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.
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Producer/Consumer Design Pattern with Classes

I'm starting a new project which involves acquiring data from various pieces of equipment using a GPIB port. I thought this would be a good time to start using Classes. I created a GPIB class which contains member data of: Address, Open State, Error; with member vis such as Set Address, Get Address, Open, Close...general actions that all GPIB devices need to do. I then created a child class for a specific instrument (Agilent N1912 Power Meter for this example) which inherits from the GPIB class but also adds member data such as Channel A power and Channel B power and the associated Member Functions to obtain the data from the hardware. This went fine and I created a Test vi for verfication utilizing a typical Event Structure architecture.
However, in other applications (without classes) I typically use the Producer/Consumer Design Pattern with Event Structure so that the main loop is not delayed by any hardware interaction. My queue data is a cluster of an "action" enum and a variant to pass data. Is it OK to use this pattern with classes? I created a vi and it works fine and attached is a png (of 1 case) of it.
Are there any problems doing it this way?
Jason

JTerosky wrote:
I'm starting a new project which involves acquiring data from various pieces of equipment using a GPIB port. I thought this would be a good time to start using Classes. I created a GPIB class which contains member data of: Address, Open State, Error; with member vis such as Set Address, Get Address, Open, Close...general actions that all GPIB devices need to do. I then created a child class for a specific instrument (Agilent N1912 Power Meter for this example) which inherits from the GPIB class but also adds member data such as Channel A power and Channel B power and the associated Member Functions to obtain the data from the hardware. This went fine and I created a Test vi for verfication utilizing a typical Event Structure architecture.
However, in other applications (without classes) I typically use the Producer/Consumer Design Pattern with Event Structure so that the main loop is not delayed by any hardware interaction. My queue data is a cluster of an "action" enum and a variant to pass data. Is it OK to use this pattern with classes? I created a vi and it works fine and attached is a png (of 1 case) of it.
Are there any problems doing it this way?
Including the error cluster as part of the private data is something I have never seen done and ... well I'll have to think about that one.
Ben
Ben Rayner
I am currently active on.. MainStream Preppers
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Producer consumer design pattern

Hello,
I am trying to build a simple program that does the following:
1. when program starts, the indicator is initialized to -999
2. when Measure button is clicked, the indicator shows random number continuously.
3. when Save button is clicked, the indicator shows 1
I use the event-based producer-consumer design pattern and it works great. However, I cannot find a way to allow the CONTINUOUSLY generating of random number when Measure is clicked. Can someone shed a light on it? Thanks a lot for the help.
Attachments:
CommandList.ctl ‏6 KB
Untitled 1.vi ‏25 KB

One simple way to do it is to use the Timeout event case. You can modify the timeout value based on what events have occurred. See attached mod.
Not saying this is the best way. Kind of depends on where the program goes from here.
Attachments:
Untitled 1-2 MOD.vi ‏27 KB

Could Buffer replace the Queue in Producer/Consumer Design Pattern

Hello,
I have a question that the task of Buffer is to store the data and the queue is also of the same so could we use the Buffer inplace of queue in a Producer/Consumer Design Pattern.
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No, those buffer examples are not nearly equal to a queue and will never ever "replace" queues in producer/consumer.
The most important advantage of queues for producer/consumer (which none of the other buffer mechanics share) is that it works eventbased to notify the reader that data is available. So if you would simply replace the queue by overly elaborate buffer mechanics as you attached to your last post, you will lose a great deal of the the purpose using producer/consumer.
So, to compare both mechanics:
- Queue works eventbased, whereas the buffer example does not.
- Queue has to allocate memory during runtime if more elements are written to the queue than dequeued. This is also true for the buffer (it has to be resized).
- Since the buffer is effectively simply an array with overhead, memory management is getting slow and messy with increasing memory fragmentation. Queues perform way better here (but have their limits there too).
- The overhead for the buffer (array handling) has to be implemented manually. Queue functions encapsulate all necessary functionality you will ever need. So queues do have a simple API, whereas the buffer has not.
- Since the buffer is simply an array, you will have a hard time sharing the content in two parallel running loops. You will need to either implement additional overhead using data value references to manage the buffer or waste lots of memory by using mechanics like variables. In addition to wasting memory, you will presumably run into race conditions so do not even think about this.
So this leads to four '+' for the queue and only one point where "buffer" equals the queue.
i hope, this clears things up a bit.
Norbert
CEO: What exactly is stopping us from doing this?
Expert: Geometry
Marketing Manager: Just ignore it.

Register event in producer consumer design pattern

Hello wire workers,
I am fairly green when it comes to dynamic event registration, so my understanding of it is probably quite flawed, but what I have implemented seems to work fine.
Assuming a fairly typical queued producer/consumer state machine architecture.
Up till now, I have done the dynamic event registration outside (i.e. to the left of) the UI loop, so all events get registered before the UI loop starts to run.
My question is:
How can I register events from inside one of the consumer loops? I cannot wire it...
Maybe an example will make my scenario more clear.
I have a DAQ loop (queued state machine). In the init state of this loop I want to register a digital input such that changes on a line are handled by my main event structure (in the totally separate UI loop). Is this possible?
I look forward to hearing positive answers
nrp
CLA

The best way to send data from the Consumer Loop to the Producer Loop is through User Events. What you will want to do is create a User Event whose data type is the registered DAQmx event refnum. Then when you register the DAQmx event in the Consumer Loop, fire the User Event with that new refnum and update the registration in the Producer Loop. That's probably hard to wrap your head around in words, so I'll attach a dummy example and pic below. Hope this helps!
VI Saved in LV85.
Message Edited by Jarrod S. on 04-21-2008 12:54 PM
Jarrod S.
National Instruments
Attachments:
DAQmx Event.vi ‏30 KB
DAQmx Event Pic.PNG ‏38 KB

State Machine and producer consumer design patterns

Hi!
In my application I am using state machine(for top level VI) and producer consumer(for subVIs) design patterns throught out.
If there is no activity on the front panel for 1min then the ideal state is invoked. Following are the situation of the ideal state:
1. It checks for any hardware present on the network . If no hardware is found on the network within 1 min then the application and LabVIEW exits.
2. While the ideal state checks for the hardware, and user clicks on any front panel button then it should exit the ideal state. The control
should be in the respective button state.
I am able to achive the point#1 mentioned above but could not get the point #2 to work. I am including the snap shot of
the state machine(default case) and prouducer consumer(deafualt state).
Each screen has the concept of the ideal state. The only additional step in the subVIs is that when the ideal state is reached and the time has expired then it will close the subVIfront panel.It will go to the exit state of the Main(top level) VI. Thus closing the application and quitting LabVIEW.
Any ideas or suggestions. Will multithreading and notifiers help?
TIA
Rachana
Attachments:
Sanpshot_Of_Design.zip ‏75 KB

Hi Rachana,
I was thinking about using event structures wherein there would an event case that would handle any value change event for any of the buttons on the front panel. But before suggesting that, I would need to know more about the Ideal State subvi( more specifically, how is it checking for any hardware present on the network and where is the one minute wait coming from?).
Please feel free to respond to this post with your questions/comments.
Regards,
Ankita A.

Producer/ Consumer Design error?

I am learning labview as I go and little errors keep stumping me. I set up the producer/consumer design and it reads the two while loops like I want. However, when I stop the program to save it to the write to measure it gives me an error saying invalid input and highlights the dequeue. I have to queues running (one for a button and one for data) and they both do this. I tried connecting it different ways but its always the same error. Any ideas?
Attachments:
2whlloopprgrm.vi ‏392 KB

Dublicate post
Stick to one post at a time. Do not post the same question on two different threads.

First attempt at Producer Consumer Architecture: continuous read and dynamically update parameters.

Hello,
I am currently working with two instruments; an Agilent E3646A and an NI 6212 BNC. My objective is to have the 6212 to be continuously taking measurements according to predefined parameters while the E3646A's parameters can be continuously updated. This simple instrument combination is intended to help me learn the necessarry achitecture; continuous measurement, dynamic output control, and more instruments will be added in the future.
I've previously posted on a similar, but more complicated, setup (http://forums.ni.com/t5/Instrument-Control-GPIB-Serial/Split-second-lag-when-controlling-two-instrum... and was advised to try the Producer Consumer Architecture. I've found relevant literature on the site (http://www.ni.com/white-paper/3023/en/, https://decibel.ni.com/content/docs/DOC-2431), searched the forums and constructed my own VI. While my first attempt at a Producer Consumer Architecture has resolved some of the issues I was having when I first posted on the subject, however, new issues have arisen with regard to reading and stopping the VI.
I am currently able to run the VI and update the device parameters. Previously, I would get a freeze while the instrument was being updated and could not toggle parameters until it was done. This has been resolved although the read only updates when a parameter has been updated although it is outside of the event structure. Furthermore the Stop button does not work in any context. I've also gotten occasional errors regarding the Deqeue Element but the bulk of the trouble is Error -200279 "Attempted to read samples that are no longer available" at DAQmx Read. I realize that there is a problem in my Producer Loop but haven't been able to figure out how to resolve it.
This is my first attempt at a Producer Consumer Architecture and already I can see that it is a powerful tool. I read as much as I could and looked at the relevant examples but expected to have some issues in the beginning. Would really appreciate any advice so I can take full advantage of the architecture.
Hope to hear from you,
Yusif Nurizade
Solved!
Go to Solution.
Attachments:
DCe3646A_DAQ6212_Prod_Con_4_13a_2014.vi ‏89 KB

Jon,
Thank you for the response and the suggestions.
I created the Local Variable for the Stop Button and moved the original into its own Event Case. It is now able to stop VI execution although I get Warning -1073676294 which I understand to be the buffer issue. The warning says that this was incurred on VISA Open for the E3646A's Initialize VI which is outside both loops, however, which I don't understand.
I tried increasing buffer size by decreasing the Number of Samples as per suggestions I found on NI forum threads. I've brought it down as low as 1 but the graph still only updates when an Event occurs and I keep getting the same buffer error. You suggested that the problem could be that they are in the same loop; does this mean that the DAQmx read should be outside both Producer and Consumer Loops or moved into the Consumer Loop? I found a couple of links ( http://forums.ni.com/t5/LabVIEW/Producer-Consumer-Design-Pattern-DAQmx-Event-Structure/td-p/2744450 , http://forums.ni.com/t5/LabVIEW/Producer-Consumer-Architecture/td-p/494883 -first response) on dealing with an Event Structure that my own VI is based. Alternatively, I found another link ( http://forums.ni.com/t5/LabVIEW/Producer-Consumer-Design-Pattern-DAQmx-Event-Structure/td-p/2744450 - first response) on a DAQmx Read with an Event Structure that places the graph in the Consumer Loop.
The core of my purpose in setting up this Producer Consumer Architecture is so that I could read continuously while the instrument parameters are updated dynamically so it's very important that I understand how this works. I am particularly struggling with how the Event Structure is controlling the While Loop's Execution.
Hope to hear form you,
Yusif Nurizade
Attachments:
DCe3646A_DAQ6212_Prod_Con_4_14a_2014_Edit_I.vi ‏90 KB

Is this a good design pattern?

Hi,
I am working with a team to create an application that can acquire data from up to 24 thermcouple channels. We plan to create a wizard to assist the user in setting up the test (specifiying stop conditions, sample rate, channels to sample, output file, and email/text notifications). After the setup, if the user does not cancel out, then the program continues. We have defined parallel loops and their roles, trying to fit into a producer/consumer design pattern. I have attached the main file which shows the structure of the program. We are trying to keep it clean and modular. Before diving into development, we are finalizing a detailed design specification so that different people can work on each of the loops' subvis. I was hoping that people could review the block diagram and offer suggestions for improvement or validate that we have chosen a good method of implementation.
Notes about the block diagram
The plan is for data driven stop conditions to be detected by Event Notifier, handled by the Main UI event, and then the Data Collector is instructed to stop (as well as the Main UI loop). When Data Collector stops, it releases queues, the queue reference becomes invalid, and then the loops with the Dequeue will be stopped.
I am not sure if an additonal event structure should be in the DataCollector to specifically handle the Stop button on the UI because if it is in the Main UI event structure, the DataCollector would not be stopped immediately. This is important to stop quickly because the sample rate could be VERY long. I understand about using an event structure instead of WAIT, but didn't know if there is an efficient way leaving it down in the Main UI event structure....????
Data Collector is the producer, it currently only collects data and passes it on. If the program is going to contain a start collection button, or pause/resume, I need to figure out how to handle this with the Main UI event structure.
Event Notifier consumes data from the Data Collector. It looks at all data and should set a flag if a stop condition is met. The flag will be handled by the Main UI event structure to tell Data Collector to stop. The event notifier also sends a notification to the Email/Text Caller when thresholds have been exceeded warranting a reason for the user to be notified via text or email.
Email/Text Caller waits for notification and then calls a subVI to send an email or text with certain data and channel information. The subVI call is lengthy, so we decided to put this in its own loop instead of a case structure inside the Event Notifier.
Report Generator logs all collected data to a file. A second queue was used, but uses the same data as the first. Is this the correct implementation since two consumers need the same data?
Main UI - handles changing of sample rate, possibly the stop button, possibly start/pause/resume of data collection. Also handles when Event Notifier sets flag for a data driven stop condition, which then should provide a mechanism to stop the Data Collector.
A big question that i have is.... Should the Main UI event structure just be in the Data Collector so that the loop can be stopped immediately? All of the UI events (stop button, start/pause/resume data collection) directly effect the Data Collector. Should the data Collector have a state machine instead?
I feel that the 3 consumer loops (Event notifier, Email/Text Caller, and Report Generator) seem like a good implementation, but i am not confident about the Data Collector and Main UI loops. Should they be combined?
Solved!
Go to Solution.
Attachments:
updatedMain.vi ‏13 KB

A couple of thoughts:
Overall it looks workable- remember to name your queus and notifiers- then you can depop the BD and launch each loop in its own sub-vi without wiring all those queues between loops- Just obtain ref to named queue.
"Data collector" needs a supervisor to control how often the data is collected- I'm thinking a QSM with "init- collect- stop" states. A simple timed loop could produce the "time to acq ticks" under control of the sample rate control. This puts both the DAQ init and cleanup functions within the Data Collector's perview so it aids coehesion and solves the exit without waiting for the loop to iterate.
I'm not a fan of duplicating data- I assume analisys is pretty much just some comparisons- so it is pretty quick- Why not append summary data to the same queue? And make it a single element queue- the report loop can then inspect the queue and look for a summary data "index" - if its greater than the last index written append the darn data to the report? Preview queue works great for these situations.
Do not combine the UI handeler with anything else! Do you see the snag?- hint: here thar be spaghetti. keep the functions seperated for solid scalable and maintainable code.
One last thought: Its getting less popular to stop loops on the error out of a dequeue. First it returns a default element that can cause your listeners some headaches. Second it doesn't let you run cleanup code. Use a stop command <enum> built into the element data as an executive to determine when to stop. We'll try to get the examples updated to show current best practices.
Jeff

Event structure doesn't handle all events from external DLL source

Hi!
I use a DLL, wich handles an USB CAN transceiver device, and generates LabVIEW user events with extcode.h and PostLVUserEvent() function.
In my vi I use an event case structure to handle theese external events. The problem is, that if the events come too frequently, then it cannot handle all events.
The vi contains a couple of parallel while loops which process the incoming data from the external event.
In another application I use the same architecture, and that works fine. This one is more complex.
Do you think I have reached the limits of the program? Or what else can be the problem?
I have tried to use a queue in the event structure, it helped a little, but yet, not all the events are handled. My external DLL sends all event, so the problem is not there.
Thanks in advance!

Hi Wasz,
Thanks for the post and I hope your well.
It is not possible for the event structure to miss events - it simply queues them up. To keep the event structure 'in time' with the code you must ensure the code to execute in each event is minimal to allow the event structure to handle the next event.
Please see this two links:
event structure buffering: leading to,
How to avoid built-up for "cursor move" events? : where somone is trying to avoid handling all events.
What sort of events are not being handled? Could it be an issue your events are not configured correctly? Maybe if you could produce a small example in LabVIEW code, so we can just test the event structure configuration... someone working around the external dll call - which you seem sure is working correctly.
In terms of queuing to process the events in other loops is certainly an option to speed up the event structure. To do this, use a produce/consumer design pattern.
hope this helps,
Kind Regards
James Hillman
Applications Engineer 2008 to 2009 National Instruments UK & Ireland
Loughborough University UK - 2006 to 2011
Remember Kudos those who help!

Producer consumer "clean up diagram" puts consumer on top of producer

I do File > New... > VI > From Template > Frameworks > Design Patterns > Producer/Consumer Design Pattern (Events)
Then without doing anything to the block diagram or front panel, I click on the "clean up diagram" which does the auto block diagram cleanup. Bam! Consumer loop is put on top of Producer.
I tried a bunch of combinations of the Block Diagram options settings (wire spacing, move controls left, etc.) , but the "clean up diagram" always wants to put consumer loop on top of the producer loop. How can I get labview to maintain the original loop positions?

There is a reason a lot of use do not like to use the Block Diagram Cleanup. It generally works for simple VIs. But I would not use it on my top level architecture.
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Recommend design pattern books

Hi
I work at a university and I am looking for some books on design patterns in LabVIEW, preferably for 2009.
I am interested in state machine and master/slave design and perhaps also producer/consumer design pattern.
Can you recommend some books on these techniques.
Simon
LabVIEW 8.6 / 2009 / 2010
Vision Development Module 8.6 / 2009 / 2010
VBAI 3.6 / 2010

The LabVIEW Style Book by Peter Blume has a chapter devoted to discussing patterns in LV. If you want to go generic and not pin yourself to one language, the classic of the subject is Design Patterns by Gamma, Helm, Johnson & Vlissides - that book has been on my shelf ever since I used it as a textbook over a decade ago.
Greg
Certifed LabVIEW Developer

Need to paste on/off button in its own event structure?

I am studying the CLD exam car wash vi attached.
First question, I noticed the Start button is pasted inside its own event structure. Is this absolutely necessary, can I just paste somewhere else?
I guess it was paste there because the Start button has no wiring so it looks neater to paste in its own event structure, am I right?
Second question, I noticed the Stop button is already wired to the red while loop stop, so pressing it would stop the vi already. But there is also the Stop value change event, this looks redundant, no? Or is it something really necessary??
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Attachments:
Car Wash LV86.zip ‏73 KB

Both are necessary.
1. The boolean's terminal while be read when the event is triggered. This read triggers the latch operation (so the button snaps back).
2. The event structure would wait for an event and nothing would happen until another event is triggered. Only after the event structure is 'run', the stop value is read.
Felix
www.aescusoft.de
My latest community nugget on producer/consumer design
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Re-triggering event structures after queue purged

I have a vi built with an event structure that allows the user to stop the tests. I am using the flush queue subvi in the producer loop to remove the remaining elements. Then the user may fix the problem and again press start, starting the tests all over.
About 50% of the time, the event structure will not restart, and the vi will need to be stopped and re-started.
There must be something I am missing that is not allowing the event structure to re-start.
Is the position of the flush queue correct in my snippet?
What could cause the event structure to not respond and how can I troubleshoot it?
Thanks.
metzler CLAD
Attachments:
producer snippet.png ‏56 KB

You should take more care about your error handling. Propably you just miss the errors that cause this behavior.
* At the while loop, use shift-registers for the error wire.
* I also recommend you to place a case around the event structure that catches errors and stop the loop.
Felix
www.aescusoft.de
My latest community nugget on producer/consumer design
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Producer/consumer w/ScanEngine issue

All,
I have a large VI implemented with producer/consumer architecture using 10 events and uses conditional disables to select the data acquisition hardware. The application is being developed in LV2009 with FPGA & RT on XP. The application is intended to acquire and control analog signals in a cRIO-9073, but early development is done using LV simulated signals. I can switch back/forth between the different configurations quickly & easily. In the simulation mode, the front panel, controls, events, etc; all behave as one would expect. When the conditional disable is changed to use ScanEngine and cRIO modules, then a very primary control fails to generate an event and does not function. All other controls that can be tested are functional and generate their events. I am at a loss and seeking suggestions on techniques to diagnose & fix the problem. The application behaves as if the 'event structure' for primary control gets corrupted or some size threshold is been exceeded. The whole concept, scanEngine based and simulated data, were functioning several days ago prior to numeruous edits and I can revert back to not using the scan engine and cRIO hardware with simulated data to validate correct function, but that is not the intended final path. At the present time, I am stuck and looking for ideas. All suggestions will be considered and appreciated. The applicatiion is company proprietary and can't be posted. Thanks in advance.

I can definitely understand your confusion, and on the surface it seems like everything should transfer over. However, when using simulated data you won't get errors that can occur once you introduce hardware. Errors can prevent events from occuring, there is also the issue of timing, to see if when using actual hardware, the timing causes issues. As I menationed before, there are a hundred different reasons why it would work with simulated data, but not actual hardware. Please let me know if you need further direction in your troubleshooting.
National Instruments
Applications Engineer

Producer/Consumer Design Pattern + DAQmx + Event Structure

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