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Chapter 5 Analog Output

– Hardware-timed acquisitions can use hardware triggering.

Hardware-timed operations are buffered. During hardware-timed

AO generation, data is moved from a PC buffer to the onboard

FIFO on the USB-621x device using USB Signal Streams before

it is written to the DACs one sample at a time. Buffered

acquisitions allow for fast transfer rates 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. After 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

three 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.

With FIFO regeneration, the entire buffer is downloaded to the

FIFO and regenerated from there. Once the data is downloaded,

w 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.

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.

National Instruments USB-621 Series User Manual

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