Jump to content


  • Posts

  • Joined

  • Last visited

  • Days Won


Everything posted by jdunham

  1. QUOTE(Jan Klasson @ Sep 14 2007, 12:09 PM) Hi Jan: Thanks for the tip. I will check out the Endevo toolkit. I have heard so much about it and I am glad to hear it could solve some problems. I think the by-reference objects will help with some other constructs I am working on. Does the toolkit also help with by-value classes? However, I do still wonder whether NI will move toward access of protected data. It seems to me that there are many cases where inheritance doesn't help much if the inherited parent data is not availaible to the children without accessor methods. It's not that you can't get the data (via methods) but standard LabVIEW clusters make access to the data so easy, that it's hard to see how the native object-oriented implementation will be worth the trouble of implementing it.
  2. QUOTE(Jan Klasson @ Sep 14 2007, 03:04 AM) Yes, I was trying to make that point. If both of my vehicles use gasoline, should it seems like it should be provided by (a property of) the parent class, and yet, since there is no protected data, only private data, the vehicles can't use it. QUOTE(Jan Klasson @ Sep 14 2007, 03:04 AM) About the session data field in the Vehicle parent class. I don't know what kind of code you need to handle the session but let's assume you put it in a separate class to use containment. Then you let Vehicle contain this class in one data field. This may be good from a design perspective (I couldn't tell without knowing more about what you do) but it doesn't solve the issue that you find annoying, i.e. child classes has to use a get method from the parent class to read out the object... back to square one:-). Yes, I tried it just after posting and realized it doesn't work. The child can't use the private data from a class it contains. That makes plenty of sense, but I have to find a different solution. Maybe I am just allergic to lots of accessor methods and I should get over it, but I see the need for many of tham as an impediment to productivity. QUOTE(Jan Klasson @ Sep 14 2007, 03:04 AM) Here is one way of solving the case with scenarios repeating in child class methods. For each dynamic dispath VI in the parent class create a static "mirror" method. The static method should be public and work like a "macro", in this case GetSession-call the DynamicDispath VI to do the job-CloseSession. ... This scenarion will double the number of methods in the parent class but you will not have to repeat scenarios in child class methods. If it is worth it only you can tell. I think you are right. Your method is probably a sensible way to go. It sure doesn't make a LabVOOP design any easier to implement when protected data would give you this for free. I actually solved my DLL problem by sticking to one class, and making similar methods for the two types of 'children' which I didn't bother to implement with objects. Thanks for your response. I would love to keep the discussion going because I feel like this situation will keep recurring.
  3. I saw a comment by Aristos Queue a different thread (see Post #11 in http://forums.lavag.org/I-want-polymorphic...lass-t8853.html) which I want to discuss further QUOTE I am trying to make some objects which have some shared properties with some different behaviors. For example I have parent class 'vehicle', and child classes 'car' and 'truck'. I don't want to strain the analogy too far, so I'll get real and say that I have DLL code which needs to open a session, common to all vehicles, and then I have different methods for car and truck which could be dynamically dispatched depending on the object on the wire. On opening the session, I get a handle, which needs to be used by all other methods in the child classes. In my first attempt, the session was in the parent private data, so I would need a parent accessor subvi call in every child call just to get the handle value. I guess that's not a deal-breaker, but it just seems like an unwieldy pattern. It would be easier to just include the handle in each of the private data clusters for the children, but that chips away at the whole point of an object-oriented design, which is to avoid duplicating common code and data structures. So on re-reading the quoted text, it seems like I could make the parent data structure include the handle, and then stuff the parent private data into the child class so that it is available where the code needs it. Does that make sense? Should the child class still inherit from the parent class? Maybe my parent class is really gasoline, used by car and truck, rather than vehicle, the generic type of car and truck. I'm not sure how I can really tell the difference.
  4. I think most, if not all, NI DAQ cards can do simultaneous input and output. But the DAQmx API was not designed to handle this as a single task. The main trick you want to use is to make both tasks share the same sample clock and starting signal when you are configuring them. There is an example VI at <labview>\examples\DAQmx\Synchronization\Multi-Function.llb\Multi-Function-Synch AI-AO.vi, however that VI only has common start signal rather than forcing the sample clock to be shared as well. That should be good enough, but it is also pretty easy to share the sample clock with the same type of DAQmx property nodes.
  5. QUOTE(Aristos Queue @ Sep 5 2007, 07:14 AM) I think Harish was talking about treating his instruments as objects, and how you would create an instrument driver using an object-oriented approach. I'm still pretty new to LabVOOP and OOP in general, but it seems like you want to think of all the stuff the instrument driver knows, but which the rest of the program shouldn't care about, and make that your class's private data. Then you would think about all the actions you perform on the instrument and make them into methods, along with other methods to do any necessary manipulation on the private data. I think it can be pretty easy to make that disctinction between public and private, because you could say "If I changed to a different instrument from another vendor, what items would change and what would stay the same?". Anything which is likely to change should be private to your object. The open question is how much benefit can you get from inheritance. It's plain that you could make a class for a generic instrument and then a generic oscilloscope which inherits from it, and then have children for different scope vendors, but if you just have one scope on your bench, it probably isn't worth the trouble to create that model and implement the hierarchy. Of course if someone reading this knows it is worth the trouble, you should use all of the time you saved to tell the rest of us the story! :worship: Jason
  6. QUOTE(yen @ Sep 3 2007, 12:06 PM) Well I built it before posting and ran it and it worked great, so I just don't know what to tell you. I looked in the LabVIEW manuals and didn't see any direct information on why this works, but I have to assume that the NI developers either knew in advance that it would be useless to stack up events for a slider control, or else they did it wrong the first time and it was fixed in an early LabVIEW 6 beta. The slider control would probably be unusable if moving its slider generated thousands of events and they were allowed to accumulate. I was a little surprised that orko's last suggestion also worked. I thought it would miss the last update, but it never does. That means that the sequence frame is not needed at all. What you can't really do is know that the slider is still in motion and only run the event case one time when the mouse up has occurred. I don't think that's possible, since there is no safe way to trap the mouse up event. Jason
  7. Here is a simple solution. The main issue is that the GUI goes dead during the waits, so the slider is not even repainted as you move the mouse. If you set the wait time above 250 you can see that the behavior is really unacceptable, but for values of 50-100, it's totally fine. The subtletly is that you can't use the NewVal from the Event Structure because that will usually be stale by the time your wait finishes. http://forums.lavag.org/index.php?act=attach&type=post&id=6834 I generally shy away from handling mouse up/ mouse leave events because there are many cases in which they don't really behave the way you would like, as orko found out with his example. Jason
  8. QUOTE(Aristos Queue @ Aug 17 2007, 01:29 PM) The point I was attempting to make is that a given VI can only be a member of one library. If two different pieces of code need to call the same function, I guess that requires it to be made into a separate object with its own lvlib (or preferably an lvclass), which can then be shared. QUOTE My personal goal is that all VIs should be owned by either an XControl library or an LVClass library. Plain libraries should only own other libraries for packaging and distribution purposes. This is in accord with the maxim "All bits of data may be thought of as part of some object; all functions may be thought of as actions taken by some object." Do I actually keep to this goal? No. LabVOOP still has inefficiencies in some places, and sometimes I get lazy. But I'm working toward that goal and do believe it is viable. Not knowing any better, I would suggest that most lvclasses should be contained in either an lvlib or lvproj to hold the class and its test harness and any examples. QUOTE All the VIs for a single project go in a single directory unless they are components intended to be shared among many projects. Within that single directory, I recommend creating subdirectories for each library. I do not recommend that you have any further disk hierarchy. If you create folders within your libraries, so be it, but don't make directories on disk that reflect those folders. Why? Because folders are the organization of your library for presentation as a palette and for providing scope for VIs. It is really annoying to want to change the scope of a VI and have to save it to a new location to keep your disk organization consistent. And it serves no purpose. If the library is a coherent distribution unit, then when you're on disk you're supposed to be moving the entire library as a chunk. Some people complain that that makes it hard to use the "Select a VI..." item in the palette. But I suggest that is what drag-n-drop from the project is for. Use the project window as a pinned palette during development. When you deploy, deploy the library with a .mnu file that is part of the regular palette hierarchy. One thing that has always been helfpul to me in the non-oo world is to have the top-level folder only contain top-level vis. That's been a self-documenting way to communicate where someone should start when attempting to take over the code. I suppose the key here is to make sure the top level folder only contains the main project, and then within the project, that only main VIs are shown outside of folders. QUOTE PS: If your clusters are already typedefs, put the typedef into the project window, popup on it and choose "Convert Contents of Control to Class." This will help you on your way to converting your project over. Great! Thanks for the help. Jason Dunham
  9. I am testing the waters for converting my large application into LabVOOP. We have a lot of clusters (typedefs of course) so the end goal is to hew to AQs advice that "all clusters should be classes". I would also like to supply more unit testing as I go along. However I feel a bit confused by one basic thing: How do I map my existing code into lvclass, lvlib, and lvproj? I get it that lvclass is the cluster and the functions which muck with its private data, and that lvlib is a namespace and has a one-to-one pairing with VIs. I am also thinking that each functional grouping should be an lvproj which includes some test VIs, or should they be in the lvlib too? If I can take similar clusters and convert them into inherited objects from a base class, then how much of that goes into lvlib? I realize there are no strict answers, but I'm having trouble getting started. Any advice would be appreciated.
  10. QUOTE(Jim Kring @ Aug 13 2007, 03:57 PM) In my app, I needed long term timed acquisition on some of the channels, so I suspect you may be able to get it working with your retries (how annoying!) whereas I could not. I presume DAQmx Read is the VI throwing the error. The read process has a property called auto-start. which basically runs DAQmx Start Task if you didn't bother to. You can turn this off, but the default is on. Since the timeout function is really for the reading of the channel not for the autostarting, it sort of makes sense that the driver does not contain code to loop on starting the task, when you are really doing a read and it is starting (and then stopping) the task as an extra favor to you. You could run your loop on DAQmx Start rather than DAQmx Read, but I guess that doesn't buy you anything except code clarity. You are still at the mercy of the shared timing subsystem. If you can get this logged as a bug against the API, then more power to you!
  11. QUOTE(Jim Kring @ Aug 13 2007, 03:09 PM) I suspect that the timers are integral to the operation of the M-series. When I did something like this, I did try to use separate tasks and never got it to work. I ended up using one task, and reading the channels independently. Architecturally it was annoying because I had to know about all possible tasks in one VI and configure them all before doing any of the reads. If you find a different way, it would be great to hear about it, but I gave up. To access the channels independently (after the global task start), just set DAQmxRead.ChannelsToRead immediately before calling DAQmx read. I was using sample timing so I also set the property node to read the last N samples which I wanted. Note that if you set DAQmxRead.RelativeTo = Most Recent Sample, and Offset = 0, it will actually wait until the next sample arrives (Traditional NI-DAQ did this too). You generally want Offset = -1 if you really want the currently available sample with no waiting. I don't know what happens if you don't request any sample timing at all, but I would read the timing from the property nodes and find out what you are getting. Good luck, and don't spend too much time without calling NI for more help.
  12. Only time for a quick note. LabVIEW only stores timestamps in UTC. In older versions of LabVIEW this was great EXCEPT that there was no easy way to get UTC out of the timestamp; it would always convert it back to your local PC time zone before showing it to you. As far as I remember, if your machine had changed itself between daylight and standard, there was a lot of potential for an hour of error in your timestamps. Newer versions of LabVIEW (for sure in 8.2) have mostly fixed this, subject to limitations of the OS. You can choose to convert the timestamps to either local or UTC representations, which is extremely helpful (if a bit late in coming). If you have discrepancies in your files, I would recommend trying to use the LV Timestamp as much as possible. You may be able to do some tricks like check the first or last times recorded as timestamps versus ascii date/times and see if there is an approximate 1-hour difference. You can also check the OS file timestamp and see whether it matches the last timestamps you recorded in the file. Good luck, and try to use the timestamps as much as possible.
  13. QUOTE(Jim Kring @ Aug 13 2007, 02:05 PM) I believe that the reserved resource is actually the analog sample clock subsystem, which can't be shared. The API doesn't assume this because it could be different for other hardware, but with E series and M series I think this is true; hence the runtime error. You would think that with all of the timers, they could run tasks in parallel, but then the signal routing definitions of those timing signals would become ambiguous. In general I would agree that the DAQmx API is not the greatest. Just try to take two slaved DAQ boards and make one task which includes all of the channels on both boards. We were kind of bummed to learn that this is totally not possible even with all of the virtual channel stuff. If there were a call which could return an immediate (non-timed) value from an analog channel, then that might work (assuming you don't care about sampling and timing, but I suspect there is no such call in the API. Last time I had to do somthing similar, I made the DAQmx task global, and then accessed the specific channels independently, which is pretty easy to do. Good luck, and thanks for all the coolness at NI Week. Jason
  14. I'm having trouble figuring out how to dynamically clone VIs in LabVIEW 8. In LabVIEW 7, we just made copies of the VI on disk with different names, and loaded them dynamically. We also used the libraryn.llb library from NI to copy the template VI out of the built executable to start making the clones, but that doesn't seem to be working with my LV 8.2.1 built executable. It seems like LV8 fixed this, with better support for reentrant VIs with separate panels, but I couldn't find good documentation or examples (or the right lavag.org thread) of how to generate an unlimited number of copies at run time. I'd rather do this the right way than fix my old code, because I'll bet it's much easier now. Thanks Jason Dunham
  15. Hi George: I have done plenty of applications with complex access to the DAQ buffer. The normal DAQ read mode keeps track of an internal read mark, but if you bounce around in the buffer you will need ot use a different mode and keep track yourself. Just set your DAQmx Read.Relative To property to "First Sample" and specifiy DAQmx Read.Offset and you will get the data you want. You will probably want to request a larger DAQ buffer size than the default to make sure the data is still available by the time your code gets around to requesting it. You may also want to set the DAQmx Read.Channels to Read property if you don't want all of the channels. You can also query the DAQmxRead.Status.TotalSampPerChanAcquired property to see which samples are currently available so you don't have to wait for DAQmx Read to return or risk a timeout error. Jason Dunham
  16. Some typedef'ed clusters can get really big when dropped on the block diagram. It can cause a big problem in a crowded case structure. It would be great to be able to replace the typedef with its icon.
  17. I think you would need to limit it to enums. I'd hate to see what labview would try to do with the case 1..1000000 Rather than being an option, maybe it could limit itself to 8 items or as many characters as fit, whichever is larger. - Jason
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.