User's Manual

Chapter 4 Analog Input
NI USB-621x User Manual 4-10 ni.com
Buffered
In a buffered acquisition, data is moved from the DAQ device’s onboard
FIFO memory to a PC buffer using USB signal streams or programmed I/O
before it is transferred to application memory. Buffered acquisitions
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 acquisition refers to the acquisition of a specific,
predetermined number of data samples. Once the specified number of
samples has been written out, the generation 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. New acquisitions will overwrite data in the FIFO before it can be
transferred to host memory. The device generates an error in this case. With
continuous operations, if the user program does not read data out of the PC
buffer fast enough to keep up with the data transfer, the buffer could reach
an overflow condition, causing an error to be generated.
Non-Buffered
Hardware-timed, non-buffered mode is not supported for USB M series
devices.
Analog Input Digital Triggering
Analog input supports three different triggering actions:
• Start trigger
• Reference trigger
• Pause trigger
Refer to the AI Start Trigger Signal, AI Reference Trigger Signal, and AI
Pause Trigger Signal sections for information about these triggers.