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Chapter 6
Digital I/O
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Buffered
—In a buffered generation, data is moved from a PC buffer to the DAQ device’s
onboard FIFO using DMA before it is written to the output lines one sample at a time.
Buffered generation typically allow for much faster transfer rates than non-buffered
acquisitions because data is moved in large blocks, rather than one point at a time.
One property of buffered I/O operations is the sample mode. The sample mode can be either
finite or continuous:
–
Finite sample mode generation refers to the generation of a specific, predetermined
number of data samples. Once the specified number of samples has been written out,
the generation stops.
–
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 several different methods
of continuous generation that control what data is written. These methods are
regeneration, FIFO regeneration and non-regeneration modes:
•
Regeneration is the repetition of the data that is already in the buffer. Standard
regeneration is when data from the PC buffer is continually downloaded to the
FIFO to be written out. New data can be written to the PC buffer at any time
without disrupting the output. Use the NI-DAQmx write property regenMode to
allow (or not allow) regeneration. The NI-DAQmx default is to allow
regeneration.
•
With non-regeneration, old data is not repeated. New data must be continually
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.
•
With FIFO regeneration, the entire buffer is downloaded to the FIFO and
regenerated from there. Once the data is downloaded, new data cannot be written
to the FIFO. To use FIFO regeneration, the entire buffer must fit within the FIFO
size. The advantage of using FIFO regeneration is that it does not require
communication with the main host memory once the operation is started, thereby
preventing any problems that may occur due to excessive bus traffic. Use the
NI-DAQmx UseOnlyOnBoardMemeory DO channel property to enable or
disable FIFO regeneration.
Digital Output Triggering
Digital output supports two different triggering actions:
•
Start trigger
•
Pause trigger
An analog or digital trigger can initiate these actions. All X Series devices support digital
triggering, but some do not support analog triggering. To find your device’s triggering options,
refer to the device specifications. Refer to the
sections for more information about these triggering actions.
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