daqmx multitask

[Gilles80]

Hi,

I'm looking for help (or at least for clarification) of the following question. I'm using Labview 7.1 with the following devices: PXI-1052 Chassis with 4 modules: 1x SCXI 1102 and 3x SCXI-1125.(2 for thermocouples and 2 for valtage). My question is the following: is it possible (and if yes how) to acquire data from the modules independently? I would like a different sampling rate for the 4 modules (all the channels on one module can be sampled as the same frequency), and also to be sure that in case of buffer limitation with one of the acquisitions, the three others are not affected. Until now I had to write the 4 Read DAQmx in a sequence loop otherwise I've got a following message "the specified resource is reserved".

I'm definitively not sure that different DAQmx could be read in parallel (in opposition as in sequence).
ThanX

Gilles

[devchander]

Hi Gilles,

In your setup, if all SCXI's are multiplexed to the same DAQ card, then you cannot acquire from them seperately using different DAQmx read.

This also means that you have to acquire from all modules at the same sampling rate.

This is bcoz all your multiplexed SCXI channels go a single ADC on your DAQ and its timing sequence will be set to scan throught all multiplexed channels with a fixed scan rate.

But, if you have a different DAQ card that 's acquiring from each Module in parallel mode.(1 DAQ card for each module), then you can acquire from each module at different sampling rate in a single VI. But again, you cannot acquire at different rate from each channel on a single module.

regards

Dev

[Gilles80]

Hi Dev,
thanks for your answer. Because my 4 modules are connected to the same DAQ (PXI-6220), I've definitively to forget my idea!

For the different sampling rates (the rate I want have some common divisors), I've got already some tricks, but it's not the optimal solutions for a memory and buffering point of view!

Regards
Gilles

[devchander]

Hi Gilles,

You can do this , acquire and save from all channels a single sampling rate and you can later(offline) extract the individual channels and decimate them to look likethey were acquired at a different sampling rates.

regards

Dev

[Gilles80]

Hi Dev,

Excuse me for reactivating this question, but I've got then another one related to the first one. Knowing that I run the 4 "DAQmx read" in a sequence loop, I would like to know how I can scan the backlog of the four DAQmx. (Note that it could be possible to use just one "DAQmx read" and using only one task, but it is really more clear like that in case of upgrade).

I know how to read one scan backlog for one DAQmx read.vi, but if I want to test it for the all DAQ system (knowing the 4 DAQmx.vi), makes it sense to scan the backlog of eachone and then to add them? If not, what is the best way to test that (and makes it sense to test that?)?

regards
Gilles

[devchander]

Hi Gilles,

From what i understood, getting scan backlogs and adding them should not give any useful info, atleast in the hardware set up you are having( 4 SXCI modules multiplexed to a single DAQ card)

regards

Dev

[Gilles80]

Hi

ThanX for the anwer. It was a little bit what I though. So, how can I determine with my configuration (4 SXCI modules multiplexed to a single DAQ and 4 DAQmx read.vi in a sequence loop) if my acquisition is too fast or involve to much processing?

Regards
Gilles

[devchander]

Hi Gilles,

This link should give you and idea about how to select sampling rates for multiplexed SCXI modules

http://digital.ni.com/public.nsf/allkb/410A70C25A4D12B486256A1E0070BDAE?OpenDocument

Note:  SCXI modules you have do not have simultaneous sampling capabilities, so ignore any notes in the link refering to it.

Regards

Dev