National Instruments NI USB-621x manual 70

Chapter 7 Analog Output

Hardware-timed generations have several advantages over software-timed acquisitions:

•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 non-buffered.

During hardware-timed AO generation, data is moved from a PC buffer to the onboard FIFO on the DAQ 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. Once 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 several different 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, new 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 will not be repeated. New data must be continually written to the buffer. If the program does not write new data to

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