syn c modules on 9178 chassic

[isitthereur]

I need to acquire at low frequencies ie 10 samples a second. I can easily achieve this using daqmx example in help, replacing virtual channel with a task, i.e. 9205 ai all diff channels. However when trying to add another module like strain 9237, sample rate will default to 1.6ks/s. I also use 9238 modules. Is there any example to get all these modules to share sample rate, ?

Regards

[Blokk]

Attach your VI, without that we have no idea what your problem is...

[isitthereur]

so using this simple program, If I was just using one task, for example all analogue diff measurements on the 9235, then I have control of clock, say 10 samples per second, if I introduce strain from different cmodule like 9237, Then clock will default to 1.6ks/s, Then as i did in this program created 2 tasks. Task 1 has clock control where task 2 (strain) defaults to 793.651 sample/s.. I tried the example in help,hardware, daqmx, synchronization, but the chassis 9178 is unsupported, So question, can i get these module to aquire at same ( controlled ) clock rate, without using software control.

[GerdW]

Hi reur,

all analogue diff measurements on the 9235, then I have control of clock, say 10 samples per second, if I introduce strain from different cmodule like 9237, Then clock will default to 1.6ks/s,

Those two modules use Delta-Sigma ADCs! (see NI9237).

- Both use their internal base clock and support different sample clock rates: you could use sample rates like 10kS/s or 5kS/s with both modules…

- Both also can use an external master base, but you must ensure to set a suitable external clock for both modules. That way you could enable other sample rates…

[Blokk]

Sorry, but I am confused a bit. Could you make a proper list of your hardware elements? So you have a single chassis (9178), USB cDAQ with 8 slots. Please state all of the modules which are present in the chassis. Please describe clearly what is your goal. What is the sampling frequency you want to share between the modules?

Also read this: https://www.ni.com/en/support/documentation/supplemental/21/signal-based-synchronization-of-analog-i...

[isitthereur]

Hardware, 9178 chassis, modules 9235, 9237 and 9206. What I am trying to achieve is to control the sample rate, for example 1, 5 or 10 samples per second. My tests are quite long , Creep tests so I dont need high frequency sample rates, especially logging data to tdms. file sizes would be too large.

[GerdW]

Hi reur,

modules 9235, 9237 and 9206. What I am trying to achieve is to control the sample rate, for example 1, 5 or 10 samples per second.

NI9235/NI9237 don't support such low sample rates - as is written in the datasheet I linked in my previous message!

You need to use a supported sample rate of those modules (like the mentioned 5kHz/10kHz)…

My tests are quite long , Creep tests so I dont need high frequency sample rates, especially logging data to tdms. file sizes would be too large.

What is the problem with reducing the acquired samples to an amount you want to log?

What is the problem to read 5000 samples per second and decimate them to 5 samples/second? (Using a mean function…)

[isitthereur]

OK, Mean function works, i ve attached file. So rate/ samples to read = samples per sec. However is there a way to only record every 20th sample etc? Would be nice to see an example of this.

Thanks again for you help.

RDG

C.

[Blokk · Accepted solution]

---

@isitthereur wrote:

OK, Mean function works, i ve attached file. So rate/ samples to read = samples per sec. However is there a way to only record every 20th sample etc? Would be nice to see an example of this.

Thanks again for you help.

RDG

C.

---

Why the mean calculation is not good for you? It might even increases the signal-to-noise ratio.

Anyway, there are many built-in things, or you make your own with a modulo devision:

edit: or, if you produce an array with the DAQmx Read function, just use the "Index Array" function to pick one single element...