In the DAQmx Timing VI, if the Cont. Sampling is on...?

Hi!
Can someone please explain to me what happens if:
in the DAQmx Timing VI, when the continuous sampling is on.. but you are asing for samples per channel? This is found in the KnowledgeBase example:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/3296ba2aef586b7386256d6d00528e3d?OpenDocument

If I understand you question correctly what continous sampling means is that the program will continously make reads what samples per channel specifies the number of samples it will make per read and then put in the buffer. Quoted from the help file..
"samples per channel specifies the number of samples to acquire or generate for each channel in the task if sample mode is Finite Samples. If sample mode is Continuous Samples, NI-DAQmx uses this value to determine the buffer size."
Let me know if you need more explanation. Have a great day.
Allan S.
Applications Engineering
National Instruments

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Doing something the DAQmx way that doesn't fit? (Triggered counter on PXI 6608)

Hi,
I want to create a counter with my PXI6608 that is triggered with an external input, then counts up. I then want to be able to read the count in LabVIEW and hence determine the time since the trigger. I have achieved this before with old-style code, but it is not clear how to do it the DAQmx way. In particular, I can't create a task because what I'm looking to do doesn't seem to fit in with any of the pre-envisaged DAQmx task templates.
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Solved!
Go to Solution.

You'll want to make an edge count task and set the source to be one of the internal timebases using a DAQmx property node (set it before starting the task):
The trigging functionality you're looking for is called an "arm start trigger" in DAQmx. It's configured through a property node (also before starting the task):
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What are the units of the DAQmx counter?

Hello,
I created this VI and several similar ones a few months ago, but, I have forgotten some of what I did back then. I believe that this VI should measure period, using the counter on the DAQ chassis. Can anyone tell me the units of the period measured? I think it is seconds, but, I am trying to figure out how to figure that out! The values of the period are generated by the DAQmx READ function in the WHILE loop. What units is that function outputting to the array? What are the units of the array elements, essentially? See the attached VI.
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Solved!
Go to Solution.
Attachments:
continuously measure period buffered_singlechannel_forum Nov 13.vi ‏33 KB

HI dav2010,
When you create a Virtual Channel, you should specify the units the counter task will use. There are several options which are listed in the help here:
http://zone.ni.com/reference/en-XX/help/370469AA-01/lvdaqmx/mxcreatechannel/#DAQmxCreateChannel%28CI...
I am not on a computer with LabVIEW right now, but I believe the default should be ticks, which is the number of ticks of your device's timebase. You can find that information in your hardware specifications.
Regards,
Chris E.
Applications Engineer
National Instruments
http://www.ni.com/support

How does the DAQmx read.vi work in producer/consumer mode

Dear all,
I have one question: how does the DAQmx read.vi work in producer/consumer mode ?
I mean if i set the acquisition samples quantity is 5000,(see the enclosed picture), how does the DAQmx read.vi acquire the samples ?
5000 samples one time ?
And how does the write. vi work ? Also 5000 samples one time ?
Look forward to your reply.
Thank you.
Attachments:
producer consumer mode.png ‏28 KB

It will read 5000 samples per channel.
The Write Measurement File just writes whatever you give it. It you send it 5000 data points, it will write the 5000 data points.
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DAQmx Timing (sample clock).vi

do u know what is the critaria to use DAQmx Timing sample clock.vi... or in what situations this vi must b used?

Hi Pritam,
Thank you for posting on the National Instruments forums.
The DAQmx Timing.VI is used when you need to perform an operation that requries sample timing whether it's Analog Input, Analog Output or Digital IO.
Here is a Developer Zone Article that explains how the Sample Timing VI can be used: Learn 10 Functions in DAQmx and Handle 80% of Your Data Acquisition Applications.
Hopefully this will tell you what you need to know.
Thank You,
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Applications Engineer

DAQmx Timing

Dear all,
I have this following VI solution that stems from the LabVIEW example for Continuous Digital Data Output. The issue concerns the System Clock - when set to a low sampling rate value (say, 1S/s), the rate seems to be much higher. The LED indicator on my BNC2120 is dimmed throughout; indicating fast switching between the Low and High?
How shall I edit the enclosed VI to actually write one sample per second? I am using the PXI-6132 card.
Thanks in advance.
Solved!
Go to Solution.
Attachments:
Digital - Continuous Output.vi ‏58 KB

Ok, when you set the clock to use the 100kHz clock, you will be sampling at the 100kHz. I am a little surpised that it won't let you use the onboard clock. Maybe the DIO can only be clocked out with an external clock?
Some tweaks you should make here:
If using an external clock, the rate should be set to the maximum expected rate of that clock. The data will be clocked out based on that clock though.
Since you are using Continuous mode, do not wire up the Samples Per Channel to the DAQmx Timing VI. All you are doing is limiting your buffer size.
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DAQmx Timing rate

I am using the DAQmx Timing control in a VI that samples data from a PXI chasis. The sampling mode is set to Finite Samples. The help states that the rate input specifies the sampling rate in samples per channel per second. Increasing this number means more samples per second. It seems to me that increasing the samples per second should reduce the overal time of the process since it is performing more actions per second. However, increasing the rate decreases the loop time of my program. What am I missing here? Am I understanding this wrong, or just implementing this strangely?

Hello,
You are right that increasing the rate means more samples per second.
It means the following: If you keep the finite number of samples to
acquire fix, then increasing the rate should decrease your acquisition
time. Suppose you are performing acquisition in a loop. If the
acquisition time decreases, then the time for a loop iteration will
decrease as well.
For example, for an acquisition of 1Hz, if the number of samples to
acquire (on the timing VI) is 1000, it means that you will acquire a
total of 1000 Samples. Since the rate is 1Hz, it will take you 1000
seconds to acquire all the samples. However, in a loop, you can use the
DAQmx Read VI to read part of the total number of samples already
available. So for example, if you specify 50 samples to read on the
DAQmx VI, every time it is called, it will grab 50 samples from the
buffer. If the samples are not available yet, it will wait until they
are availble or there is timeout and return. Now if you have a
process in the loop that take more than 50 seconds to comple before you
read again, then there will always be at least 50 samples in the buffer
from the second iteration. From there on, if you increase the rate, it
will just increase the number of samples availble to be read, but it
will not necessary make the other process in the loop faster. So the
DAQmx Read will always have the 50 samples it needs, but the other
process will still take time. So in this case, changing the rate from
1Hz to 100Hz will seem to have no effect on the entire process.
The important thing is to understand what those numbers actually mean.
-Sample per channel in the timing VI represents the total number
of samples to be acquired by the board on each channel of the task.
-Rate represents the rate at which the samples above are acquired
-Sample per channel in the DAQmxRead VI represents the number of
samples you want the VI to grab from the buffer. Those samples
have already been acquired and stored on the computer.
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"""Error -200621 occurred at DAQmx Wait Until Done.vi:2
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DAQmx :: Continous reading of the last 10 values of a continous data-aquisition at a fixed point

Hello,
I am using a NI-PCI 6259 for generating and aquiring data.
The other configuration is:
- LabVIEW 7.1
- DAQ 7.3
I need to measure three anaolg channels, and generate an analog output. Additonally I need to read some values over the RS232. The whole activity should repeat every 30ms. To get this, I am using a timed loop at the moment. I generate a clock with counter 0 and read it with counter 1 for the timed loop. Inside the timed loop I start/stop a finite aquisition for the three analog channles and the RS232 reading. The fastest aquiring supported by the RS232 is an iteration every 30ms. Without the analog aquisition the system runs without any dropped loops. By adding the analog aquisition the timed loop dropps 2-5 loops every 1000 iterations.
I think the problem is the repeated start/stop of the aquisition. Maybe a solution is a continous aquistion by reading out the last 10 values of the buffer every start of the timed loop. But all my tests failed, because of the FIFO design of the buffer. When I start a continous aquisition outside the timed loop and read 10 values inside, I always have the first 10 values of the buffer, not the last ones.
Does anyone has an idea, how to read loop the last 10 values of a continous aquisition inside a timed ?
Thanks a lot for your help.
Best regards,
Michael
Attached you will find the actual state of the testprogram. Because the aqusition should be in the last quarter of the generated output value, the values of the loaded waveform (output) are four times resampeled and the clock has four times the speed of the aquisition, which generates a timing event every four clocks.
Attachments:
Datenerfassung.zip ‏3047 KB

To address your main issue you might try using the DAQmx control Task VI right after you call the Timing VI (outside the timed loop) and set its value to commit. This allows for more efficient starts and stops. Some other things I saw in your program which might make it a little easier and use less HW. First, you shouldn't need to create a CO task on ctr0 and then feed it to the timed loop using ctr1. Why don't you just use the Frequency personality of the timed loop? That way you will only use one counter. If you want to use the 2 counter method then to avoid using external wiring as the source for counter 1 in the timing source VI use CTR0/internaloutput. This is an advanced property for the terminal I/O control so you will need to right click on the constant and select I/O filtering. Finally, when you specify your create channel for the AI Voltage why are you doing this in a for loop. You can just have one channel string Dev1/Ai16:18 and that will create all 3 channels with the same information. Hope this helps
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How do you configure multiple PXIe chassis to share Tclk synch with the 6672 timing module?

Hi,
I have two PXIe chassis, each with a number of digitizers (configured through NI-SCOPE). Each chassis has a PXIe 6672 timing module. I have downloaded the sample VI from:
http://zone.ni.com/devzone/cda/epd/p/id/5254
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National Instruments

In the DAQmx Timing VI, if the Cont. Sampling is on...?

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