© National Instruments Corporation 19 NI cDAQ-9178/9174 User Guide and Specifications
• Continuous—Continuous generation refers to the generation of an unspecified number of samples.
Instead of generating a set number of data samples and stopping, a continuous generation continues
until you stop the operation. There are three different continuous generation modes that control
how the data is written. These modes are regeneration, onboard regeneration, and
non-regeneration:
– In regeneration mode, you define a buffer in host memory. The data from the buffer is
continually downloaded to the FIFO to be written out. New data can be written to the host
buffer at any time without disrupting the output. There is no limitation on the number of
waveform channels supported by regeneration mode.
– With onboard regeneration, the entire buffer is downloaded to the FIFO and regenerated from
there. After the data is downloaded, new data cannot be written to the FIFO. To use onboard
regeneration, the entire buffer must fit within the FIFO size. The advantage of using onboard
regeneration is that it does not require communication with the main host memory once the
operation is started, which prevents problems that may occur due to excessive bus traffic or
operating system latency. There is a limit of 16 waveform channels for onboard regeneration.
– With non-regeneration, old data is not repeated. New data must continually be written to the
buffer. If the program does not write new data to the buffer at a fast enough rate to keep up
with the generation, the buffer underflows and causes an error. There is no limitation on the
number of waveform channels supported by non-regeneration.
Analog Output Triggering
Analog output supports two different triggering actions:
• Start Trigger
• Pause Trigger
• AO Start Trigger
• AO Pause Trigger
An analog or digital trigger can initiate these actions. Up to two C Series hardware-timed digital input
modules can be used in any chassis slot to supply a digital trigger. An analog trigger can be supplied by
some C Series analog modules.
AO Start Trigger Signal
Use the AO Start Trigger (ao/StartTrigger) signal to initiate a waveform generation. If you do not use
triggers, you can begin a generation with a software command. If you are using an internal sample clock,
you can specify a delay from the start trigger to the first sample. For more information, refer to the
NI-DAQmx Help. The NI-DAQmx Help is available after installation from Start»All Programs»
National Instruments»NI-DAQ»NI-DAQmx Help.
Using a Digital Source
To use ao/StartTrigger, specify a source and a rising or falling edge. The source can be one of the
following signals:
• A pulse initiated by host software
• Any PFI terminal
• AI Reference Trigger
• AI Start Trigger
The source also can be one of several internal signals on the NI cDAQ-9178/9174 chassis. Refer to the
Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more information.
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