Multithreaded analog input and output and serial communication during the AIAO

Question

I need to read in analog data on 4 channels (4000 scans/s on each channel for a total of 250ms) and while this is happening send out two signals: A trigger pulse (which I currently send on an analog channel, using channel DAC0OUT) and two ascii commands using the serial port. My problem is that the data acquisition VI I am using is a blocking VI, it runs to completion before allowing the two signals to be sent. I am using the VI titled, "AI Acquire Waveforms (scaled array).vi" I don't need to look at the acquired data until after the whole operation is complete. Any help anyone can offer would be much appreciated.

Answer 1

Hello Mark,
The AI Read VI will block other operations when waiting for data. I would suggest to reduce the number of scans to read (this is an input for the AI Read VI) to help with handling multiple operations simultaneously. By reducing the number of scans to read by the AI Read VI, it won't have to wait for the whole buffer to be read in before "unlocking" the CPU for other operations. You could place the AI Read in a loop and acquire smaller chunks of the data at a time. For instance, your buffer size is probably 1000. Instead of calling the AI Read VI once and waiting for all 1000 scans, you could place the AI Read in a for loop and acquire 200 scans at a time (have the for loop run 5 times).
Or you could set up the AI Read VI to read 0 scans the fir
st time and then just read whatever is in the backlog on subsequent calls. There is a backlog output to AI Read; you could use this with a shift register within a loop to read only the data that is already in the buffer. For both of these cases, I would place a wait in the loop (25-100 ms) to give the CPU time to do other things.
Another method is to use an occurrence. Place the Functions >> DAQ >> Misc >> Set DAQ Occurrence.vi on your block diagram and set the "DAQ Event" input to "Fire every n scans" (the actual string is a little different). Then put in a Wait on Occurrence function (Functions >> Advanced >> Synchronization >> Occurrence) When you use an occurrence, you are waiting for the driver to tell you when the data is acquired, rather than constantly querying the driver, so it is much faster.
One of these suggestions should help.
Let me know if you have any additional questions.
Regards,
Todd D.
NI Applications Engineer