Stop using that code you keep showing -- it is fatally "broken" (because you are asking it to do something that makes little sense, and the way to find out about that is to "see for yourself", which is precisely what I'm trying to get you to do without my "doing it for you", which is not my style of teaching).
- Make two copies of your "Newcontrol" routine. Call one "Analog Input", call the other "Analog Output".
- Open "Analog Input". Remove everything associated with the AO code. Remove the code involved with timing (near the top). Remove everything having to do with saving data.
- Now scale everything down so that the Error Line and the Task/Channel line run horizontally without bends. Get rid of most of the blank space. You should be able to get the VI to take about half the height of a laptop screen and about 2/3 the width, so you can "see everything". Add back the missing Error Wires connecting the DAQmx functions on the left.
- Just to remind yourself, wire a constant to the DAQmx Timing VI. Note that it will default to 1000, meaning that since you did not specify it, DAQmx will try to read 1000 Samples if sampling continuously. This is a Good Thing!
- Now fix the DAQmx Read. Set it to Analog, Multiple Channels, Multiple Samples, 2D Dbl. This will "break" the 1D Dbl wire going to the Chart (just keep one Chart). Delete the broken wire, and rewire the Chart with the 2D Wire.
- Here is a subtle point. DAQmx is sampling 2 Channels at 500 Hz for 1000 points. Think about how you would save this in a 2D Array. The first row is the first sample of 2 channels, and the 1000 row is the 1000 sample. But a 2D Chart wants to see 2 channels of 1000 samples, which it thinks of as 2 rows, 1000 columns. Do you know how to transpose the data for the chart? [Hint -- read the previous sentence out loud].
Run this VI. If you have a waveform generator, use it as an input to one of the A/D channels. If not, simply touch one of the A/D channels to inject 50 or 60 Hz "noise" into the system.
So what about generating an analog signal to "drive" the system? It depends -- do you want the "driving signal" to depend on the analog response you are measuring (above)? And if so, how quickly do you want it to respond? If you are trying to build a closed-loop control system, this is a much trickier problem that requires specialized (and fast) hardware, such as an FPGA that can respond almost instantaneously, which leaves DAQmx out. Otherwise, take a separate While Loop and build your Analog Output code following the same principles as outlined above. Note that, again, you can pre-compute 1000 points of your Waveform, start it going, and while it is outputting the first set of points, you are generating (and doing a DAQ Write for) the next bunch of points, making the output continuous and uninterrupted.
Do these one at a time. Keep your VIs compact and neat. Don't add Bells and Whistles at the beginning of your code development -- wait until "something works".
Bob Schor
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