Multiple finite sampling

[PB8]

Hello everybody, I am developing a test application in VB 2010 with the following configuration:

- NI cDAQ 9184 rack

- NI-9205 32 analog inputs module

I tried the example AcqVoltageSamples_IntClk in which a finite number of samples is acquired. I discovered that the operation

Dim data() As AnalogWaveform(Of Double) = reader.ReadWaveform(samplesPerChan)

instead of lasting about (number of samples / sampling frequency) takes something like (number of samples / sampling frequency) + 300 ms.

I suppose that the 300 ms are needed for starting and stopping the task but for me this is unacceptable because my application consist of a sequence of steps in which I put the device under test in some condition and I take 100 samples at 5 kS/s. This correspond to 20 ms and I cannot loose 320 ms every time.

My 1st question then is: is it possible to start the task when the application starts and then do several reads of a FINITE number of samples? I tried but the 2nd time that I call the ReadWaveform methods it generates an exception.

I even tried in a different way. When the application starts I create and I start a continous sampling task that fills a buffer at 5 kS/s. Then, for example, I do a sequence like this:

1) put 1 V on analog input 1

2) empty the buffer

3) wait for the sample count to be >= 100

4) read and treat the samples

Point 1 is done through an electronic switch so I can consider it instantaneous. Even this solution does not work in fact I always get samples to 0 V. If I add a 50 ms delay between point 1 and point 2 it works i.e. I get samples to 1 V so it seems that the first 100 samples put in the buffer were sampled before the moment in which I emptied the buffer.

My 2nd question then is: what is going wrong? How do I manage a scenario like this? I simply have to take several "photographs" of the analog inputs.

Thank you in advance everybody for you polite attention!

[Valerio.S]

Dear PB8,

I found a forum that seems to have your issue that it may be useful for you.

http://forums.ni.com/t5/Multifunction-DAQ/9188-200msec-delay/td-p/1703884

Regards,

Valerio