External timming

[GPGPR]

I used Ni examples that suited my needs: Cont Acq&Graph Voltage-Ext Clk-Dig Start.vi.

I input a source clock of 10Hz(100ms period on PFI0) and specified to the Daq-mx timming VI a rate of 10Hz.

In the loop, when i order to read 10 samples, the time stamps from the Analog 1D Wfm NChan NSamp read fonction give an evenly time space time stamp of 1 sec... Which is fien because i want to read 10 samples every second.

But, when i put the get time function, i can see that the loop iterate every 500ms!?!?

How can i get 10 real samples every 500ms if the sampling rate is 10Hz??

Is there a double reading of some sample?

How can i get the newest 10 samples every second.

Is this configuration set automatically buffered operations?

Any hints?

Regards

[eesen]

What number did you input "# sample"  for "DAQ read vi ".

If you want to read 10 sample every second you can do such a configuration:

Sample rate=10 hz

# sample to acquire= 10

[GPGPR]

Thats exactly what i do...

[Ed W]

Hi,

I’ve setup a similar system here, using a PCI-6251 and DAQmx 8.5, and I could not reproduce the behavior that you’re seeing.  The time string is updated once per second.  A picture of my block diagram is attached.

What device and driver version are you using?

Have you verified that your signal is truly 10Hz – could it be 20Hz instead?

Best regards,

Ed W.

Applications Engineer
National Instruments

[GPGPR]

Ed,

The Daqmx driver version is 8.3. I,m using PXI-6220. I verified the timming signal and i have 10Hz.

I talked with technical support from NI. While talking with your guys, i was trying different configuration and here is what i got.

In the example, The Active edge terminal on the Daqmx timming(Sample Clock) function is not wired(Default:Rising). I wired it with Rising as value; nothing chaged, the bug was still there.

Then, i changed the value to falling and it starts working!? I changed back to rising and still working!

I closed Labview and retry, everything is fine. I rebooted the PXI main frame, no more problem as well.

I find that disturbing, i did not expect such a behavior and i dont want that happen again.

Should i check for this problem while acquiring(Time consuming)?

Can i be confident enough that everything will goes right?

Any advices?

Regards,

Guillaume

[Ed W]

Hi Guillaume,

It’d be very difficult to determine why you were seeing this behavior, especially since the problem is now fixed on your system.  There are no known issues with this type of acquisition in DAQmx 8.3, so you can be confident that your system will work in the future. 

Best regards,

Ed W.

Applications Engineer
National Instruments