Acquiring multiple DMMs

[gmonti]

Dear LabView users,
I am really new to labview and I need to implement a vi where I have to acquire data from 10 DMMs (NI PXI 4071). I do not have any special requirement in term of syncronization, I have just to launch a waveform acquisition for the 10 boards with a given sample frequency and number of samples to be acquired (same settings for all the boards). I found the following example:
http://www.ni.com/example/30496/en/
for acquiring waveform data from an NI DMM. The idea was just to duplicate the code 10 times to be able of acquiring the 10 DMMs. But I've got really a weird behavior, namely, I cannot read more the 4 DMMs at the same time. Any idea why?

Thank you in advance for the help.

Giuseppe

P.S.
I'll post the implemented vi tomorrow.

[mikeporter]

Define "weird". What happens when you try to read 5? Errors? Slow performance? Program crashes?

Mike...

[gmonti]

Dear Mike,
you're right I have to provide additional details on the setup. To check the behaviour of the cards, I use the same input signal - a 5 V pulse of 1 s width- during a 10 s acquisition at a sample frequency of 1 kH. As in the NI example I put 10 displays to verify the presence of the pulse. Moreover, I added code to write the data from each board in a separate txt file. Then, when I run the vi it seems that there are no errors but just 4 cards get the pulse. The other six boards just get noisy signals (that I can see in the file). I also verified that it was not an issue related to the wiring. Indeed, If I just initialize 4 DMMs (e.g. DMM1-2-3-4 or DMM2-5-8-9 and so on so forth ......) the acquisition is fine and the pulse is steadily there. Sorry if I do not attach the vi by now, but I will have access to the code tomorrow (on Monday :)).
What do you think?

Thank you in advance for any help

Giuseppe

[johnsold]

Giuseppe,

Without seeing the code this is just a guess. I suspect that the acquisitions do not all start at the same time and that the pulse has ended before the fifth through tenth instruments start so they are measuring baseline noise after the pulse has ended.

Can you generate a ramp signal which lasts 30-60 seconds? Feed that to all the instruments and run the VI again. The voltages measured will give an indication of when the acquisitions occurred. If some DMMs report only noise, then you know they are not measuring the signal for whatever reason.

Lynn

[crossrulz]

This sounds like a situation where you really do want syncronization.  I would recommend setting up a trigger between all of the DMMs and send the trigger right before sending your pulse.

[mikeporter]

This is where you can take advantage of the pxi chassis' triggering capabilities.

Mike...

[gmonti]

Dear all,

Lynn was definitely right .I just generated a saw tooth signal 1 Vpp with a period of 10 s. The DMMs settings were 1 kHz sample rate for 10 kSample acquisition (range 10 V). Indeed, I was able to check the starting time of the DMMs, all of them were properly acquiring but the lack of synchronization led to a huge difference on the starting time of the boards (please have a look to fig.1a and b). Therefore, it is clear that I have to use a starting trigger for the DMMs. Could you please be so kind then to provide me any example of synchronization (through PXI bus)? I am using a PXI-1044 chassis with a PXI-8108 controller. Moreover, please find enclosed the vi that I wrote. In particular, I attached the whole .dll from NI with my file DMM_Fast_Acq_10B_v2.vi.

Thanks again for the great support.

Giuseppe

P.S.

Should I also use a control on the data ready from the DMMs?

[gmonti]

Hello everybody,

I solved the problem using this NI example:

http://www.ni.com/example/28378/en/

Thanks again for the support. 

Regards

Giuseppe