Sample delay from software to hardware

[tschmid]

Hi,

I am interested to find out the time it takes for a sample generated in LabView until it hits "the air" (channel output) of one of your DAC cards? I couldn't find any data on this in the different product specifications nor in the forums.

Thank you very much,

Thomas Schmmid

[Brian F.]

Hello tschmid.  The time that a value takes to appear at the analog output of a DAQ board can be divided into 2 distinct parts.  The first part is the time it takes to get from the software to the buffer on the DAQ board.  The second part is the time it takes to progress through the buffer on the DAQ board to the output.  The first part is nondeterministic since there is no way to benchmark this process, but should be very quick since all the software does is send the output value to the board.  The second part can be found in the user's manual of a specific DAQ device under "Maximum update rate" for analog output channels.  For example, assuming one channel is being used, the PCI-6251 has a maximum update rate of 2.8 MS/second.  Thus, if you have a clear buffer, it will take this second part 1/2.8M seconds to execute.  If the buffer contains 99 values before your desired output, the second part will take 1*100/2.8M seconds since 100 values have to be written to the output in order to see that value that was just written to the board from software.  There is a way to ensure that your onboard memory doesn't slow down this process too much.  There is a property node in DAQmx called AO.DataXferReqCond which allows you to chose one of the three following options: "On Board Memory Empty," "On Board Memory Half Full or Less," or "On Board Memory Less than Full."  Choosing the "On Board Memory Empty" option will allow a more responsive output assuring that the current value written from software will be seen on the board's output quickly, but it will limit the achievable update rate.  However, if you are concerned with seeing one value appear on the board in the shortest amount of time, "On Board Memory Empty" is your best option.  I hope this helps and if you have any additional questions, I would be happy to address them.  Have a great day!

Brian F

Applications Engineer

National Instruments