Handling DAQmx Sampling Rate within Event Structure

Hello all,
I've been programming in Labview for a while now, but have recently been introduced to Event Structures. They are useful, however I think in the program below, the event structure is causing me trouble.
VI (Written in Labview 8.0): http://myweb.dal.ca/~jwoodacr/Pendulum.vi
The program itself is fairly straightforward, and by no means complete. One event allows the user to acquire data in real-time (Preview Data Event), and uses the Wait VI to control speed (I know this is not particularly accurate, but the preview does not have to be). The other event is meant to collect data with a well defined time interval between data points.
The problem is, I can press the "Collect" button once, and the "Collect" event will run fine with the values of "Time to Collect" and "Data points / sec" that were input by the user prior to pressing Collect. But, if one wishes to press collect again, no matter how you change the "Time to Collect" and "Data point / sec" controls, the express VI "DAQ Assistant 2" will only remember the very first values you gave it, and will output data with these parameters. So, if you change "Time to collect" to from one (1) to two (2) seconds after having run it initially with one (1) second, it will continue to remember the one (1), and collect data for only one second.
I've attempted to replace the express VI with all the proper DAQmx VI's used to collect in an identical fashion, but those also do not update past the initial conditions that are set. If you enable "Highlight Execution" and watch as the task proceeds, the numbers that are sent to the "DAQ Assistant2" VI are all as they should be, based on everything input on the front panel. It simply seems to ignore them upon execution.
This seems an odd problem, but I am hoping someone may be able to shed some light on it, or has experienced something similar in the past.
Thanks in advance.
Cheers,
Jeff

The DAQ Assistant only initializes the first time it is called. Thus each subsequent call, it does not care about the inputs into it. Another option is use DAQmx VIs and use a property node to modify whichever propreties you want. You must stop the task before changing its properties.
Brian R.
District Sales Manager
Washington DC
National Instruments

Similar Messages

While loop with stop button within event structure locks up front panel.

I am not sure if this is a bug with my program or a bug within LabVIEW. If you believe that this is a bug with my program then I will post my program to be looked at.
The problem I am having is there is a while loop within an event structure that fires when a particular value changes. Once the "Activate" button is pressed the while loop within the event structure starts going with a polling frequency of 1hz (1000ms wired to the "wait till next ms multiple" vi). There is a "Deactivate" button that is wired to the stop control of the while loop and an outter while loop that resets the event structure so that the activate button is being listened to again.
Once inside the while loop, however, none of the button are responsive within the front panel. The VI continues to run, and only 60% of my CPU is being consumed, but none of the button or scroll bars work. The only way for me to terminate the program is with the "Abort" button next to the "Run" button. If I remove the event structure so that the while loop in question runs as soon as the program starts, the front panel remains responsive. I've inserted probes within the while loop and verified that it is not running prior to the "Activate" button being asserted, and it is running after the assertion of the "Activate" button with the expected polling frequency set by the "wait till next ms multiple" vi.
Can anyone help? Do I need to post my code?
-Nic

It is typically not a good idea to stall an event structure by placing loops inside event cases. What good is an event structure if it is not free to repond to events?
Have a look at some alternative solutions, such as in the following link:
http://forums.ni.com/ni/board/message?board.id=170&message.id=224817#M224817
LabVIEW Champion . Do more with less code and in less time .

Determine index of array within event structure

I am doing some testing with dynamically registered events. For these testing purposes, I have 2 separate arrays of boolean references hooked up to the dynamic input terminal. The event structure is executing like I expect it to. The only problem is that I want to know the index within the array for the event that triggered the value change event. I would have thought I could use the CtlRef node to determine this, but instead it returns a reference to the actual boolean control that triggered the event.
I could write a subroutine that searches for the label of the boolean that triggered the event within the arrays, but that is not efficient or good practice.
So for instance, if I click on Boolean 2, I'd want the event structure to know it was array #1, index 1. If I click on Boolean 6, I'd want the event structure to know it was array #2, index 2.
Is this possible?

That is a very good idea. The caption is good to use for the label you show to the user. You can change it to show different things, even programmatically, such as if you need to make your application more international and have it display controls in different languages to different users. It lets you use longer or more descriptive names, without having those long names eat up block diagram space.
Another thing I've done is take the control reference and search for it among the array of control ireference. Search 1-D array will give you the index of the control within the array. Then I can use that index and apply it to a different array and index out a value from that. It could be a name, or perhaps a numeric value I use as a multiplier or something like that. Just make sure there is a one to one correlation between the references in the array and whatever is the other array you are getting additional data from. For what you have drawn now, it could be a little bit complicated because you actually have two different 1-D arrays of references. But if you concatenated those arrays together, you'd have a single 1-D array you can search.

Case structure & event structure

1.What are the disadvantages of case strucure?
2.What are the advantages of event structure?

Are these test questions? I could name 10 for each, but why are you asking?
While there are often many ways to accomplish the same thing, Case structures and Event structures serve different purposes. Straight from the Help file:
Case Structure
Has one or more subdiagrams, or cases, exactly one of which
executes when the structure executes. The value wired to the selector terminal
determines which case to execute and can be Boolean, string, integer, or
enumerated type. Right-click the structure border to add or delete cases. Use
the Labeling tool to enter value(s) in the case selector label and configure the
value(s) handled by each case.
Event Structure (Not in Base Package)
Has one or more
subdiagrams, or event cases, exactly one of which executes when the structure
executes. The Event structure waits until an event happens, then executes the
appropriate case to handle that event.
Right-click the structure border to add new event cases and configure which
events to handle. Wire a value to the Timeout
terminal at the top left of the Event structure to specify the number of
milliseconds the Event structure should wait for an event to occur. The default
is –1, indicating never to time out.

Adjusting the sample rate

How do I adjust the default sample rate setting to 44100?

Blastbot wrote:
If you are not going to answer this because it is on an older product, that is lame. I tried to get on a forum for said older product, but that is grayed out from the list of products you have a forum on, which is also lame.
Well, I certainly don't wish to be described as "lame," Blastbot, as it is a personal fear of mine. The forum for previous versions of Audition is available at http://forums.adobe.com/community/audition/audition_previous but since we're here and my earlier obligations today have wrapped freeing up a few minutes of my time, I'll be happy to walk you through the process for Audition 3.0.
In Audition, click Edit > Audio Hardware Setup... In the dialog that pops up, you'll see the current hardware sample rate listed and a Control Panel button that will display the device configuration panel for the selected driver. You should be able to change your sample rate by clicking that button, depending on which audio driver is selected. The default "Audition Windows Sound" driver, which is generic and best suited for audio devices without their own drivers, defaults to the Windows sample rate. If you have a good audio device with its own, native ASIO driver, you can usually modify the sample rate within their control panel.
Since you're apparently running Windows, although I'm not certain which version you have, you may need to change the sample rate of your audio device at the Operating System level. You can usually reach this menu by opening the Windows Control Panel and launching "Sounds and Audio Devices" (on Windows XP) or "Sound" (on Windows Vista and Windows 7.) Here, you'll need to navigate to your playback device selection, open the device properties, and adjust the default sample rate that you desire.
More detailed information can be found in the Adobe Audition Help documents either from the Help menu item, or the Help button located in the bottom-right corner of the Audio Hardware Setup window. Additional information about Sample Rates can be found on Wikipedia at http://en.wikipedia.org/wiki/Sample_rate
I sincerely hope this helps answer your question and relieves me the burden of lameness, at least as far as this issue extends.

Handling windows messages within an event structure

Hello all.
I've got an event driven VI which makes use of an event structure. I'd like my VI to be able to respond to some custom windows messages (i.e. WM_USER) sent from a separate process that listens on a telemetry card (sends a message when it receives data).
I have tried modifying the Windows Messaging Queue VI example provided by NI so that the VI will process windows messages during the timeout event in the event structure but this doesn't seem to work.
Any suggestions?
If labview events and windows messages don't mix then I guess I could do away with the event structure and go entirely with windows messages. This could be done by defining some custom messages that replace the labview events (e.g., W
M_STOPBUTTON_PRESSED = stop button, value changed event) and have the VI post these messages using PostMessage from User32.DLL and then catch these messages in the same manner as the NI Windows Messaging Queue VI.
My preference is to make use of the event structure if I can because its more inline with the labview methodology.
This is really interprocess communication question so if there are other ways of achieving the same result (sockets, named pipes, shared memory?) I'm keen to listen.
cheers,
Novak.

These 'sockets' sound interesting. What exactly are they? Could you direct
me to an example?
Thanks
Denis
"VI Guy" wrote in message
news:[email protected]..
> LabVIEW Events and Windows messages don't mix. To implement as such,
> you would have to do as you describe.
>
> Using sockets as an inter process communication scheme works great.
> And on a single machine, they dont leave the computer (i.e. no round
> trip on the network). Also, its cross platform compatable, not to
> mention it adds the possibility of being able to seperate your
> processes across several machines. Add to this that sockets are as
> easy to code as serial communication (with the benefit of "sleeping"
> waits instead
of "serial polling") and you get a solution fits many
> applications and needs.
>
> Get the idea I am sold on sockets?
>
> Good luck.

Producer/Consumer Design Pattern + DAQmx + Event Structure

Dear community users,
at the moment I try to implement the Producer/Consumer Design Pattern into my current project. For this purpose I am trying to make a "minimal-example" in which I use the Producer/Consumer Design Pattern for the data acquisition and analysis (display in GraphXY, calculation etc..) process. In the Producer Loop I perform the data readings via DAQmxRead.VI and in the Consumer Loop, in this example only, I display the data in a GraphXY. At the same time I want to control the data acquisition via an Event Structure, for example Start Measurement/Stop Measurement Buttons. To configure the measurement time, for example if the user only wants to measure 150 sec, I use the Time Out Event of the Event Structure. This is a technique which is often used, when data acquisition and Event Structures are used at the same time (Excuse me if this is wrong!). Due to understand the Producer/Consumer Design Pattern I looked up in the Template\Framework of LabVIEW.
In the attachment you will find my "minimal-example", which -sadly-, not working. What should I do ?
Thank you.
Best regards,
tugrul öztürk
Attachments:
daqEngine_PCP.vi ‏35 KB

Your VI will never stop due to the Ok Button that stops the producer loop is read OUTSIDE of the loop. Add an event case for it.
Move the Start and the Ok Button inside of their respective event cases. This will allow the latch to reset when they are read.
Change the Ok Button to a stop button.
Use a Greater Or Equal instead of Equal for the measurement time comparison. What if somebody randomly changes it to 0 on you? Again, you will never stop.
Is the DAQ only supposed to read for "Measurement Time". Right now, it constantly reads. So if you don't use the DAQmx Read, you will get a buffer overflow error. You need to put the DAQmx Start Task inside of the Start event case and then the Stop DAQmx Task inside of the Timeout event case when the measurement time is exceeded. You should also perform the read regardless inside of the Timeout event case.
I HATE telling the consumer loop to stop by destroying the queue. You can look data that way. Instead, send something an empty array. In the consumer, check for an empty array. If it is empty, stop. Destroy the queue only AFTER BOTH LOOPS HAVE COMPLETED.
You should also report your errors in some way. Simple Error Handler at the minimum.
Since you are using Continous Samples, don't limit the buffer. Leave the Samples per Channel unwired on the DAQmx Timing VI.
There are only two ways to tell somebody thanks: Kudos and Marked Solutions
Unofficial Forum Rules and Guidelines

DAQmx in Event structure / avoid multiple events

Hi all
I've started creating program for acquisition and analysis of data . I don't have much experience in LabVIEW, but it's important to me to start creating this application in good way. Can you give me some advice about things below?
I prepared template with Event structure, based on some article .
First question is how exactly place DAQmx blocks in it. Now they are all on one state of case structure "test", but I'm not sure that's correct because most of the example on NI side have initialization (like sample rate, number of saple) outside of the loop? It's important to make it possible to change parameter between measurements.
Second. How to avoid multiple events. In example: button TEST is pressed, measurement is taken. This measurement take a long time. I've started bored, and some other buttons (maybe TEST ) accidentally were pressed. When measurement is completed it would be good idea to destroy this events associated with accidently pressed buttons. It is possible to do it? What's the best way when there's a lot of buttons(when analysis part of program will be added)?
Best Regards
PS. sorry about my english
Attachments:
pgm.vi ‏33 KB
Enum_Events.ctl ‏5 KB

Dear Finch!
Welcome to NI Forums!
My first advice regarding your code would be to use shift registers instead of Queues as your state storage, since (as you've said) there is no reason to store multiple events that have happened. The state machine design pattern, which I strongly recommend in this scenario is built into LabVIEW, you can use it as a template if you go to the New.. menu.
Please check out these materials for further discussion about state machines.
You are correct in that most of the DAQmx VIs can be placed outside of the loop, only DAQmx Read (the function we actually use more then once) must be placed inside the loop, the rest can stay out. If you want to modify some parameters (like Timing) mid-execution, you only have to stop the task, set them, then start it again.
This can be easily done in a different state, which can execute only when some parameters have changed.
Also, if you want your user to be unable to interact with certain controls while the test is being taken, you can programmatically disable them with a Property Node.
Please get back to me if you have any other questions.
Best regards:
Andrew Valko
NI Hungary
Andrew Valko
National Instruments Hungary

NI-DAQmx frequency sampling rate

Hi there!
I'm working on setting up a data acquisition Labview VI, to measure different signals on a test rig.
I'm using the NI-DAQmx assistance (the Express VI?) to continously measure analog signals (Variable current, voltage and temperatures). This is working just fine, and i can change the sampling rate by writing to the express VI. The idea is, that the user can change the sampling rate from around 1 to 500 Hz.
We do however have a sensor that transmittes digital signals (a frequency), and are using a NI-9423 module to "read" it. As this is a digital signal, another NI-DAQmx express VI is needed to handle it (that's ok), but so far we can't figure out how to alter the sampling rate - it's apperently locked at 1kHz.
Being that we want to merge the analog and digital signals to one array, we are recieving overflow errors from the "analog" DAQ, if it's not set at exactly 1kHz.
So, in short - is it possible to change the sampling rate of a DAQmx recieving frequencies? So that we to DAQ assistences have the same sampling rate?
Help would be greatly appreciated!
- Nicklas
Attachments:
DAQissue.PNG ‏64 KB

Unlike voltage measurements, which tend to be (more or less) instantaneous, frequency measurements take a finite (and often variable) amount of time.
If it is a slow signal then you measure the number of counts of your reference clock that occur in one period of your input signal. As your input signal varies in frequency, so does the measurement rate. If it is a fast signal, you can either measure how long it takes to get n cycles or your input (again variable) or you could count how many cycles of your input occur in a fixed time period.
The NI help on frequency measurements describes three different ways you can configure a counter to measure frequency.
The long and short of this is that generally counter measurements come at variable measurement rates which can be problematic to fit in with a fixed rate loggin system. If the measurement period is much smaller than your desired rate then you can wait and trigger a measurement at regular intervals. If not, you can let the counter run at its own rate, placing the latest result on a notifier, and in another loop just read the latest measurement from the notifier each time you want to record a result. Depending if you counter is running faster or slower than your desired logging rate you will end up with either missed samples or repeated samples. There are inherant timing inaccuracies in both approaches because, unlike analog measurements, the counter measurement is not made at 'that exact time, now!' but over a period of time which may be long or short compared to your logging rate.

How to handle event structures for two buttons and two counters.

     *I have two buttons A and B, and have to compare the difference in hits between them. So do I place an event structure within a while loop for each button (2 buttons, 2 event structures, 2 while loops), or does it need to be one event structure with cases for both buttons whithin one while loop?
     *How do I pass the values of buttons A and B's hit counters out of the while loop in order to calculate the differences between them after each increment, not just the final count after the stop button is hit?
    *How would I make one stop button work to handle both buttons A and B?   I tried to place the actual stop button in one button A's structure and its global in B's structure, but It wont seem to let me use a local variable for a latchable control
Checker

You should probably have not started a new thread and waited until your question in the original thread was answered. You can modify altenbach's example for a second button. You have a single event structure with a new event for the second button. You have a second shift register to track the number of times the second button is pressed. Inside the while loop but outside the event structure, you just have a subtract function in or to report the difference. do not modify the posted example for the stop button. It works perfectly as is.

Daqmx "Done" event not triggering event structure

I would like the user to be able to abort my daqmx task by pressing a button.
I am attempting to use the method in this VI:
https://decibel.ni.com/content/docs/DOC-15815
It utilizes an event structure with the daqmx Control Task VI to abort.
The example VI above works for me, on my system with my USB-6356 device, as expected.
When I attempt to incorporate the same method into my VI it does not--despite having an identical arrangement, so far as I can tell.
I believe that my problem lies with the event structure, and the dynamic registration. I am unfamiliar with this and must be using it incorrectly.
I have attached my VI, though it is cumbersome. The code which I am struggling with is in the "Acquire" case. As far as I can tell, I am doing exactly the same this in this particular case as in the example mentioned. Yet in the example, the event structure completes, or aborts, whatever you want to call it, right after the "Wait Until Done" VI executes. In my own VI, the "Wait Until Done" and "Clear Task" VIs execute, but the event structure does not register it.
Here are my specific questions: is there anything extra or special that one must do in order for the Task to properly trigger a "Done" event and pass through the event structure? At what point is the daqmx task "Done"?
thanks,
Matt
Attachments:
BoomTubeMain.vi ‏175 KB

natasftw,
My event structure is identical to theirs; what you are seeing are the two different cases that exist in the event structure. In both the example and the VI, there is a <task out>: Done and a "Dynamic Event": Value Change. In the dynamic event case, I have my abort button and relevant code--this I believe is the code you were referring to, which you thought I was missing... Regarding my multiple event structures, they should operate completely independently as they respond to different events.

How can I programmatically determine the capabilities of a card under NI-DAQmx (e.g. max sample rate, number of AI/AO/CTR channels, etc.

Is there a DAQ_Get_Device_Info() equivalent for NIDAQmx? I need to iterate thru all the devices on my system, and build up a list of device capabilities. The system may include M-series and E-series cards.

Attached is a program I've used in the past to determine number of AI channels. It could be easily modified to check for AO or digital or counter. Also, there is a ton of properties that you have access to (i.e. max sample rate, max/min voltage inputs, etc.) that are accessed as properties of the type of channel, or timing properties, as opposed to properties of the board. Check out the DAQmx C Reference Help (Usually at Start>>All Programs>>National Instruments>>NI-DAQ). Expand the NI-DAQmx C Properties, and look at the List of Channel Properties, and Timing Properties, etc.
-Alan A.
Attachments:
Device_Info.vi ‏25 KB

Event structure to detect value change of a control within a cluster in an array

I have 1D array that contains a cluster. The cluster contains a numeric and a boolean control.
If the user starts to edit the numeric control value i would like to call one subVI, and if the boolean control value is changed, call a different subVI.
The array control on the front panel allows the user to edit a number of the array elements.
I would like to use an event structure to detect a value change in the cluster. When editing the Events, in the Event Sources panel i get the option to select only the array, not the cluster within the array or the controls within the cluster. Can the Event structure be opened up to show controls within clusters and arrays?
The solution i am using is to detect a mouse up event on the array and then use property nodes to determine if the key focus is on the numeric, and a case structure to determine which subVI to call. This works, but is there a better (simpler) way?
Thanks, Blue.

Thanks for the responses guys.
The tricky bit was that i wanted the numeric control values to flag they were going to be edited, so i could call a subVI, before their values were changed by the user. This is done by using the key focus property node, - i need to detect changes on the fly rather than post the event. Probably didn't make this clear enough in my original post.
The array is of variable size depending on if the user decides to insert or delete elements. The user also has the option to click and edit the array without having to do to much scrolling through the array index, as the FP shows several elements at a time. The Event Structure does a good job of automatically determining which element in the array is being edited, and returning those values to the property nodes. Turned out simpler than i thought it might be at one point!
Cheers, Blue.
Message Edited by BlueTwo on 01-15-2009 06:52 AM
Attachments:
evstrct1.jpg ‏63 KB

Can I use an event structure within another event structure

Hi
Can use an event structure within another event structure?
Thanks
Yasamin
Solved!
Go to Solution.

I attached my Vi from my old project (MY OLD VI). It is worked correctly.
But I decided that use event structure instead of case structure.
I attached my new VI as a picture and as a LabVIEW VI too (network_analyzer_8510_920527 Frequency Sweep.vi )
My graphs show by pressing "case 5: button (Bottom of page right hand).
Then I want to show each of graph in a large window( in a new VI) by clicking a key (for example "show 1").
Thanks
Yasamin
Attachments:
my VI.JPG ‏256 KB
network_analyzer_8510_920527 Frequency Sweep.vi ‏72 KB
MY OLD VI.vi ‏69 KB

Is there a way to add an event to the event queue handled by the event structure?

I am using an event structure to control the flow of a program. In one case I want to change the state of a switch and then have the event structure catch this exception and handle it. It works if the user toggles the switch, but doesn't seem to when it is done programmatically. Any suggestions.

Hello,
LabView User's manual says:
"Use Events for Direct User Interaction Only
Events wait only for direct user interaction on the front panel. Events do not work if you use VI Server, global variables, local variables, and so on to change VI or front panel objects programmatically."
However, you can have a look in Example Code, there's some applications generating events programmatically, maybe that helps you. (Search for event structure in support).
Hope that helps,
SBC

Handling DAQmx Sampling Rate within Event Structure

Similar Messages

Maybe you are looking for