using different AI DAQ rates at the cDAQ-9172 device at the same time?

[zahnrad]

Hi, we are using the cDAQ 9172 device  with two 9233 and two 9215 modules. Is it possible to use these 4(2) modules with different AI rates at the same time - within one VI?
f.ex.: 2 x 9233  at  50k  and 1 x 9215 at 1k 

Following the 9172 User Guide there is only one Ai timing engine .... so is there no possibility to do so?

The 9215 runs at a max. rate of 100k - if i use it with the 9233  module(max. rate 50k) is the max. "system" rate then 50k?

Thank you!

bob

[Andre_Saller]

As there is only one AI timing engine, all onalog inputs have to be sampled at the same rate, so it is not possible to sample one channel 1 with 50k and channel 2 with 1k. You will have to sample all channels with 50k (in this example). You can use LabView to extract every 50th value of channel 2, to get data thast looks like being sampled with 1k.

As a note: You can use different timing for Analog Input, Analog Output, Counter/Timer and Digital I/O, but all Analog Input/Analog Output/Counter/Digital

As you have to use one samplerate for all modules, your maximal sampling rate when using multiple devices in parallel is defined by your slowest I/O module, or by the maximum system-throughput of the cDAQ-9172 (3,2 MS/s), so you are right in your second question: The maximum rate is 50k in your example.

Hope this helps!

[zahnrad]

Thank you for these qualified posting!

Is there any other hardware (i think like cRIO?) wich is able to do multirate AI by the described method?

Thank you again

bob

[Andre_Saller]

You could use multiple PCI boards (M-Series Data Acquisition boards) and synchronize these. You could set a different sampling rate on every single board. CompactRIO would be an option as well, but you have to know that you would need LabVIEW FPGA and LabVIEW RT to program the FPGA in the backplane and the Realtime-Controller. You would need some experience with these tools.

For a data-logging application, I would recommend to use multiple boards.

[zahnrad]

Thank you again Andre!

is it also an solution to use 2 cDAQ9172 backplanes and synchronize them over pfi (2x9401)?
(backplane 1 with 50k for each module, backplane 2  with 10k for e. m.)

bob

[Andre_Saller]

Yes, thats an good idea as well. You can use two backplanes, and synchronize these. Best would be to use a counter, configure it as a freuqnecy devider, and use the devided (by 5 for your example) signal as the sample-clock on the second backplane.

[KyleBryson]

Bob,

CompactRIO or an R-Series board will give you true multi-rate sampling with the modules that you currently have.  For your application (with the 9215 at 1kHz), the software solution using LabVIEW or your development environment to decimate the data should work very well.

The CompactDAQ Advisor will show the maximum analog input sampling rate available for any combination of modules:

http://ohm.ni.com/advisors/compactdaq

You are correct, you could also synchronize two chassis at different rates using 2 NI 9401 modules in the PFI-capable slots.

Regards,

Kyle
National Instruments

[zahnrad]

thank you Andre, thank you Kyle!

these are really useful informations ... !!!

[manliff]

I think I am experiencing different results than indicated in the current thread.  I am using NI 9237 (bridge-pressure xducers), NI 9211 (thermocouples), and ni 9205 (voltage) simultaneously. The 9237 has minimum dac rate of about 3800 Hz while the NI 9211 has maximum of about 15 Hz.   I am setting the sampling at the 3800 Hz rate and the system fills in the 9211 data with redundent values to match a psuedo 3800 Hz rate.  I should say the 9211 does actually specify a "maximum rate" but lists the "conversion time" which limits the practical rate to the 15 Hz rate.  The other data looks quite valid for 3800 Hz.

using CompacDAQ; NI cDAQ-9172.

Just observations,

Manliff,

Message Edited by manliff on 05-24-2007 10:22 AM

[NI-Bongo]

Hi All,

Yes, the 9211, 9217, and 9219 all produce redundant data.  The reason for this is because the sampling rates of these devices are several orders of magnitude slower that the other Compact DAQ devices.  In general all modules share the same sample clock and the sampling rate is limited to the slowest module, but these modules are an exception as it would considerabley degrade the performance of the other modules in the chassis if these modules were the limiting factor.

Regards,