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Chapter 2
Using the PXIe-4309
Sample Timing Modes
The PXIe-4309 supports four timing modes that fall into two categories: software-timed and
hardware-timed.
Note
Software-timed, hardware timed single point (HWTSP), and external sample
clock use single sample acquisition to minimize latency. Refer to the
section for more information.
Software-Timed Acquisitions
Software controls the rate of the acquisition. Software sends a separate command to the
hardware to acquire each sample. In NI-DAQmx, software-timed acquisitions are referred to as
having on-demand timing. Software-timed acquisitions are also referred to as immediate or
static acquisitions and are typically used for reading a single sample of data.
Hardware-Timed Acquisitions
A digital hardware signal (AI Sample Clock) controls the rate of the acquisition. This signal can
be generated internally by the PXIe-4309 or provided externally.
Hardware-timed acquisitions have several advantages over software-timed acquisitions:
•
The time between samples can be much shorter.
•
The timing between samples is deterministic.
•
Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or HWTSP. A buffer is a temporary storage in
computer memory for to-be-transferred samples.
•
Buffered
—In a buffered acquisition, data is moved from the onboard FIFO memory of the
DAQ device to a PC buffer using DMA before it is transferred to application memory.
Buffered acquisitions typically allow for much faster transfer rates than HWTSP
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 acquisition refers to the acquisition of a specific, predetermined
number of data samples. Once the specified number of samples has been read, the
acquisition stops. If you use a reference trigger, you must use finite sample mode.
–
Continuous acquisition refers to the acquisition of an unspecified number of samples.
Instead of acquiring a set number of data samples and stopping, a continuous
acquisition continues until you stop the operation. Continuous acquisition is also
referred to as double-buffered or circular-buffered acquisition.
If data cannot be transferred across the bus fast enough, the FIFO becomes full. For
more information refer to
NI-DAQmx Help
.
