Large number of samples by FIFO - avoid the saturation of the buffer target

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  I use the DAQ assistant for analog voltage of an input OR data acquisition card. What is the difference between the rate and the number of samples in the DAQ assistant and what are the units of the two?

  Thank you.

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  If number of samples is 100 and the rate is 1000 samples per second, then the acquisition would take 0.1 second (100 / 1000).

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• all samples n transferred from the buffer

  Hi all

  I have a question for every N samples transferred DAQmx event buffer. By looking at the description and the very limited DevZones and KBs on this one, I am inclined to believe that the name is perfectly descriptive of what must be his behavior (i.e. all samples N transferred from the PC buffer in the DAQmx FIFO, it should report an event). However, when I put it into practice in an example, either I have something misconfigured (wouldn't be the first time) or I have a basic misunderstanding of the event itself or how DAQmx puts in buffer work with regeneration (certainly wouldn't be the first time).

  In my example, I went out 10 samples from k to 1 k rate - so 10 seconds of data. I recorded for every N samples transferred from the event of the buffer with a 2000 sampleInterval. I changed my status of application of transfer of data within the embedded memory in full with the hope that it will permanently fill my buffer with samples regenerated (from this link ). My hope would be that after 2000 samples had been taken out by the device (e.g., take the 2 seconds) 2000 fewer items in the DMA FIFO, it would have yielded 2000 samples of the PC for the FIFO DMA buffer and so the event fires. In practice it is... not to do so. I have a counter on the event that shows it fires once 752 almost immediately, then lights up regularly after that in spurts of 4 or 5. I am at a loss.

  Could someone please shed some light on this for me - both on my misunderstanding of what I'm supposed to be to see if this is the case and also explain why I see what I see now?

  LV 2013 (32 bit)
  9.8.0f3 DAQmx
  Network of the cDAQ chassis: 9184
  cDAQ module: 9264

  Thank you

  There is a large (unspecced, but the order of several MB) buffer on the 9184.  He came several times on the forum, here a link to an another discussion about this.  Quote me:

  Unfortunately, I don't know the size of this buffer on the 9184 on the top of my head and I don't think it's in the specifications (the buffer is also shared between multiple tasks).  This is not the same as the sample of 127 by buffer slot AO which is present on all chassis cDAQ - controller chassis ethernet / wireless contains an additional buffer which is not really appropriate I can say in published specifications (apparently it's 12 MB on the cDAQ wireless chassis).

  The large number of events that are triggered when you start the task is the buffer is filled at startup (if the on-board buffer is almost full, the driver will send more data — you end up with several periods of your waveform output in the built-in buffer memory).  So in your case, 752 events * 2000 samples/event * 2 bytes per sample = ~ 3 MB of buffer memory allocated to your task AO.  I guess that sounds reasonable (again would really a spec or KB of...) I don't know how the size of the buffer is selected).

  The grouping of events is due to data sent in packages to improve efficiency because there is above with each transfer.

  The large buffer and the consolidation of the data are used optimizations by NOR to improve flow continuously, but can have some strange side effects as you saw.  I might have a few suggestions if you describe what it is you need to do.

  Best regards

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• samples per channel and the number of samples per channel

  in my DAQ mode samples finished program, there are two screws: timing and read.vi DAQmx DAQmx.

  I have to set the parameter to "samples per channel" DAQmx timing.vi and 'number of samples per channel' on DAQmx read.vi... Is there a relationship between these two?

  My laser runs at 1 K Hz. I want to go to the wavelength, wait for a number of shooting lasers, read the data and move on to the next page...

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  Lei

  In your case, the VI will acquire the lesser of either:

  The "samples per channel" that you have defined on the timing DAQmx VI

  -OR-

  The number of iterations of your for loop (N) times the 'number of samples per channel"that you have defined on the DAQmx read VI

  The "samples per Channel" VI DAQmx of timing for a finite acquisition dictates how many samples the DAQ hardware should acquire in it's onboard buffer before indicating that the acquisition is complete.  "The number of samples per Channel" on the read DAQmx VI dictates how many samples the DAQmx driver must return buffer on board the aircraft to your application.

  Let's say the "samples per channel" on the calendar DAQmx VI is set to 50.  Thus, the card will acquire 50 samples and place them in the edge of the buffer, then stops.  Suppose we have the 'number of samples per Channel"on the DAQmx reading VI the value 3 and what we call the VI in a loop For which runs 10 times.  Thus, every time the DAQmx lu VI is called, it will wait until there are at least 3 samples in the buffer, and then return these three.  We call the VI a total of 10 times, then we will answer 30 total samples.  Thus, the last 20 samples acquired the card remains in the buffer and are destroyed when the task is disabled.

  Now let's say that we increase the "number of samples per Channel" on our DAQmx Read VI at 10.  VI Read will wait until 10 or more samples are in the buffer, and then return these 10. Thus, we will be back all 50 samples map acquired by the 5th iteration of the loop For.  The 6th time we call him VI DAQmx Read it expires, because there will never be another 10 samples in the buffer, and the VI returns a warning.

  This clarifies things?

  The purpose of this behavior is to allow you to both set the total number of samples that the DAQ hardware will acquire and also control how much of these samples is returned whenever you call the DAQmx Read VI.

  Kind regards

• How to abort Continuous Acquisition (which is set to acquire a constant number of samples per channel each iteration)?

  Hi all

  I would like to continuously acquire an input channel analog and, if necessary, be able to stop the acquisition as quickly as possible.

  I use a card NI USB-6289. I implemented a DAQmx (voltage) analog input task, set the clock sampling DAQmx 'continuous samples' and the rate required (IE 1000, which is 1 kHz). For the acquisition, I used a while loop with inside the DAQmx Read with Terminal 'number of samples per channel', wired (IE 1000). The while loop waits until 1000 samples are acquired and therefore continuously reads the channel in uniform batches of 1000 samples every second ticking. The while loop can be easily interrupted when the DAQmx has completed the acquisition of the lot being 1000 samples, but it takes the DAQmx finish their current task.

  My problem is how to stop the loop while the DAQmx Read lies in the middle to get 1000 samples WITHOUT delay all 1000 samples are received? Is it possible to interrupt the DAQmx Read?

  I could reduce "the number of samples per second", increasing the responsiveness of the while loop, but this is not the solution I prefer. I tried to destroy the task (outside the while loop), but that does not stop immediately the acquisition, Read DAQmx still finish acquire 1000 samples. I've included an example Subvi. When I was using a PCI card, I used a timed while loop with inside the DAQmx Read with Terminal 'number of samples per channel' wired-1 (= which means ' read everything in the buffer "). In this case, it was easy to stop the acquisition at any time: the timed, while the loop is abandoned and the acquisition has ceased immediately. But it does not work with an NI USB-6289 map (see thread http://forums.ni.com/ni/board/message?board.id=170&message.id=386509&query.id=438879#M386509) because of the different way the data are transferred to the PC.

  Thank you very much for your help!

  Have a great day,

  LucaQ

  Hi LucaQ,

  Your solutions are decrease of the number of samples, or record the time you want to stop and remove samples that have been acquired beyond this time. There is no other way to stop the actual reading out of the hardware store.

  Flash

• Total number of samples per channel

  Hello

  I was wondering if you can call a function that returns the total number of samples per channel read. I try to get a precise timestamp.

  I use DAQmx ANSI C developer environment.

  According to NOR:

  Option #1:

  Try and do exactly what the driver is doing.  This will require you to do exactly what you do in the displayed example.  Call time current system immediately before calling the DAQmx Read and subtract dt * x where x is the number of samples already acquired.  It will take you to know exactly how many samples were acquired.  It can be found by calling the Total property of the samples by chain acquired immediately before the DAQmx Read.  This is to introduce some points for innaccuracy.  For example, you're time system is already innaccurate to a certain amount.  In addition, it takes some time between the system times call, calling the total number of samples acquired and calling the DAQmx reading.  If 2 samples are acquired between the call to the time system and the total number of samples acquired, you could be shifted a few samples.  For the slower clock rates, you will have more precision.

  I try to attempet to program this solution, but cannot find the total number of samples per channel acquired property. If anyone can help me, your help will be very appreciated.

  Thank you for your help,

  Vladimir

  Hi Vladimir,.

  Here is the property you are looking for (as the C reference help):

  

  You will be able to get accurate relative timestamps, since we know that our acquisition is based on a sample with a certain dt clock.  The value of initial time must still come from the OS.

  Best regards

  John

• PCI-6070E: maximum number of samples (analog in) by Hz?

  In MAX and LV8.6 I have converted 1kS (analog in) at a rate of 1 kHz (the 6070E is estimated to be 1.25. MECH / s), therefore, seems OK.

  However, why conversion fails to 10kS to 100 Hz, just the same 1 MECH/s?

  Something to do with the maximum size of the buffer of an 6070E internal?

  If so, is Labview 8.6 fast enough to read and store 1 MECH. / s adjacent to the coast of the 6070E although 6070E data acquisition presents data of Labview 1000 pieces/s.

  I've run an AMD64 x 2 @ 6 GHz @ 4 GB RAM @ 2 TB HDD, must be fast enough.

  I certainly don't miss something, but what?

  In the data acquisition textbooks, I couldn't find lights on this topic.

  Thanks for any explanation.

  I'm having a hard time to understand what you are doing. You say "conversions fail to 10kS to 100 Hz, comes to be the same MECH/s 1." What is the actual sampling frequency you are using? Is - this samples 100/s? And you ask 10 k samples? If so, why are you doing it like that? If you want 10 k samples at this pace, you should probably ask 100 samples and make multiple readings. To get 10 k samples at a frequency of 100 hz, which would take 100 seconds, wouldn't you get anything until the 100 seconds is upward and probably the buffer overflow. Take 100 samples at a time and repeat 100 times would give you the same number of samples, but you get the data every second and not needing a such great buffer.

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  Hey iron_curtain,

  You are right that the movement of large blocks of data can lead to a more efficient use of the bus, but it certainly isn't the most important factor here. Assuming of course that the FIFO on the FPGA is large enough to avoid overflowing, I expect the dominant factor to the size of reading on the host. In general, larger and reads as follows on the host drive to improve throughput, up to the speed of the bus. This is because as FIFO. Read is a relatively expensive operation software, so it is advantageous to fewer calls for the same amount of data.

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