low frequency sampling with multiple channels

[Thijs]

Hi,

I am trying to measure 8 different channels (all analogue input 0-10V on my DAQCard 6062E) with a sampling rate of 1 Hz (one sample every second) continuously, for a data monitoring program.

If I try to use the example Cont Acq&Graph Voltage-Int Clk.vi (Example under: DAQmx, Analog In, Measure Voltage.llb) and change the channel parameters to a list of global channels, I manage to do this. However, if I change the sample rate to 1Hz, and the samples to read to 1 I get an error message 200284.

I tried many solutions, but I seem to be stuck either to only 4 channels, which works fine, or to log at an extremely high frequency, which is unnecessary (and unwanted, because then I have to average again to get one value for each second) and would consume only more memory.

Any ideas what I am doing wrong and why I cannot log at low sampling rates are most welcome!

Thanks,

Thijs

[Sarah_Coleman_NIUK]

Hi,

The following KB refers to this error:
http://digital.ni.com/public.nsf/websearch/FEF778AD990D5BD886256DD700770103?OpenDocument

Try the suggestions in this and let me know how you get on. If they don't help, post your code for me to have a look at.

Kind regards
Sarahb

NI | UK

[Thijs]

Thanks for your reply. I had read the webpage, but it does not seem to solve the problem.

I can easily solve the problem by logging ath 1000Hz or higher and take the average over 1000 points, but I like to understand why labview cannot handle low frequencies. Sure the sampling of different channels cannot take too long that it takes longer then 10 seconds (timeout value) to show any results on the screen!

In my idea, because I give labview so much time to log all the channels, the logged values (at 1Hz) are the (almost) realtime true value of the signal (no delays in signal measuring and plotting on my computer screen).

regards,

Thijs

[Sarah_Coleman_NIUK]

Hi Thijs,

I tested this on my PC with a PCI card and it worked fine. The problem could be with the DAQ card you're using. I'll try testing it with a PCMCIA card we have here.

Sarahb

NI | UK

[Sarah_Coleman_NIUK]

Hi Thijs,

I tried your code on a laptop with a PCMCIA card and I get the same error. The problem appears to be with data transfer across the PCMCIA bus.

I would suggest doing single point acquisitions in a loop. You can add a timer to the loop to control the frequency. You'll have to change the graph to a chart to be able to see the data. I've attached an edited example that works on my system.

Kind regards,
Sarah

[Thijs]

Hi sarah,

I have difficulties opening your attached vi. I am running labview 7.1. (labview says it is not a vi).

Maybe a compatibility problem with labview 8...?

Could you try to save the vi under a different version?

thanks,

Thijs

[Sarah_Coleman_NIUK]

Sorry about that, I'm used to programming in 8!

Attached is the 7.1 version.

[reddog]

If you still want to use hardware timing, try looking at this knowledge base.  I'm pretty sure this will fix your problem.