The DAQ device samples the DIO
lines on each rising or falling edge of a clock signal, di/SampleClock.
PFI 1
(AI Start Trigger)
AI Sample Clock
DI Sample Clock
AI Start Trigger initiates
AI Sample Clock and DI Sample Clock.
