In the previous post, we started looking at how to deal with the situation where you have reentrant VIs that need to have UDEs that are specific to particular instances of the VI. That post covers a lot of the basic theory and design issues, so if you haven’t read that post, please take a few moments to check it out as much of the following won’t make sense without that background.
Filling in the Blanks
The basic approach we took in designing our example consisted of first defining the program’s overall intended operation. With that theory sorted-out, we then began creating the program’s high-level structure, being sure to leave blanks or prototypes as placeholders for details that we hadn’t yet defined. At one time this sort of approach would have been considered heresy. The feeling was you hade to have everything worked out in detail before you wrote even one line of code or created even a single VI.
Clearly there are a lot of practical problems with this approach, but for one of the biggest (in terms of long-term impact on the project) is that it leads to a break-down in the walls that information hiding is trying so hard to erect. Think about it for a second, if an integral part of designing a process launcher includes figuring out the internal operation of the processes it is going to launch, it’s going to be very difficult to keep that knowledge from contaminating your launcher design. Or to put it another way, there will be a strong tendency to create code that depends on the process VIs behaving in a particular way.
You can see a better approach in the way we designed the launcher for our reentrant VI. Last time we defined a VI that (beyond the mechanics of how to launch a reentrant VI) only concerned itself with the data that the process VIs need to do their work. Based on a black-box description of what the routines were supposed to do, we able to create the logic for providing that data without worrying about what the routines were going to do with it. This is a Very Good Approach.
One of the “blanks” that we left to be filled until now was a VI called
DAQ Operations.lvlib:Interval Registry.vi. When we first considered this VI’s external persona we noted that, when passed an enumerated value
Check and an interval, it would return a boolean flag indicating whether or not that interval already existed. However, we made no assumptions about how it might go about completing that task. Having taken, so to speak, a step inside the veil we can look at how it works.
This is the logic that completes the
Check function, and as it turns out, there isn’t really very much to it. In fact, we see logic that implements a simple FGV. All the function in question really does is search an array of intervals to see if the one we asked about exists. Given this level of simplicity, a logical question would be, “Why bother with the subVI? The process manager has a loop, why not just use it to maintain the list of active intervals?”
This approach would handle half of the problem quit well – the half dealing with whether or not a particular interval was launched. But if you think about it, that isn’t what we really need to know. Operationally, we don’t care if a particular interval was ever launched. We want to know whether the interval is still running, and the interval’s current state is something of which the manager has no knowledge. Remember that when we defined the rules governing the high-level behavior of the acquisition subsystem, we said that when an acquisition process sees that it has no addresses left to poll it will shut itself down. How is the process manager supposed to know that happened?
Beyond this practical consideration, there is also a conceptual problem associated with storing the interval list in the process manager. You might not realize it, but all data has an associated “scope” that defines the area, or context, which owns the data. If we store the interval list in the process manager we are, conceptually, giving ownership of a piece of information that should belong to the subsystem as a whole, to one specific VI. Moreover, that one specific VI would then be required to manage all accesses to that data. Now while that functionality could certainly be implemented, it would be at the cost of additional complications in the form of additional signalling, and event handlers to respond to those messages.
By contrast, moving the array into a FGV that any VI in the subsystem can access, makes the data immediately available to the subsystem as a whole: No event handlers, no added signalling. As we get into the VI that performs the (simulated) Modbus IO
Collect Data.vi, we see how the FGV’s remaining two functions work.
Next, let’s look at the reentant VI (
Metronome.vi) that is responsible for triggering acquisitions at a fixed interval. You will recall that the VI is passed three parameters when it is launched: The desired interval between acquisitions, the event to fire when the interval times out, and the event that will fire when the interval is shutting down. Here is the logic for initializing the VI.
We see that the interval value drives the
Timeout node of an event loop, the shutdown event is registered, and the event that this VI will fire, is simply passed into the event loop. As you might expect, the event loop itself only has two events. Here is the
No surprises here: all the event needs to do is fire the acquisition event – which this code does with alacrity. The shutdown event (which handles both types of shutdown) is likewise uncomplicated:
It destroys the two dynamic UDEs and stops the event loop. The acquisition event is destroyed because once a shutdown is initiated, it is no longer needed. The interval stop event is destroyed because it is only fired in the acquisition VI, so by the time we get to this point in the code it has already been fired by the only other VI that needs it. The small delay between the two operations is to ensure that the acquisition VI has time to start shutting itself down.
Getting into the Publishing Business
The last code we need to look at is the reentrant VI that reads the simulated data and publishes it for use (
Collect Data.vi). Again starting with the initialization logic, we see something very similar to the metronome VI:
The main difference is that this VI needs to register to receive both dynamic UDEs. Although the VI will be generating the
Delete Interval event it also needs to be able to respond to it because that context is the best place put the logic for deinitializing any acquisition logic that might be used. Another conceptual difference from the metronome is that this VI relates to the interval input in a way that is fundamentally different. For the metronome the interval is a number that has a specific meaning: it is the number of milliseconds between triggers. For this data collection routine however, it is simply a number that provides it way for it to identify itself. The broader point is that you need to remember to manage the expectations when different processes look at the same value and see different things.
Because this VI is going to be acquiring data, and performing other operations, its event loop is also more complex. Let’s look at the 5 events it will handle:
Timeout – This application is only generating simulated data, but in most other situations, there will need to be some sort of initialization performed, like opening DAQ references or establishing connections to one or more Modbus devices, and this event is a good place to handle such initialization. The way this logic is written, it is easy to make the initialization run just once, but still have it readily available if it ever needs to be run again.
However, there is other initialization that needs to be performed even for simulated data. Specifically, if the initialization logic completes with no errors, we need to tell the rest of the application that this process is open and ready for business. To record that state change, we use our “repository” VI again, but this time running the
…which is the very soul of simplicity.
Start Addresses – This event’s purpose is to maintain the process’ internal list of addresses in response to the user starting additional addresses. In completing this work, there are two cases that it will need to address.
First there is the case where the interval number matches the value that the process is using to identify itself. In that situation the code needs to add the new addresses to the existing list, or more correctly, it needs to add addresses to the array that don’t already exist in the array. This logic protects the logic from what would be a very common operator mistake: adding addresses tht already exist. The second situation is one where the interval number does not match the value that the process is using to identify itself.
In this case, we need to enforce the rule that only one process can be polling a given address. Hence, the logic needs to remove from its array any addresses any addresses contained in the new event. Of course this operation raises the spectre of the entire polling array being emptied out, with the contingent requirement that the interval shut itself down. The logic handles that scenario with a two-step process. First it tells the rest of the application that its stopping by calling the registry VI using the delete logic.
After removing itself from the registry, the VI fires the
Stop Interval UDE to close both itself and its associated metronome process.
Stop Addresses – This event has logic that is similar to the previous event’s delete logic, but is simpler because the only thing that matters is whether the indicated address is in the process’ address list.
Get Data – This event generates an array of simulated data and passes it to the static
Publish Data UDE, along with an array of addresses associated with the data values. In addition for troubleshooting purposes, the code also writes the data to a front panel table.
Stop Interval; Stop Application – Finally, this event stops this VI if either the interval or the application as a whole is shutting down.
As the comment says, if the acquisition logic needs to be deinitialized, this in the place to put that logic. But why is the code interested in the front panel’s state? Although this VI usually runs in the background unseen, there are times that you want to be able to view its front panel so you can verify its operation. In this example, I implemented that functionality using the VI properties to force the front panel open when it starts running. This logic checks to see if the front panel is open and, if so, closes it.
Testing the Code
So with all the code implemented, here are the Subversion links to the application an the toolbox of reusable code:
The first thing I would recommend after downloading the code, it to go through it while re-reading both this post and the previous one. Often times it is easier to understand things when looking at the code, that are otherwise a bit obscure when all you have are pictures of the code.
Next run the top-level VI (
Dynamic Registration.vi). When the front panel opens, click on the
Add Addresses button to define some addresses. For the purpose of this example, I created a simple dialog box that lets you specify a starting address, the number of consecutive addresses to collect, and the sample interval. The starting address needs to be between 40000 and 49999, the number of consecutive address must be less than 1000 and the sample interval needs to be between 300 and 5000 (milliseconds). These parameter limits are set in the dialog box code – feel free to change them as you desire. Likewise, the output of this dialog box is a list of addresses, so you can also change the selection interface if you so desire.
To get started, define 5 addresses starting at 40000 with a sample interval of 1000-msec. You should see the front panel of acquisition VI pop open showing the 5 addresses being updated once per second. In addition, the main GUI should show data from the same 5 addresses.
Add Addresses button again, but this time define 5 addresses starting at 40008, and updating every 2000-msec. A second acquisition VI windows will open showing the 5 new addresses, and the main GUI will show a total of 10 addresses with the results changing at different rates.
Let’s next see what happens if you specify addresses that are already being polled. Click the
Add Addresses button one more time and define 5 addresses starting at 40004, and updating every 3000-msec. This action defines a range where 40004 is already being polled once a second and 40008 is being polled every 2 seconds. In response you will see a third acquisition window open, but you will observe that one address is removed from each of the two existing polling lists, and that the main GUI shows a total of 13 addresses being polled.
Finally, to test the auto-shutdown operation click the
Delete Addresses button and in the resulting dialog box, tell the system to delete 5 addresses starting at 40008. Because the 2000-msec interval is emptied out, the acquisition VI window associated with that interval will close. Finally, the polling list for the 3000-msec interval will be reduced by one.
The Big Tease
So that’s about all for now on this topic, but what’s in store for next time? One of the things that I like to talk about in this venue are things that can cause unexpected complications, so next time I’m going to discuss what happens when a DLL misbehaves as you are trying to close it.
Until Next Time…