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Chapter 4 Analog Input

• Hardware-Timed Acquisitions—With hardware-timed acquisitions,

a digital hardware signal, AI Sample Clock, controls the rate of the

acquisition. This signal can be generated internally on your device 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 are 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. After the

specified number of samples has been read in, 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 acq

uiring a set numb

er 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.

National Instruments USB-621 Series User Manual

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