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This solution might also apply to other similar products or applications. Solution Timed loops enforce strict timing, and while loops execute as fast as your CPU will allow. A timed loop is close to deterministic because it will iterate at the period you specify although it is not completely deterministic since Windows is not a real-time operating system.
A timed loop is generally used when there is a need for multi-rate timing capabilities, precise timing, feedback on loop execution, timing characteristics that change dynamically, several levels of execution priority, or precise determinism in a real-time operating system. Open a service request. Purchase or renew support services.
Was this information helpful? Not Helpful.I have two while loops running in paralle. One while loop is not timed, the other while loop is set at 0. Both loops seems to be running conccurrently both graphs update normally. Then, I changed the while loops into timed loops and did nothing else. Now, only one loop updates once every 5 sec? The other one is completely dead graphs not updating. I have a stop button for each of the loops, so I stopped the loop that was updating, and the other loop remains dead.
I was expecting them to behave the same as they did with just while loops. What is happening? Most likey you are making a simple mistake and it would be trivial to troubleshood of you would attach your two code versions before and after. It is an RT system, it is the R I don't know how realistic it is to post all the files needed to run this thing.
But here are the pictures. Basicly, I just right click on the loop and select "Replace with timed loop". The front panel have graphs. This is completely stupid. I linked the stops together. I didn't do this before, and did it after, forgetting that the loop depends on it. In this use case i think it would be appropriate to use a local variable of the stop button in the other loop. Yes, you created a data dependency between the two loops.
You could place a local variable of the stop button inside the other loop. Don't forget to set it back to false outside the loop.
Also remember if you're on RT, there's more than one choice of internal clocks. That can be a "gotcha" with timed loops on RT.
Error -808 While Using Timed Loops or Timed Sequences in LabVIEW
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So I was wondering if you had any idea what the problem could be? You don't want a "Wait" function -- that function halts execution until that node finishes, which means your While Loop will only execute 1 time, and that 1 time will last ms. You can drag this image out of your browser some browsers may require you to save the PNG to disk first onto your block diagram to recreate the code if you have LV or later, as noted by the version number in the top-right corner.
Another option to resolve you problem can be using a timed loop. These are specifically designed for timing tasks. The benefit of a timed loop is that you have more accurate control over when it stops.
Synchronization in LabVIEW – Part 1
The "downside" of a timed loop is that it behaves more like a for loop where each loop takes a specific time interval, ussualy I find that more a benefit than a downside:. I also put in the previous suggested solution from srm for comparison. On my pc the timed loop stops with ms accuracy while the while loop has a delay of several tens of miliseconds. The actual reason your bool value is false is that the output of the Wait ms VI is not the number of milliseconds the VI waited but "milliseconds timer" which is the number of milliseconds since windows booted up or something to that effect.
Others are correct to point out that a 6 second wait in the loop will prevent your loop from completing execution and repeating until the 6 seconds are over. Learn more. Asked 3 years ago. Active 3 years ago. Viewed 5k times. Barbara Stefanovska Barbara Stefanovska 21 1 1 silver badge 2 2 bronze badges. Posting a picture of your current code always helps the question. This gives you a PNG file that includes metadata that others can use to drag onto their block diagrams and recreate your code.
Active Oldest Votes. What you want is the Tick Count function.There are lots of ways to move data between loops in LabVIEW and to send commands along with the data to tell the receiver what to do with those data.
The QMH is a combination of a producer event handler, which pushes user messages onto a queue, and a consumer with a state machine embedded in the consumer loop, such that the consumer loop can push its own messages onto the queue. In the example there, the queue data type is an enum indicating the message to handle. Since nothing else is pushed on the queue, the consumer loop awaits further instruction, which must come from the producer loop.
An obvious benefit of this architecture is the responsiveness of the UI while the app is busy performing some task. Some earlier implementations of a QSM were done with a single loop i. All you readers should know by now that it is better to create a queue data type that holds both the message and the data upon which to act or to describe the message payload more precisely. The element holding the data is often a Variant, allowing almost any type of data to be passed in the queue.
There is no reason that queue usage needs to be confined to this producer and consumer loop. Since a queue can be named, access to a queue can happen anywhere in any VI. Thus, other parallel loops can communicate with this consumer loop by pushing messages on the queue. Likewise, user events can be created to cause the consumer loop to response to events other than the press of a button on a UI.
An alarm handler might be best suited for monitoring an event created when the application detects a problem. For example, an alarm event would cause the producer event case to analyze the alarm, determine the best course of action, and push the appropriate messages onto the queue for handling in the consumer loop. It is interesting to see how far the adoption of good programming practices has come in the last several years. Note how some developers think the use of a QSM is overkill for situations where sequential flow i.
Granted that they may be an acceptable statement for a simple app, but an app hardly ever stays simple. They use Ethernet to communicate between a single writer and an associated reader. They provide lossless data transfer and are robust to connection interruptions.
As such, they are essentially queues that operate across the Ethernet. Network Streams define a connection between a single writer and a single reader. Such a point-to-point connection implies that data transfer is in one direction. An application might use multiple Network Streams. A typical arrangement is to have a Network Stream that moves data between machine A and machine B and another Network Stream that goes the reverse direction B to A.LabVIEW uses data flow to sequence code execution.
This approach offers inherent multitasking capabilities, including parallel tasks managed in multiple while loops. However, almost immediately, a desire arises to communicate between parallel tasks.
So, these tools are very useful in designing automated test systems. The first group pauses execution of a piece of code until data is available or a condition is met.
LabVIEW Timed Loop Slower than While Loop
For example, perhaps you need to wait on a temperature controller for an aircraft hydraulic system to reach a certain pressure before moving a part and the pressure is being controlled in a parallel loop. Note that Notifiers are supersets of Occurrences, since they can pass data, whereas Occurrences cannot. The second group manages access to a piece of code based on the actions of multiple data sources. Another blog post will cover the second group. They are event-driven and consume very little CPU usage: your paused code essentially goes to sleep.
Notifiers are a sophisticated version of Occurrences. In fact, NI recommends that you use these instead of Occurrences. Notifiers are found in the sync palette as shown next.
Notifiers have the Timeout and Ignore Previous features similar to Occurrences. They can also be named which allows a Notifier to be found elsewhere in your code without needing a connecting wire. Plus, they can pass data from the producer to the consumer and are thus defined with a data type. An example of usage follows in the 2 block diagrams. The first is the main VI. The data type associated with this Notifier is a string. See the slave diagram below for details.
When the main VI is stopped, the Notifier is destroyed. The slave loop then simply waits for the Notifier to be set. When the Notifier is set, the Message Box displays the string passed by the Notifier.
Note that there is no wiring between this loop and the main loop. And, yet, the main VI affects the execution of the slave by forcing the slave to delay execution until the main alerts it via the Notifier. This feature can simplify the structure of an embedded control application. Also, the data type in the Obtain operation can be anything available in LabVIEW, such as a cluster, array, or variant.
Because of this flexibility, the Notifier is a much improved Occurrence. Queues pass data between two processes: one process pushes data onto a queue enqueue and the other consumes data from the queue dequeue. The consumer process is delayed until data is available in the queue, making queues a mechanism for synchronization.
The number of elements available in a queue is set when you obtain the queue.
By default, this size is unlimited i. However, this behavior embodies poor programming practice because an application could die a slow death as memory becomes completely consumed. A better approach sets a nominal queue length and monitors for full queue status.
A full queue implies the consumer cannot consume elements as fast as they are pushed by the producing code.Looking at the scheme i am sure there will be lots of eyebrows that go up as to why a Timed Loop is used instead of a while loop. I am coming to that point only. I just ran a serious of rough evaluations on a Intel Pentium dual core PC 1. Following were the observations : I dont claim these to be any kind of bench mark.
I just want to know if there is anything more than the demand on CPU that is a concern when using Timed loops on regular OS. Go to Solution. It will wait for the samples to arrive - and this will take always the very same time as this is hardware-triggered! This way your DAQ loop will also be "rock steady" as much as possible…. You are waiting more than anything, so I would not expect any real differences in your CPU usages here.
However, some things the Timed Loop does:. Sets everything inside of the Timed Loop to be a single threaded process. You cannot do anything in parallel inside of the loop. Sets a priority to the loop that I have seen mess with Windows starve OS processes that the rest of my application needed. Generally adds more overhead for little to no gain on Windows. Especially in your case, as was already stated, the DAQmx Read will be what determines your loop rate.
So just keep it simple and use the standard While loop. The Timed Loop was designed for "time critical" code. Again, your DAQ is already handling anything that is time critical in the hardware mainly the sample rate. So we don't really care about the software being time critical. The software just has to be fast enough to keep the buffer from getting full. So, again, the Timed Loop just does not make sense in a non-RT environment. Inside of a Timed Loop parallel processing is not possible!
Well its this kind of an insight I was after Also if there was one thing that I "thought" was an advantage of TL over WL is its ability to be prioritized!!
And the rest of TL in Sub Vis which consume this data are set for 75, 50, 25 priority depending on requirement. But what I gather is And one last thing. So which do you think is the best way to pass the same to another While loop in a Sub Vi? The options that I know are :. Missing an occasional data is not a big concern as the process like temperature does not vary so fast.
I am big fan of them. Till date have never tried Notifiers or Channels The Streaming channel would work equivalent to a queue. Depending on where you are sending the data, I have also used User Events. But the queue would be my first choice.
United States.I would like to know how Labview handles multiple "while loops" by default? I am not very experienced with Labview and definitely have no experience with the fundamentals of loop timing and such.
I've done a search of the forums for "multiple while loops" and I couldn't find any simple answers. To give an example which will hopefully clarify exactly what I'm asking. If I drop two independent while loops into a VI, independent in the sense that neither while loop requires any information from the other to proceed to some conclusion, in what order will they execute?
How can I control this order? Although I believe the winner is practically arbitrary and you can't depend on the order. You're better off forcing an order if it's important to you. One advantage to the first option is that the error cluster is passed throughout the program. You are passing your error cluster throughout your program, right?Using LabVIEW for Loops
Ahem, I haven't bothered with error clusters in general so far, though I did use it once in the same manner as your first example I was copying a labview example. Is the use of sequence structures as opposed to "data dependency" a purely stylistic choice or are there programmatic reasons efficiency etc for using data dependency? Natural data flow sequences nodes from the "natural flow of data" i. Artificial data flow refers to the use of wires connected to nodes that are not required however, in connecting the nodes you have created a data flow dependency albeit a fake or artificial one.
Finally you can use sequence structures - stacked or flat - these can sequence nodes that are independent of each other. As each subsequent frame will not execute until the previous one has completed. You will find a plethora a reasons on why people consider the use of sequence structures to be bad style e.
Using error clusters and connecting them up is a great way not only to create a basic error handling scheme but to sequence nodes too. But, one of them will start first. And which one does is indeterminate and unreliable.
LabVIEW Basics - 11 | Passing Data Into and Out of Loops
If you have a producer and a consumer loop, and the consumer is set to stop when the queue it's consuming is empty, you want to make sure the producer starts first. That's what I was going for. If it happens to start in the proper order in development giving you a false sense of confidence in your code you won't be able to count on it working as an executable or on another target.
I think. I have an alternative to your forth example. Don't look under the hood, it has a sequence structure. Well, technically that would now come under my first example - natural data flow.
An easy way to enforce ordering of parallel loops is to use timed loops instead. Wiring a different value to the offset input of each loop will cause them to start at different times. I've always been under the impression stacked sequence structures are considered bad style but flat sequence structures are okay, even when using multiple frames. Did I miss the announcement? Well I am under the impression the flat sequence structures are better than stacked sequence structures, namely because all the code is visible, you don't need to use sequence locals to pass data and the data is available to other nodes after a frame executes rather than then entire structure.