Bon jour, Manon.
It has been many years since I studied Le Français in high school, so I will continue in English.
There is a very useful NI White Paper that I read, and finally learned and understood how to use DAQmx. Do a Web search for Learn 10 Functions in NI-DAQmx and Handle 80% of Your Data Acquisition Applications,
So I'm looking at your code, and noticing a few things that are "missing" or at least unclear to me, some related to your question about Finite vs Continuous sampling.
First, it appears that you are sampling a single channel, as your Physical Channel input has only a single Dev1/ai0 entry. You are missing a DAQmx Timing function (which should go before the Start Task function). Your DAQmx Read is 1D Wfm NChan/NSamp, but you only have a single Channel defined.
So here are some questions:
- What sampling rate are you using for temperature? Note that temperature tends to vary "slowly", so you might want a "slow" sampling rate, like 1 Hz, or even 0.01 Hz (one reading every 100 seconds). With such slow rates, you might want to acquire one sample at a time, particularly if you want to "look at them" (via a Chart) or respond "quickly" when it exceeds some limits.
- You set the timing with the DAQmx Timing function. What is your sampling rate, and how many samples do you want to acquire before you "do something with them" (such as check for values too high or too low)? I'm guessing you'll sample fairly slowly, and will want to process every sample. To process every sample, you should use either Continuous Sampling with a sample size of 1 or Hardware Timed Single Point (if your device supports this). This function comes just before the Start DAQmx function. You also specify the sampling Rate here, so putting in "1" means you'll take one temperature reading per second.
- If you are doing a single sample, your DAQmx Read will specify Single Channel Single Sample, and will probably be a Dbl. If you don't need to immediately see every sample (but could collect, say, 10-100 at a time at some faster rate, say 10 Hz), you could try 1 Chan N Samp Waveform, which gives you the Time information that you might want. Just be aware you will not "see" your data until the sample has finished.
For example, suppose you say "I want to sample at 10 Hz and take 100 samples at a time". Your DAQmx Read loop (if you set Continuous Sampling at a rate of 10 with 100 samples/channel) will sample for 10 seconds, deliver a waveform (if you asked for it) with 100 data points, and loop waiting for the next sample 9.998 seconds later (actually, it's probably 9.99998 seconds, as enqueuing a Waveform takes very little time).
Set your Waveform Queue to hold a single Waveform, as you are sampling a single channel.
See how that works.
Bob Schor
