Compatibility of throughput of NI cDAQ 9181 with laptop/desktop

[ManasRanjanPattnayak]

Dear members

I am completely new to this domain. Therefore, kindly help me in choosing my proper DAQ devices.

I want to buy the NI cDAQ 9181 with an ethernet port and NI 9222 C series voltage input module. I am planning to use 2 channels from NI 9222 with a sampling rate of 400 kHz/ch/s.

So, will the throughput of NI cDAQ 9181 be 400kHz/ch/s * 2 ch * 8 Bytes/s = 6.4 MB/s ? If the calculation is correct, can a normal laptop/desktop with a GBE-based ethernet port and common hard disk be used for acquiring the data continuously for 3-4 minutes?

Kindly help.

Regards

Manas Ranjan Pattnayak

[GerdW]

Hi Manas,

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@ManasRanjanPattnayak wrote:

I want to buy the NI cDAQ 9181 with an ethernet port and NI 9222 C series voltage input module. I am planning to use 2 channels from NI 9222 with a sampling rate of 400 kHz/ch/s.

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The cDAQ9181 only offers a 100Mbps Ethernet port as written in its datasheet!

The NI9222 offers 500kS/s/ch in CompactDAQ devices as written in the datasheet.

So your cDAQ only offers upto ~10MB/s data transfer speed, while you request 400kS/s * 2ch * 2bytes/sample = 1.6MB/s (the module offers 16bit ADCs!). I guess the Ethernet port is fast enough for your requirement…

(I would probably put the cDAQ9181 on its own dedicated LAN port on your computer to minimize influences/disturbances from network handling.)

[ManasRanjanPattnayak]

Dear member

Thank you so much for your kind reply. I want a clarification on a similar post in NI forum.

Link to the post: https://forums.ni.com/t5/LabVIEW/How-to-estimate-data-acquisition-storage-requirements/td-p/3079210

In this case, the ADC resolution is 24 bit; but in the calculation of the throughput, it is mentioned as 8 bytes/sample. Why is it so?

Considering the above post, why in my case it will be 2 bytes/sample?

Kindly look into this, and clarify.

Sincerely

Manas Ranjan Pattnayak

[GerdW · Accepted solution]

Hi Manas,

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@ManasRanjanPattnayak wrote:

In this case, the ADC resolution is 24 bit; but in the calculation of the throughput, it is mentioned as 8 bytes/sample. Why is it so?

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Because in that post the amount of data is calculated for saving in a (TDMS) file, which stores data using 8 bytes/sample (aka DBL datatype)!

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@ManasRanjanPattnayak wrote:

Considering the above post, why in my case it will be 2 bytes/sample?

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Because DAQmx will transfer raw data (I guess), which should be just 2 bytes/sample for a 16bit ADC.

(For bigger ADCs, like 18-24bit, most  probably 4 bytes/sample are transferred…)

I'm not an expert in these specific DAQmx internals (you might call NI tech support): when you need a "worst case" calculation you might assume 8 bytes/sample. But I guess DAQmx is optimized for data transfer using also slower busses (100Mbps Ethernet, 480Mbps USB2.0) and so will only transfer the minimum amount of data.

[ManasRanjanPattnayak]

Dear member

Thank you so much for your reply. You are correct. The raw data will be saved as 2 bytes/sample and TDMS will be saved as 8 bytes/sample.

So, if I am saving as raw data, the throughput will be 400000*2*2 = 1.6 MB/s and in TDMS format, this will be 400000*2*8 = 6.4 MB/s.

Considering the communication rate of NI cDAQ 9181 to be 10 MB/s, I am guessing, the above throughputs will be sufficient for my application.

Sincerely

Manas Ranjan Pattnayak