AO amplitude transitions from 0 to 10V (and back to 0 again) within running VI

[DEF]

Hi,

I have a NI PCI-6225 multifunction DAQ that I am using for Geophysical measurements.  I am applying 10V from one AO channel and measuring voltage differences with 49 AI channels.  For the method I am using I must follow these steps:

1.  Set voltage out = 0V

2.  Take Self Potential measurements from all AI channels

3.  Turn the AO channel ON with an applied voltage = 10V (1Hz square waveform)

4.  Take measurements from all AI channels

5.  Loop back to step # 1

At the present, I have the VI set up to apply the AO of 10V and to take measurements over time.  The only thing missing is to take the Self Potential measurements while the applied voltage is at zero volts, run the VI in its current configuration, and then to start over again.

I am using the Express VI DAQ Assistant to set up all of the electrodes in the array for the applied voltage and the measured voltage and am able to do everything needed except include the applied voltage to zero for the Self Potential measurement.

I have attempted to use the DAQmx subVI's "Stop" and "Clear" (and any other combination of subVI's) to allow for the channel to be freed up within the VI, but the output channel then gives an error signal in the next cycle of the loop.

I am not sure what else to do, so any suggestions will be greatly appreciated.

Thanks

[devchander]

attach your vi or a pic of your block diagram

we should be able suggest better after taking a look at it

[DEF]

Here is the VI in which the problem is occuring.  I have this VI as a subVI within a timed loop that will execute every 60 seconds. 

[DEF]

One additional note:

I have also attempted to use:

1.  "Stop" at both Outputs only

2.  "Control Task" to unreserve the channel

3.  "Reset Device"

Every time I get an error message:

1.  reserved channel

2.  missing string in write subVI (part of the DAQ Assistant subVI)

3.  cannot clear when task is running

Thanks for your time,

Dylan

[Neil_S.]

Dylan,

Thanks for posting to the NI Forums.  I took a look at your VI and it looks like you are using continuous mode to output your voltages.  From the sound of your application this is not necessary.  NI's DAQ boards actually will retain the last voltage written to the board until either you write a new value or you reset or power cycle the card.  Therefore to output 0VDC continuously you only need to do a single finite write of 0V.  The same applies to 10VDC.  You can simply write 10V once and the board will keep the voltage at 10VDC until you write something else to the card.  This will also resolve the errors you are getting since the task will complete after the DAQ Assistant finishes executing.  Let me know if you have any further question and good luck with your application.

Regards,

Neil S.
Applications Engineer
National Instruments

[DEF]

Thanks.  It is now working as desired.