National Instruments DAQ X NI 634 Series, Руководство пользователя

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5-3
X Series User Manual
Analog Output Data Generation Methods
When performing an analog output operation, you can perform software-timed or
hardware-timed generations.
Software-Timed Generations
With a software-timed generation, software controls the rate at which data is generated. Software
sends a separate command to the hardware to initiate each DAC conversion. In NI-DAQmx,
software-timed generations are referred to as on-demand timing. Software-timed generations are
also referred to as immediate or static operations. They are typically used for writing a single
value out, such as a constant DC voltage.
Hardware-Timed Generations
With a hardware-timed generation, a digital hardware signal controls the rate of the generation.
This signal can be generated internally on your device or provided externally.
Hardware-timed generations have several advantages over software-timed generations:
• The time between samples can be much shorter.
• The timing between samples can be deterministic.
• Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or hardware-timed single point (HWTSP). A buffer
is a temporary storage in computer memory for to-be-transferred samples.
• Hardware-timed single point (HWTSP) —Typically, HWTSP operations are used to
write single samples at known time intervals. While buffered operations are optimized for
high throughput, HWTSP operations are optimized for low latency and low jitter. In
addition, HWTSP can notify software if it falls behind hardware. These features make
HWTSP ideal for real time control applications. HWTSP operations, in conjunction with
the wait for next sample clock function, provide tight synchronization between the software
layer and the hardware layer. Refer to the
NI-DAQmx Hardware-Timed Single Point
Lateness Checking document for more information. To access this document, go to
ni.com/info and enter the Info Code daqhwtsp .
Note (NI USB-63
xx
Devices) USB X Series devices do not support
hardware-timed single point (HWTSP) operations.
• 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 DACs 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.