Shared variable question

Post Author: despec
CA Forum: General
Hello to the forum once more,I am trying to use a shared variable as a "switch" in a sub report to either turn on/off suppression of a text label and the associated field. If the user selects a "summary" report, I set a shared variable as 0; a "detail" report, I set the shared variable to 1 (I created a formula called "note_flag" and shared numbervar note_flag=[the parameter as selected by user]).In the subreport, I define a formula called "sub_note_flag" with the following declared: shared numbervar note_flag;But the value of the shared note_flag in the subreport is always zero. Not sure what's going on. I have the subreport within the "detail" section of the main report. I've tried setting the shared note_flag with other values from the main database I'm using, but the value passed to the subreport is still zero.Any ideas what I'm doing wrong?Thanks,David

Post Author: wolfgang666
CA Forum: General
Typically when I have had problems with shared variables behaving as you are indicating it is because of the placement within the report. First thing I would do is display the value of the variables on your forms. Ensure the value is setting itself as you would expect in the main report to eliminate that as a possibility. If it is then ensure the variable in the main report is located above the details section where the subreport is located. Hope that gets you pointed in the right direction.

Similar Messages

Question about shared variables and report generator

I have a project with about 200-250 shared variables and at every 12 hours i want to make a report containing information about some boolean and double front panel variables linked to the shared variables... Now, i've tried with both the "Write Trace to SpreadSheet File" function from DSC->History and with different functions from NI Report Generation Toolkit. My problem is this: i would like to get my xls file to contain just the timestamps when my shared variable changed value. As it is, the number of rows in the file depends, obviously, on the sampling interval given as input. As an example, in 12 hours, my shared variable would change value maybe 5-6 times. Thus i would like my xls file to contain just 5-6 rows with those 5-6 changes. If i set the sampling interval to say, a matter of seconds, that would mean too much unneeded information in my xls file. Instead, if i set it to let's say 15 minutes, i would probably lose the moment the shared variable changed value. Is there a way for me to achieve my desired functionality with functions from DSC or Report Generation Toolkit? Thanks in advance, Sabin

Hi Mike and thanks for the reply. Actually, my first idea was exactly this: in an event case structure, whenever my variable changed value, i would retain the relevant data in an array and then at the 12 hour period write all of it in a report. However, my boss deeemed it memory unefficient and compelled me to use only one function. Thus my predicament... If you have more advice on this new info, i would be grateful... Cheers, sabin

Why should you explicitly open and close shared variable connections?

I'm looking into switching over from the old Datasocket API to the new Shared Variable API for programmatic access to shared variables, and I noticed that LV doesn't seem to have any problems executing Shared Variable Reads & Writes without first opening the connection explicitly. That is, I can just drop in a shared varaible Read VI, wire a constant to the refnum input, and it will work. I'm wondering, then, what benefits are offered by explicitly opening the conenction ahead of time...?
I guess I could see some cases where you want to open all necessary connections in an initialization state of a top-level state machine, particularly if you want to use the "Open & Verify Connection"---so you could jump straight to an error case if any connections fail. But other than that, why else might one want to explicitly open the connections.
And, along those lines, are there any problems with implicitly opening the connections? One reason why I am hesitant to open them explicitly is because for one of our applications, we need to be able to dynamically switch from one variable to another at runtime. It would be nice to just switch the variable refnum (wired to the input of the Read function), without having to manually close out the old connection and open a new one. A quick prototype of this seems to work. But am I shooting myself in the foot by doing so?
Thanks in advance.

I'd expect there's a very small number of people at NI that would know the answer to the detail you're asking for. But, let's try to extrapolate from this rather old post to see if we can understand what they're forming their impression on.
The shared variable has to have some sort of reference going on in the background. It looks like they're calling this a connection. It's how LabVIEW knows where to find this variable on the network. We can also see this reference certainly exists if we're opening/closing the reference in the explicit method. You see the connection as just a string referencing the variable by URL. This "reference" has to be stored somewhere. No matter how we're looking at this, we're aware there's a reference of some sort stored.
Now, we'd want to look at what would cause this reference to go away. If you open/close explicitly, it's easy to see it goes away at the close. If it's implicit, when would it make sense to close it out? The VI can't be expected to guess where it's done being referenced and close it out. This puts us into a situation where the soonest it could close is when the VI ends. From my experience, references tend to be wiped when you close out LabVIEW. It's this kind of idea that makes the FGV possible. I wouldn't be surprised by Morgan's claim here.
If we look at scalability, you're talking about two different topics. You're talking about adding an extra open, close, read, etc rather than just a few wires. That certainly would look a mess. In terms of the dynamic swap that was being discussed, we wouldn't be adding all of those. The concern would be if enough connections were opened it'd start to behave similar to a memory leak. This could be something that works with a smaller number of variables. If you continue to scale, it becomes problematic. This is why they suggest it's not scalable.
To your questions:
Does LV allocate some kind of session in memory using that string as a lookup?
It would HAVE to allocate some memory to hold that string. Otherwise, it'd be pointless to even have the reference.
Does it reuse that session if other parts of the application reference the same variable, or does it create a unique session for each referencing call to "Read Variable.vi"?
I would expect this to be "it depends." With the implicit method, I would expect it to open a new reference in each point it isn't wired. This is similar to int x,y = 5; x and y share the same value but are their own unique memory location. If you wire the reference to the other points, it should use the same reference. This would make more sense to me than the program seeking out any other potential usage of the variable in the application to see if there's already a reference open. I could be wrong, though.
And what resources does this "connection" actually represent?
At best, this is just the string. At worst, it's the string and the TCP socket. I'd lean towards the first. Opening and closing sockets should be relatively easy in most applications. But, it also wouldn't surprise me if it holds the socket.
I'm sure others have a better understanding than I do. But, that's what I'd expect for anything you've asked.

Print-time charting with shared variables on a report without groups...

hello all,
I have an interesting conundrum; I am working on a report (Crystal XI) with no groups to it, just several subreports. Presently the report is run weekly and sent to the manager of one of our departments.
The report itself showcases all areas of the department with total tasks, longest outstanding task, average days outstanding, MTD tasks completed, and YTD tasks completed. Due to the variety of tables the data pulls from, I have it set up as subreports that simply display the data.
All total, there are 20 different shared variables that have been obtained from 10 different subreports. I have the shared variables set as the running totals from the subreports (hence, all are print time). Since the running totals and all the subreports are time based and are not actually linked to ANYTHING on the main report, I am struggling with how to set up the formulas described in the white paper, "Charting on Print Time Formulas".
The setting up of the two formulas ({@onchngof} and {@showval}) will be different for my situation, as all of the subreports run off of date information relative to when the report is ran. I've got all of the subreports that the shared variables originate in early in the report; but where to from here?
Would I set up maybe a date grouping or some other non-essential consistent grouping to make it work? The way this report is, it really doesn't need grouping, because the main report is just a display agent for the subreports...
As always, any help is greatly appreciated!

Let me ask couple of questions before trying to resolve this.
1. The chart you are trying to built is placed in which section and is it on Main report or sub-report of its own. This is needed because if you are trying to access shared variables values in Main report, the section should be below the sub-report placed sections.
2. Try pacing few test formulas in Main report and make sure the correct values are being pulled in on Main report.
If you are successfully pulling the variable values on the report, try creating a simple chart in report footer with the values you have pulled and see if its taking you somewhere...
Would need more information on how those variables are being accessed form sub to main or sub to other sub for example are you making an array of those variables and splitting or bringing it in as separate...

How do I setup shared variables between executables created in sepparate projects

Hello,
I have several sepparate projects with their own respective executable files and I would like to be able for these executable files to all share the same variable (one program controls the value of the variable, while the others read from it).
I got this setup to work on my personal computer (by being able to access variable manager, etc), but I need to deploy these executables on different computers that don't have the labview development program. What steps do I need to do in order for me to be able to put these executables on any computer (I'm assuming I need to setup a path for the shared variable that is always in the same folder, etc)
Thanks
Vlad
Solved!
Go to Solution.

Hi Vlad,
I think this article may answer some of your questions regarding shared variables in deployed applications.
http://zone.ni.com/devzone/cda/tut/p/id/9900
It sounds like you already have your executables built, but this article may answer some questions about deploying them to other machines.
http://zone.ni.com/devzone/cda/tut/p/id/3303
Jeff S.
National Instruments

How to use shared variables to address multiple Watlow controllers on the same COM port

Hello,
I am trying to use LabVIEW 2010 to control 4 Watlow temperature controllers on one COM port. 3 are Model 96 and 1 is an EZ zone controller. Each controller has a unique modbus address, and I am trying to read from and write to individual registers (such as closed loop setpoint) using shared variables. I am getting return data when reading (although the data appears to be invalid), but am unable to change the value in the register by writing. How can I be sure that the Modbus server is sending commands to the correct controller?
Chuck
Solved!
Go to Solution.

Peter,
Thanks for the reply. I have actually solved that problem. I realized that the Modbus server address has to be the same as the controller's Modbus address.
I have, however, run into another problem. Perhaps you could help me with that. I have a system with 4 Watlow controllers, 3 are series 96 controllers, one is PID only and 2 are ramping. The 4th controller is an EZ zone. I am using RS485 for communications and the controllers are all wired in parallel for communications and power.
I have set up 2 Modbus servers for 2 of the controllers.
This is the first I have ever worked with Modbus based communications. I have successfully programmed using the Modbus read/write VIs, and am wanting to move to shared variables. My questions right now revolve around addressing, Modbus I/O servers and COM ports. Specifically, at this point, I know the addresses need to match up between the server and the slave device (Watlow controller in my case), how many servers can I create and use on one COM port? If the number is limited, is there a way I can specify an address that I want the server to talk to? Will the broadcast mode work to request data values from the controllers?
I'd appreciate any information you can help me with, or if you could point me to some sort of concise 'How-To' for Modbus communication.
Thanks.
Chuck

How to add a new Shared Variable programmatically to an existing and deployed library?

Hi there!
I am trying to accomplish this on both LabVIEW 8.6 and LabVIEW 2010 and seems like it's not any different in this situation.
My case: A project has a library with 4 Shared Variables (SVs). The library and the variables are deployed (visible in Distributed System Manager 8.6/2010). I want to add 2 more variables into this library. It is possible to do this manually from Project Explorer window's options menu. But while running an application it has to be done programmatically. Can I provide the library reference to the one currently existing without creating a new one?
"Create Or Add Library To Project" function in the Datalogging and Supervisory Control (DSC) toolkit does not help in this case. That function, as the name suggests, just tries to create a new library in the project.
Two possible methods:
The function "Add Shared Variable To Library" (DSC>EngineControl>Libraries & Processes) needs a library reference, which could be provided via "CreateOrAddLibraryToProject" function, but this function tries to create a new library, and if I provide the path of the existing library it throws an exception that the library already exists in the project (yes, as I wrote above, I need to add new variables to an existing and deployed library).
OR
The function "Create Shared Variable" (DSC>EngineControl>Variables & I/O Servers) will add Shared Variable in a process, and not physically in a library file, and the problem (limitation) with this approach is that it doesn't allow to add complex data type Shared Variables (for example in LabVIEW 8.6 it has only 4 datatype options in input parameter, and even in LV2010 it does not have "Image" datatype that I need).
The scond method is my preferred method as it allows to work on Online Shared Variables and doesn't create them physically in libraries (and this is good as the variables' scope remains only till the Variable Engine is running). But it doesn't support advanced data types, and the first method is powerful in terms that it supports to virtually any datatype, it just seems tricky to get that reference to the library.
Any tips?
Thanks ahead!
Vaibhav

<<<<>>>>
As the above two images show, while a library does not exist, it is easy to use the "CreateOrAddLibraryToProject.vi" which will add a new library to the project (if it doesn't exist on file system, a new library will be created) and that way, using the reference (the green wire going out from the function and the Case Structure, can be used to add Shared Variables to the library. The problem was what to do when a library already exists in the project, how to add more variables to it. I was looking for a way to get a library reference, and somehow I could not see it inside a Project's property (VI Server functions). Hence I posted the question. And upon continuing my search, I found it on the Application's property list.
I hope it was useful for someone else as well. And thanks for the replies. Please share a better idea if you have.
Vaibhav

Upgrade to 8.5.1 on all computers using shared variables?

Hello,
We are currently using DSC 8.5 and hosting the shared variable engine on one 'server' computer with other computers subscribing to network shared variables. Some of the computers are Windows XP and others are RT.
When we upgrade to DSC 8.5.1 can we just upgrade the 'server' computer and leave all other computers at 8.5 or do we need to upgrade all systems to 8.5.1 at the same time? Will computers running 8.5 be able to read/write network shared variables hosted on the DSC 8.5.1 system? Is such a progressive upgrade not advised? Thank you in advance.
- Chris White
ThinkG Consulting LLC

Hi Chris,
The variables pass through the Shared Variable Engine on your server before they are read by other subscribers. Therefore, you do not have to upgrade the DSC on all your subscribers. I hope that answers your question. For further reference on how network pulished shared variables operate as well as the role of the SVE, please refer to this link.
Ipshita C.
National Instruments
Applications Engineer

About the performance of using shared variables

Hi
My system has almost 200 I/O points,I want to the host communicates with PLC using OPC(I had create a OPC I/O Server).
If I use shared variables bind to OPC data items, there must be almost 200 shared variables.
And my puzzle is that what's the performance of using these shared variables,will they eat up memory and processor?
Another question is that if an shared variable will be time-lapse(i.e.,if an OPC data item value change feedback to program block with 0 delay or not),if so ,then how many millseconds it will delaies?

here is a paper about the performance of shared variables.
Steve Bird
Culverson Software - Elegant software that is a pleasure to use.
Culverson.com
Blog for (mostly LabVIEW) programmers: Tips And Tricks

Shared variables update in QuickClient but not in Variable Manager

Greetings,
I am using LV 8.6 Developer with the DSC module installed. Also, I'm using WinXP Service Pack 3.
I am trying to create a VI that will interact with an OPC server connected to some equipment.
I've created shared variables to read the value of tags in my OPC server exactly according to the instructions on this page:
http://zone.ni.com/devzone/cda/tut/p/id/6346
For some reason though, the value of the shared value never updates to reflect the value on the server.
When I follow the tag on the OPC QuickClient, it updates just fine, but somehow the value of the shared variable isn't updating.
I also tried retreiving this data using the DataSocket method in the "Multiple OPC Items Monitor.vi" example, but running that VI crashes LabView every time.
Also, I can create a shared variable and write to tags using a VI, but I can't read them.
It seems like I'm *this* close to getting the shared variables to work. Any suggestions?

Hi Rob,
To answer your questions...
DataSocket: NI doesn't give me the option to investigate the error when I restart the program. I did a search for .cpp files, but none turned up. I did collect the error report that Windows generates, and I attached it. I'm not sure if that would be something useful or not.
SharedVariables: I went to use the Distributed System Manager to take a look at the variables. For both my shared variable as well as the tags on the I/O data source, the data type is listed as "advanced" and the quality is "unknown bad." I collected the logs from the I/O server and attached them. Oddly, when I look through there, I see some Quality=BAD and some VT_ILLEGAL flags. The deadband is set to zero, and the update rate is 1,000ms.
Also, I've configured the DCOM and Local Security Policy settings as described in numerous support articles, and my Windows Firewall is turned off. The OPC server and LabView reside on the same computer.
I suppose the confusing thing is why the QuickClient works fine, but the SVE doesn't.
I've tried this on three different computers with the same results in all cases. Currently, LV and the OPC server is the only software installed on any machine. The only other OPC server I have access to is the LV server, and it seems to work okay.
Thanks,
Wes
Attachments:
a793_appcompat.txt ‏133 KB
screen.JPG ‏105 KB
OPC I-O Server Diagnostic Information.txt ‏69 KB

Shared Variables - Properties

Hello,
I have a couple of questions about properties of Shared Variables (SV). They can be configured either by using the dialog box (right-click on SV -> Properties), or programmatically via the "SharedVariableIO" property node. Have a look at the NI-Examples to see how SV can be created programmatically by using the DSC module.
1) The dialog box offers a property "Variable Type" which can be "Network-published", "Single-Process" or "Time-Triggered". The "SharedVariableIO" property node gives no access to this property. Does anybody know why? The same question can be rephrased: How can I create a "Single-Process" SV programmatically?
2) The "SharedVariableIO" property node has two items, "Network.OnScan" and "Network.ConnectionType", which have no direct correspondance to any of the properties available in the dialog box. What is the exact meaning of those properties? The on-line help isn't of much use here...
3) Mutliple SV can be edited by using the Multiple Variable Editor (Tools->Shared Varaible->Multiple Variable Editor). A cool feature is the import/export of CSV-files. I am interested in programmatically creating SVs from the information stored in such a CSV-file. Reading the file is no problem, but connecting the file entries to the properties of the "SharedVariableIO" property node would be quite some job. In principle NI has already solved that within the Multiple Variable Editor. I guess there is a bunch of VIs somewhere below "....Program Files/NI/.../vi.lib that do exactly this thing. Does anybody know, if these (which ones?) are available for public use?
Regards,
Dietrich

dietrich wrote:
1) The dialog box offers a property "Variable Type" which can be "Network-published", "Single-Process" or "Time-Triggered". The "SharedVariableIO" property node gives no access to this property. Does anybody know why? The same question can be rephrased: How can I create a "Single-Process" SV programmatically?
You can't. The single-process shared variable is an old style LabVIEW global placed under the shared variable abstraction. There is currently no way to create those on the fly. Time-triggered also cannot be created programmatically. I confess I don't know enough about them to know the reason why.
dietrich wrote:
2) The "SharedVariableIO" property node has two items, "Network.OnScan" and "Network.ConnectionType", which have no direct correspondance to any of the properties available in the dialog box. What is the exact meaning of those properties? The on-line help isn't of much use here...
Network.On Scan = read/write hardware? - this allows you to programmatically control when you are reading from or writing to configured hardware.
Network.ConnectionType = connect to hardware even when no one is viewing me? - this has 2 choices, UpFront & OnDemand, UpFront means that the variable will be connected to hardware even when no one is connected to the variable. This is useful when you want to reserve hardware or when logging is enabled. OnDemand means that you disconnect from hardware whenever no one is reading the variable.
dietrich wrote:
3) Mutliple SV can be edited by using the Multiple Variable Editor (Tools->Shared Varaible->Multiple Variable Editor). A cool feature is the import/export of CSV-files. I am interested in programmatically creating SVs from the information stored in such a CSV-file. Reading the file is no problem, but connecting the file entries to the properties of the "SharedVariableIO" property node would be quite some job. In principle NI has already solved that within the Multiple Variable Editor. I guess there is a bunch of VIs somewhere below "....Program Files/NI/.../vi.lib that do exactly this thing. Does anybody know, if these (which ones?) are available for public use?
I would advise against using any of these. National Instruments can change, or remove, those underlying files at any time as a result of modifications to the MVE. That could cause issues for any code using those sub-VIs.
Regards,
Robert

DSC: Importing Shared Variables results in error -1950679010

This post is more of an "FYI" rather than a question. I am using LV DSC 8.2.
ASIDE: I ran into this problem because I was trying to solve a related problem with Deploy Library.vi (I had dependant libraries that deployed manually, but not programmatically). Eventually, I found the answer in http://digital.ni.com/public.nsf/allkb/8EF71E1DDDC36C908625716900594B50 (not a very satisfactory answer however, IMHO).
During my solution search, another hint was provided with http://forums.ni.com/ni/board/message?board.id=170&message.id=177268&requireLogin=False which hints that using a "Network" defined SV, as opposed to a "Project" defined SV, could help. Then I discovered the difference between NetworkrojectPath and Network:URL in the SV property definition. Then I realized that NetworkrojectBinding is exposed in the Import fields (using the Multiple Variable Editor to do the importing of course), and this gave me the idea of being able to define a Network SV using the Import function.
Unfortunately, it does not work. In the csv file I set NetworkrojectBinding to FALSE, Network:UseBinding to TRUE and provide a valid Network SV path, but this results in error -1950679010: "Shared variable is bound but path or URL is not specified", which occurs when trying to save the LVLIB. It would be nice to know why this doesn't work, as I think it should. I think it is a problem with the SV import mechanism.
David Moerman
TruView Technology Integration Ltd.

Yes, you can do this programatically. I've attached a small chunk of example code (LV 8.5) that creates just 1 variable, as well as an image of it in case you are using an earlier version of LV. If you want to create multiple variables for the same library just put the "AddSharedVariableToLibrary" in a loop, of course.
David Moerman
Attachments:
Create Library and SV.PNG ‏48 KB
Create Library of SVs - BASIC.vi ‏24 KB

Modbus Ethernet read and write to a Eurotherm 6180XIO Modbus server using LV8.2 shared variables

I am having EXTREME difficulty trying to establish communications with a Modbus device using LV8.2 shared variables. The device is a Eurotherm 6180XIO Datalogger configured as a Modbus master. The PC and a cFP-1804 are slaves. All IP addresses are set correctly. This approach using shared variables would seem simple, but I can't find any examples or proper guidance on how to get it working. I am trying to avoid having to mess around with TCP/IP, OPC, or any other old-fashioned method.
I have read many threads on related topics but none directly apply to this situation. I have created a library containing a Modbus I/O server and shared variables bound to read and write holding registers. I have followed all recommended tips for creating such variables but I can neither read or write data. All data types are U16 due to Modbus protocol limitations. I have also applied the LV x10 factor in the most significant digit in the register offset (6 digits instead of 5).
I have a cFP-1804 on the same network which reads into the datalogger OK. The registers I use are 31000 (for CH0 on module 0, 31002 for CH1, etc) and the data can be read as FLOAT32. I have updated the firmwate on the 1804 to the latest level. I cannot even get shared variables to read SGL values. Using registers 301001 for CH0 and 301002 for CH1 I can only read U16 values, and not a 2-word SGL.
Third party Modbus simulation software is able to write to and read from registers very easily, but not LabVIEW.
Some questions are:
- do I use a Modbus master or slave as an I/O server in the library as a target for binding the shared variables?
- is there some other wierd translation in register offsets between LabVIEW and traditional Modbus?
- is this actually possible using shared variables or am I wasting my time?

Sending the whole 60-character string using a string or array would be the most efficient. I have tried both methods, and these only cause the datalogger to flag a message log but no text is displayed.
For a string variable, I have used the following binding "My Computer\Modbus Test.lvlib\ModbusServer6180\442305", where ModbusServer6180 is a Modbus I/O server configured with the logger IP address, and 42304 is the register offset at the start of the text block in the logger. I need to write to 30 consecutive registers starting with this one. I am not using buffering and have not enabled single writer.
Can anyone confirm whether this method should work in 8.2?
Does the string need a special termination character?

Using Shared Variables and Initialize Front Panel Binding (to PSP)

Hi,
I use LV DSC RT 8.2.1
I have a Vacuum System That includes signals from - Pumps failure, Valves status, Vacuum gauge, start Pump ...
Each signal is read by a FieldPoint.
All the relevant FieldPoint Channels are read by the Server (a computer in the Ethernet Network) and published to the network in the form of Shared Variables.
I have a client VI that is reading the Shared Variables published by the Server using Front Panel Binding.
Problem :
Some of the Bindings are in the mode 'Write &Read' and that causes some initialization problems.
For example - Valve #1 is Open, and then a User start running the Client VI, (the Valve #1 Status mode is 'Write & Read")
if in the VI the status of Valve #1 is closed (before running it) then the Valve status is changing to Closed.
I want the Client VI to first read the Physical status of the instrument and then to change the Value if the User changes it.
But that's seems to be a problem when using Front Panel Binding... (is it?)
I know I can Deploy a lvlib in the Client Side and Item Bind to the Shared Variables or Use DataSocket.
(Is DataSocket is a Reliable method when connecting to Shared Variables? What are the disadvantages when using DataSocket?)
What is recommended by those of you that are experienced or by NI ?
Sincerely Yours,
Amitai Abramson.

Amitai Abramson,
Hello and thanks for using the NI Forums.
I'm glad that you've read the Using the LabVIEW Shared Variable Tutorial on our website. Check out these other resources:
Network Variable Technical Overview
Troubleshooting Network-Published Shared Variables
Why Do I See Unexpected Value Change Events for Shared Variables Using LabVIEW DSC?
All You Need to Know About Shared Variables
Creating a Value Change Event for Shared Variables
Alternative Method for Using Shared Variables Between Systems in LabVIEW 8.x
What Is The Difference Between Using Shared Variables And DataSocket VIs To Access OPC Tags?
The issue that you are seeing by having "Write & Read" bound items on both the server and client side is essentially a race condition, you don't know which one is being read/written at what time. To resolve this issue I would take a look at some of the documents below.
Using a Local, Global, or Shared Variable in Parallel Loops Can Cause Race Conditions
Using Local and Global Variables Carefully
Tutorial: Local Variable, Global Variable, and Race Conditions
Locking a Shared Resource or Variable in LabVIEW Using Semaphores
You mentioned not wanting to have two sets of shared variables (one on each side), but this is a great method to resovle this issue, that, or you can develope some sort of hand shaking to prevent these race conditions.
I would suggest that in the future when using these forums you try to ask only one question per thread and make it more concise. It's hard to tackle multiple questions and such broad questions as "I want to know all the ways that I can connect to Shared Variables, and I want to know the advantages and disadvantages." I suggest this because we want you to get your questions answered and more concise questions will result in quicker and better answers.
Message Edited by Ben S on 10-01-2009 06:05 PM
Ben Sisney
FlexRIO V&V Engineer
National Instruments

Serializing read/write operations of network-published shared variables

Hi all,
I'm developing a distributed application (PC + CompactRIO), and using shared variables (SVs) for inter-device communication. Here's my journey so far:
Intended procedure
PC parses file
PC writes processed file data (custom cluster, large) into the 1st SV
PC writes a "grab data" signal/command (enum) into the 2nd SV
cRIO polls the 2nd SV
cRIO sees the command, then reacts by reading the 1st SV
Steps #2 and #3 were sequenced, using error wires.
Unexpected results
Even after the command is transmitted and the cRIO sees it, the cRIO could not read the data (which I wrote BEFORE the command) -- LabVIEW reported that the buffer was empty.
The operation succeeded when I placed a wait (5 seconds) between steps #2 and #3.
Questions
What am I doing wrong, and how do I achieve my desired outcome?
Is SV I/O asynchronous by design?
Is it possible to use event-driven programming to handle SV access? (i.e. does LabVIEW signal when the new SV value has propagated across the network?)

BillMe wrote:
Why do you have to "notify" the other end that data is available? The subscriber can simply sit in a loop doing a timed read just as if using a queue or notifier IPC. If it doesn't time out, you got new data. If it does time out, do some other processing if needed and then loop back for another timed read.
My sytem architecture is command-driven -- the cRIO listens for instructions from the PC interface (sent as an enum via one SV), and performs tasks (motor control) in response. One of the commands (the one described in this thread) happens to be "download a new motion profile from the PC, by reading the other SV". Given that the cRIO is already polling the command channel, I felt that there's no need to also poll the data channel (especially since the "download" command is issued very infrequenty). Plus, I thought that polling two channels would increase the chances of race conditions or illegal state transitions, particularly if the app is developed over a long term.
I am also new to LabVIEW, so my current programming style will heavily reflect my C++/Qt background while I get a feel for LabVIEW's strengths and weaknesses -- Qt is a heavily event-driven framework (even for networking!), where polling often means you're doing something wrong.
Still, thank you for pointing out that I can use timeouts to determine if new data has arrived -- my subscriber current writes a null command back into the SV when it has consumed the command, but you showed me that I don't have to.

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