Hi Mr_Nice,
Thank you for posting on the NI Forums.
I have taken a look at your code and have tested your VI. The VI seems ok, but I see the same error as yourself.
Although your VI itself will function fine, I think the issue resides with the DAQ timing settings you have specified. The default settings in your VI are buffer size=100, rate=1000, and number of samples=100. Using these settings means I receive error -200279 after a few seconds. You noted you were wishing to acquire at 20kS/s, using this sample rate the error occurred almost immediately.
Although I may be using settings slightly different to your own, it is these values that will govern whether you see this error. Error -200279 means the DAQ card is filling its buffer more quickly than your code can empty it. This means that after a time the new datapoints generated will be written over old data points, meaning you will lose data. This is known as a buffer overflow/overwite.
The buffer size setting specifies how many datapoints (per channel) your system can store in the buffer. Rate specifies the rate at which your card will sample and push a datapoint into the buffer (again, this is per channel). Number of samples relates to how many datapoints the DAQmx Read VI will pull out of the buffer on each iteration of your code (yet again, this is on a number of samples per channel basis).
The first step to rectify your issue is to ensure that your buffer is large enough to store intermediary datapoints while your VI iterates. For an acquisition rate of 20k I have set this buffer to be 10000 samples per channel.
The second step is to ensure that the datapoints are being read as quickly as they are generated by increasing the 'number of samples' setting. If you think it, any VI will have a maximum speed of iteration (dependant on the PC it is running on). If you were to pull only 1 sample on every iteration of your VI you will most likely find that you are not removing enough datapoints quickly enough and see an error (-200279). However, if you increase the number of datapoints you will find that you will increase the rate at which datapoints are being removed. For an acquisition of 20k I have set this value to 5000.
It is worth noting that the more you do in your code (e..g writing to file(s)), the slower it will iterate. This will mean you will have to increase the number of samples to read from the buffer to avoid a buffer overflow.
By using the settings I have noted I found your code runs fine and without error. I have attached your VI, but have modified the default values to the above. Please give this a go and let me know how you get along.
Thanks,
Rob
National Instruments | Northern California
