National Instruments NI cDAQ-9181 User Manual

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National Instruments NI cDAQ-9181 User Manual Manual pdf 52 page image

3-2 | ni.comChapter 3 Analog OutputHardware-Timed GenerationsWith a hardware-timed generation, a digital hardware signal controls the rate of the generation.This signal can be generated internally on the chassis or provided externally.Hardware-timed generations 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 triggeringHardware-timed AO operations on the cDAQ chassis must be buffered.Buffered Analog OutputA buffer is a temporary storage in computer memory for generated samples. In a bufferedgeneration, data is moved from a host buffer to the cDAQ chassis onboard FIFO before it iswritten to the C Series modules.One property of buffered I/O operations is sample mode. The sample mode can be either finiteor continuous:• Finite—Finite sample mode generation refers to the generation of a specific,predetermined number of data samples. After the specified number of samples is writtenout, the generation stops.• Continuous—Continuous generation refers to the generation of an unspecified number ofsamples. Instead of generating a set number of data samples and stopping, a continuousgeneration continues until you stop the operation. There are three different continuousgeneration modes that control how the data is written. These modes are regeneration,onboard regeneration, and non-regeneration:– In regeneration mode, you define a buffer in host memory. The data from the buffer iscontinually downloaded to the FIFO to be written out. New data can be written to thehost buffer at any time without disrupting the output. There is no limitation on thenumber of waveform channels supported by regeneration mode.– With onboard regeneration, the entire buffer is downloaded to the FIFO andregenerated from there. After the data is downloaded, new data cannot be written tothe FIFO. To use onboard regeneration, the entire buffer must fit within the FIFO size.The advantage of using onboard regeneration is that it does not require communicationwith the main host memory once the operation is started, which prevents problems thatmay occur due to excessive bus traffic or operating system latency. There is a limit of16 waveform channels for onboard regeneration.– With non-regeneration, old data is not repeated. New data must continually be writtento the buffer. If the program does not write new data to the buffer at a fast enough rateto keep up with the generation, the buffer underflows and causes an error. There is nolimitation on the number of waveform channels supported by non-regeneration.