USB 6211 Analog Output Mode

[Grenka]

Hi everybody!

According to the specifications sheet of NI USB-6211 Device, Section Analog Output, the analog output can be processed in three modes:

AO waveform modes:

• Non-periodic waveform
• Periodic waveform regeneration mode from onboard FIFO
• Periodic waveform regeneration from host buffer including dynamic update

Source: NI-USB-6211 spec

In my waveform generator written in LabVIEW I use to generate frequences from 20 Hz up to 1000 Hz.

The signal is periodic, so the programm calculates only one cycle of waveform and use regeneration mode.

For 1000 Hz the settings are as followed * : SampleClockRate=800000 S/s,  #s 800

For 100 Hz: SCR=800000 S/s,  #s=8000

For 50 Hz: SCR=800000 S/s, #s=16000

and so on.

My question: In what mode the device generates the wave exactly? Obviously one ot the last two. Does it switch to the last mode automatically, as the number of samples is greater than 8191 (Output FIFO size)? If not, how can I control this behavior?

* My second question refers to the maximum output rate of USB-6211 device. According to specifications it has a maximum output rate of 250 kS/s. As playful as I am, I tried to set greater rate to see what happens. Got it up to 800 kS/s. Maximum rate accepted without error was 816326 S/s, but there was no measurable difference between 800 kS/s and the maximum. The rate stayed effectively at 800000 S/s.

Do I misunderstand something here?

[HoussamK]

Hi

could you please post your  VI? Are you getting any DAQmx-error?

[HoussamK]

Hi

more information about the three AO waveform modes:

• Non-periodic waveform
• Periodic waveform regeneration mode from onboard FIFO
• Periodic waveform regeneration from host buffer including dynamic update

For few tasks you might need to specify a different condition other than the default for the DAQ board to request data from the buffer or perhaps write data to it. One of the examples would be if you want to perform continuous analog output and change the output waveform while the DAQ board is still outputting data., so when trying to change the output on the fly you are a little limited. The following document gives an example of how you can change the output on the next cycle of the while loop:

Data Transfer Request Condition for Continuous Analog Output using NI-DAQmx

[HoussamK]

you can find a good example using  LabVIEW 2011

\Program Files\National Instruments\LabVIEW 2011\examples\DAQmx\Analog Out\Generate Voltage.llb\Cont Gen Voltage Wfm-Int Clk-Non Regeneration.vi