DAQmx :: Continous reading of the last 10 values of a continous data-aquisition at a fixed point

Question

Hello,
I am using a NI-PCI 6259 for generating and aquiring data.
The other configuration is:
- LabVIEW 7.1
- DAQ 7.3
I need to measure three anaolg channels, and generate an analog output. Additonally I need to read some values over the RS232. The whole activity should repeat every 30ms. To get this, I am using a timed loop at the moment. I generate a clock with counter 0 and read it with counter 1 for the timed loop. Inside the timed loop I start/stop a finite aquisition for the three analog channles and the RS232 reading. The fastest aquiring supported by the RS232 is an iteration every 30ms. Without the analog aquisition the system runs without any dropped loops. By adding the analog aquisition the timed loop dropps 2-5 loops every 1000 iterations.
I think the problem is the repeated start/stop of the aquisition. Maybe a solution is a continous aquistion by reading out the last 10 values of the buffer every start of the timed loop. But all my tests failed, because of the FIFO design of the buffer. When I start a continous aquisition outside the timed loop and read 10 values inside, I always have the first 10 values of the buffer, not the last ones.
Does anyone has an idea, how to read loop the last 10 values of a continous aquisition inside a timed ?
Thanks a lot for your help.
Best regards,
Michael
Attached you will find the actual state of the testprogram. Because the aqusition should be in the last quarter of the generated output value, the values of the loaded waveform (output) are four times resampeled and the clock has four times the speed of the aquisition, which generates a timing event every four clocks.
Attachments:
Datenerfassung.zip ‏3047 KB

Answer 1

To address your main issue you might try using the DAQmx control Task VI right after you call the Timing VI (outside the timed loop) and set its value to commit. This allows for more efficient starts and stops. Some other things I saw in your program which might make it a little easier and use less HW. First, you shouldn't need to create a CO task on ctr0 and then feed it to the timed loop using ctr1. Why don't you just use the Frequency personality of the timed loop? That way you will only use one counter. If you want to use the 2 counter method then to avoid using external wiring as the source for counter 1 in the timing source VI use CTR0/internaloutput. This is an advanced property for the terminal I/O control so you will need to right click on the constant and select I/O filtering. Finally, when you specify your create channel for the AI Voltage why are you doing this in a for loop. You can just have one channel string Dev1/Ai16:18 and that will create all 3 channels with the same information. Hope this helps
StuartG