Chapter 5 Analog Output© National Instruments Corporation 5-3 NI USB-621x User Manual– Hardware-timed acquisitions can use hardware triggering.Hardware-timed operations are buffered. During hardware-timed AOgeneration, data is moved from a PC buffer to the onboard FIFO onthe USB-621x device using USB Signal Streams before it is written tothe DACs one sample at a time. Buffered acquisitions allow for fasttransfer rates because data is moved in large blocks rather than onepoint at a time.One property of buffered I/O operations is the sample mode. Thesample 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 ofsamples has been written out, the generation stops.Continuous generation refers to the generation of an unspecifiednumber of samples. Instead of generating a set number of data samplesand stopping, a continuous generation continues until you stop theoperation. There are several different methods of continuousgeneration that control what data is written. These methods areregeneration, 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 continuallydownloaded to the FIFO to be written out. New data can be written tothe PC buffer at any time without disrupting the output.With FIFO regeneration, the entire buffer is downloaded to the FIFOand regenerated from there. Once the data is downloaded, new datacannot be written to the FIFO. To use FIFO regeneration, the entirebuffer must fit within the FIFO size. The advantage of using FIFOregeneration is that it does not require communication with the mainhost memory once the operation is started, thereby preventing anyproblems that may occur due to excessive bus traffic.With non-regeneration, old data is not repeated. New data must becontinually written to the buffer. If the program does not write newdata to the buffer at a fast enough rate to keep up with the generation,the buffer underflows and causes an error.Analog Output Digital TriggeringAnalog output supports two different triggering actions:• Start trigger• Pause trigger
