Shared variable vs. data binding

Hi,
I try to write a program which communicates motor drives through ethernet and I use OPC server and shared variables created using PLC. I created front panel indicators in a while loop with 50 ms cycle to read motor encoder position, status etc. When I try to do this with wiring shared variable to indicator and with changing the data binding property of the indicator browsing the same variable from the project library, data binding seems to respond slower than wiring the shared variable to indicator directly. Can anyone explain why?

Hi mhn05,
I'm not exactly sure why it would be operating slower. Do you have code that you can upload that benchmarks or tests the speed of these, side by side?
Thanks,
Dave T.
National Instruments
FlexRIO & R-Series Product Support Engineer

Similar Messages

Shared variable, missing data, the same timestamp for two consecutively data

hello
I have a problem with missing data when I read from a network published shared variable.
Host VI:
In a host VI on my Laptop (HP with WinXP Prof.) i'm writing data to the shared Variable "data". Between two consecutively write operations is a minimum of Milliseconds waiting time. I use that because I want to be sure that the timestamp for each new data value is different then the preview one (resolution shared variables is 1 ms)
Target VI:
the Target VI on a cRIO-9012 realtime device is reading only new data in the way that it compares the timestamp of a new value with the timestamp from the last value.
Problem:
rarely, I'm missing a datapoint (sometimes everything works fine for several hours, transferring thousands of data correctly before suddenly the failure happens). With some workaround I'm able to catch the missing data. I've discovered that the missing data has the exactly same timestamp then the last readed datapoint, therefore is ignored in my "legal" data.
To sum up, the missed value is written to the shared variable in the host, but the target ignores him because his timestamp is wrong, respectively the same as the last value has, despite the host waits every time for a minimum of 10 milliseconds before writing a new value.
Note:
The shared Variable is hosted on the Laptop and configured using buffering.
The example is simplified only to show the principle function, in real I use also a handshaking and I secure that there is no over- and underflow.
Simplified Example:
Question:
Has someone an idea why two consecutively data can have the same timestamp ?
Where is the (wrong) timestamp finally coming from (system?) ?
What would be a possible solution (at the moment with shared Variables) ?
-> I tried a workaround with clusters where each data gets a unique ID. It works but it is slower than comparing timestamps and I could get performance problems.
Would it change something when I host the shared Variable on the RT-System ?
Thanks for your help
Regards
Reto
Solved!
Go to Solution.

Hi Reto,
I had a look on your modified Example.
Because the Shared Variables didn`t work like Queues or Notifiers (No Event or Interrupt when a new value has been written. And for sure the´re not possible over a network) you will see the issue, that the code is reading the values more often with the same timestamp (Polling Problematic) if the reader is faster then the writer. And because the timestamp is written with the value you´re able to program like you do. Filter out whats duplicated when you have the same timestamp.
Everything is described in here:
http://zone.ni.com/devzone/cda/tut/p/id/4679#toc1
Laurent talked about a second depth of buffer. Please have also a look at the link. Somewhere in the middle of the tutorial you see the explanations of Buffer and RT-Buffer.
Regarding your question: Would it change something when I host the shared Variable on the RT-System? --> No
In my experiences, you should consider to place the Shared Variable Engine after asking some questions regarding the application.
You will find the Answers to this 3 Questions also in the link:
Does the application require datalogging and supervisory functionality?
Does the computing device have adequate processor and memory resources?
Which system is always online?
And you`re right the smalles time interval you can see in the timestamp is 1ms.!
What you also can do is working with an enabled "timed out". This might be more performance efficient than reading the timestamp.
What I don`t know and not find up to now, is if LabVIEW or the OS adds the timestamp. It´s taken from the system time, this looks like LabVIEW is taking the value and adds it.
I hope this helps
Alex
NI Switzerland

Using TCP or shared variable for data transfer

I am trying to send a large amount of numbers from a real-time module to a host computer. These numbers have been arranged into a large array, such as an array with 10s of thousands of points. The time critical portion of getting the information has already been done, so the data transfer back to the host VI is not time critical. I know I will need to break the large array down into smaller arrays and then reform the large array after all the information has been sent. I know how to use both TCP and shared variables with FIFO. What I am unsure of is which one is better to use for this application. I do not know what the maximum size arrays I can send through either.
Also, from what I have gathered from using LabView is that the sender has to be listening for a connection before the client opens a connection, or else it will throw an error. When I tried breaking it down into 50 points, if i did not wait long enough in the host VI or if I did not put a long enough wait function in the RT loop, and error would throw, so it would take a long time to transfer the data when it worked properly.
Any help or suggestions is appreciated, thanks.

Regarding the array size question, there is no real limit (other then the amount of memory in your system) to the size of data that you can transfer in a single block using either TCP or the Shared Variable. In your case you can easily transfer an array with 10's of thousands of data points in a single write operation. Both TCP and the Shared Variable will automatically handle breaking up the data for the maximum packet size on Ethernet and then reconstitute the array on the receiving end. In LabVIEW you will simply get back the array as a whole without needing to worrying about how the data is broken into smaller packets on the Ethernet.
I tested the attached example which transfers 400kB per block (50000 Doubles) without any problems. You do need to have the Server (in this case RT) running first before the client (Windows) can connect.
Message Edited by Christian L on 02-09-2007 11:34 AM
Christian Loew, CLA
Principal Systems Engineer, National Instruments
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Attachments:
TCP.JPG ‏44 KB

Use of Shared Variables for data transfer from a RT traget to a desktop application

Hi,
I want to adopt the shared variables to share data in an existing distributed application comparable with the T3 Benchmark configuration in this document:
[1] http://zone.ni.com/devzone/conceptd.nsf/webmain/5B4C3CC1B2AD10BA862570F2007569EF
The current implementation uses a well tuned solution with RT FIFOs and TCP/IP communication with a desktop-PC which monitors and stores the data.
The desktop application runs with a much slower execution rate than the TCL. So the TCP/IP packets are used to buffer the data with a package size dependent on the data send rate.
To use the shared variables instead of this RT FIFO + TCP/IP implementation I need to read the shared vairable buffer at once each time the desktop application reads the shared variable.
But according to this quotation from [1]:
"With buffering, you can account for temporary fluctuations between read/write rates of a variable. Readers that occasionally read a variable slower than the writer can miss some updates."
this seems to be impossible. Am I right?
Are there any suggestions to circumvent this problem? Or are shared variables not made to share data between unsynchronized processes without data loss?
Regards Till

You can use the error-cluster to detect the end of your queue. The erroroutput of the shared variable will return a -2220 Warning, if it reads a value it has read already before. I attached a modified example and saved it for 8.2 I hope you can open it.
Attachments:
readbuffered82.zip ‏51 KB

Binding Shared Variable to Source

Dear all,
I'm currently working on a project that requires the use of shared variables. The shared variables will be network published and programmatically created using properties nodes. However I encounter one error and one problem in the process of doing so.
1) I programmatically bind the shared variable directly to the fieldpoint channel (data item) by writing the URL \\10.0.0.1\FP\1AIO\00 when i create the shared variable. The shared variable can be created successfully but when i deploy it, it generates an error which seems to tell me "it is out of range". Can anyone help me to rectify this problem?
2) The 2nd question will be, as my fieldpoint is connected to a DHCP server. At the start when i create the shared variable programmatically, i could bind the shared variable directly to the data item by writing the URL to its property. However, during the process of deployment, the dynamic IP address assigned to my fieldpoint may change . How can I ensure that the shared variable continue to bind to the right fieldpoint channels respectively? even though the IP address changes?
- Or is there any method that I could view all the network data items in a table? a list? or an array? so that I can link them to the URL.
I tried using the data socket URL.vi, I can only select the data item one by one.. this will be tedious for 100 or 200 I/O points..
Will greatly appreciate for any advice and help. Thanks in advance.

Try this.
Give your fieldpoint controller a computer name through MAX. Then instead of referencing it by 10.0.0.1 in the URL's of the shared variables reference it by computer name. So \\MyFPmachine\FP\1AIO\00 instead of \\10.0.0.1\FP\1AIO\00.
Dynamic IP's can definitely cause headaches, and it would be best to use static IP's if possible, but doing this method should help. It seemed to work for me. I found when you browse for the channel in LV, it wants to give it the IP address by default, but it was possible to edit the string and paste in the computer name.
Message Edited by Ravens Fan on 01-10-2007 11:01 AM

Shared variable architecture for distributed crio system

I have a distributed system consisting of 20 cRIOs and one central server running Windows XP. Each cRIO is a mix and match of AI, AO, DO, DI and TC modules. There are around 200 AI and TC channels and close to 100 AO, DO and DI channels. I need to acquire data from all the AI & TC channels at 10 Samples/sec and log them in the Server. In some cRIOs I have 2 to 3 PID loops, in addition to the data acquisition or generation code. Since a cRIO chassis can have only eight modules, certain PID loops will have to use data of channels that are available in other cRIOs. cRIOs can be turned ON and will be running even without the control server, but facility and test will be started only through the control server.
We have decide to use LabVIEW DSC for this application and shared variable for data transfer, logging and alarm. We are using the following SV architecture.
1. Each cRIO will have its own RT shared variables for AI, AO, DI, DO and TC parameters
2. Each AI and AO parameter shared variable in RT will have its corresponding host shared variable which will be bound to the RT Shared variable. In host shared variable we are enabling logging and alarm option for variables corresponding AI parameters.
3. In cRIO RT, data is read from the FPGA (using FIFO every 500ms, and we get 5 Samples for each channel) in binary format and scaling is done. After scaling we are updating the corresponding AI RT shared variables using DataSocket write in a loop. In any cRIO there will be only a maximum of 32 AI or TC parameters.
4. The server has a mimic panel where we need to display the status of DIs and allow the user to turn ON and OFF DOs. For the we decided to use Front panel DataSocket binding.
I would like to know whether there is any better shared variable architecture that we can follow. Considering the number of variable that we are using, will the current architecture cause any performance issues?
Thanks!
"A VI inside a Class is worth hundreds in the bush"
യവന്‍ പുലിയാണു കേട്ടാ!!!

Hi Mike,
Looks like you are lucky as you can make use of the great feature available in 8.6. Over here I have 8.5.1 so got to do some of the things the 'hard' way But I must say, 8.5.1 is the so far the best version of LabVIEW I have worked after 7.1 as the crashes are less and less frequent.
BTW I faced some problems directly dragging and dropping an RT SV node to Windows. If the RT code is running and if you try to run the Windows code, it was asking to stop the code running in RT. That's one reason I had to duplicate some RT Shared variable in host and bind them to corresponding RT Shared variables.
This is the first time I am handling this many number of shared variables. In case I come across some issues, I will post them here.
"A VI inside a Class is worth hundreds in the bush"
യവന്‍ പുലിയാണു കേട്ടാ!!!

Using Modbus TCP I/O Server with new DSC Shared Variables in LabVIEW 8.6

Hello,
I'm using LabVIEW 8.6 and want to communicate with a Beckhoff BK9000 Ethernet TCP/IP Bus Coupler via Modbus TCP. Instead of using the NI Modbus Library, I've tried the new LabVIEW 8.6 feature "DSC Shared Variables" as described at the bottom of this page: Latest NI LabVIEW DSC Module Features and Demos. Reading of analog input bus terminals works fine. However, I haven't figured out yet how to write on an anolog output bus terminal with these shared variables.
It's about a 16 bit analog output and I need to write to the registers 0x1121 and 0x0801. It works with the NI Modbus Library (just using function code 6 and choosing the registers), but on the other hand I don't know which shared variables I have to choose for these registers. I've tried several data items (e.g. 400001 upwards as well as 402049 for 0x0801) but none of them worked. I would be glad for a short explanation - thanks in advance for your support!
Regards
utechle

The Beckhoff documentation says, that holding registers start with 0x0800. I've checked this by using the NI Modbus Library. I used the "MB Ethernet Master Query.vi" togehter with the function code 6 for "Write Single Registers", changed the settings of the starting address to hexadecimal view and entered 801 (since it starts with a control byte in 0x0800 and the data out word follows in 0x0801). Furthermore, I had to address register 0x1121 in the same way for resetting the watchdog. As I've mentioned in my first post, this method works fine. However, I haven't found out yet which shared variables i have to use for accessing these registers.
On the other hand, it's no problem to read data from analog input bus terminals using shared variables. They start with 0x0000 (status byte) and 0x0001 (data in word) and I can read data with the shared variable and data item 300001, respectively.
Message Edited by utechle on 01-27-2009 11:12 PM

How Can i retain the Shared Variable Values after PC rebooting

Hi all,
I am facing a paculiar problem with Shared Variables. I have an application in which shared variables are used for data communication.
One of the application requirement is to retain the variable values eventhough PC is rebooted or started after crashing.
As per the my understanding, the variable values will retain eventhough the PC is rebooted. But here i can observe a paculiar problem like some library variables are retaing the values while some others not. I enabled logging for all the variables.
I tried many ways. like logging enabled, logging disabled, changing variable names, changing process names etc... But i am not getting a consistent behaviour.
I hope some you can help me in solving this issue.. "How Can i retain the Shared Variable Values after PC rebooting"
Thanks and Regards,
Mir

Hi Blackperl,
Thanks for the post and I hope your well.
What do you mean by not getting consistent behaviour.. this will all depend on excatly when the crash happens i.e. before the write or after.
Surely a better method would be to log the data to a file during the reboot...
I beleived the value read back
will be the default value for the shared variable's data type.
The LabVIEW DSC 8.0 module adds more functionality to the shared variable, including initial values and alarms.
If you enable an initial value on a shared variable, when the variable
engine comes back on-line it will default to this value. Setting a bad
status alarm for the shared variable is also a good way of handling
this type of event. Additionally, if you are using a LabVIEW Real-Time
target such as Compact RIO or Compact FieldPoint, it is appropriate to
consider hosting the shared variable engine on the real-time target.
These devices have watch-dog capabilities and are typically the
hardware controlling the critical pieces of an application. Most
Windows or PC-based targets do not have these fail-safes.
I guess, if you could explain to me again that would be great. From my point of view, if I have a cRIO and a Windows PC. If the windows PC crashes, the cRIO will still update its shared variables. Then once the PC has started up its own shared variable engine, and the bindings are loaded, it will once again continue to update its copies of the variables.
Please let me know what you think,
Kind Regards,
James.
Kind Regards
James Hillman
Applications Engineer 2008 to 2009 National Instruments UK & Ireland
Loughborough University UK - 2006 to 2011
Remember Kudos those who help!

Shared Variable Crashing LabVIEW

This is just one of those projects that has a brick wall at every turn so far. Two computers running LV2009. Computer A hosts Data Write Shared Variable. Computer B hosts Data Read Shared Variable. Data Read is bound to Data Write. Everything was working fine, and then late yesterday afternoon, I don't think I had changed anything on the shared variable, I start up my VI on Computer B, and LV crashes. The VI starts to run until the first execution of Data Read, and then poof, everything open in LV dissapears. The problem is one hundred percent repeatable since then.
Here is what I know:
1. An error indicator hooked to Data Read shows -1950679034 (Shared Variable has no value) for about a second before the crash. Everything stops executing as soon as the warning appears.
2. The Data Write is single writer, but it is also occasionally read in the VI that it is written in. It is not being read when the crash happens. I can however perform the read on Computer A without incident.
3. Not sure if this is related, but if I open the VI on Computer B, and then open the project, I am not able to drag Data Read from the project onto the block diagram. I also cannot perfrom any operations on the Shared Variable (like right click, ignore timestamp). I have to close the VI and reopen it from the project. Thinking back, this problem may have started when I opened the VI without opening the project.
I am torn between abandoning Shared Variables and trying to figure this out. I would definetely prefer to use the Shared Variable, especially because I need some buffering. I'm guessing I can buffer things some other way. Not sure if I can remotely access a queue, or ... ?
Solved!
Go to Solution.

I looked at STM, but I think the Data Socket VI's may be a better fit for me. It seems like Data Socket and shared variables are sort of intertwined, but I'm hoping DS doesn't suck as much as the SV seems to so far.
I think NIC means network interface card? Anyway, I don't really know how many I have. The Data Writer is on a PXI8108 with Windows, the reader is an old crappy Dell that I inherited with the desk.
I'm also not sure about the logos.ini file you referred to.
My intention is to transmit an array of double waveform that is about 15 - 20 channels acquired at 100 - 1000 Hz. I think the Data Socket looks like the simplest way to do this, and I could live with 100 Hz if speed is a problem, but I sort of don't think it will be. I'll try tomorrow unless somebody has a better suggestion.
By the way, I saw in another post a similar problem with Shared Variables that is supposedly fixed with the new patch for 2009, but I'm not sure I trust SV's because this will, for about thirty seconds of its lifetime, be one of the most critcal applications I've written, so I want confidence that it will work for those thirty seconds.

Unable to read shared variable in signal express

Dear all,
I am trying to read a shared variable with data type 'Array of Double Waveform' and the signal express gives a network variable error stating
"The data type of specified shared variable xxxxxx is not supported".
could any one explain why is this not supported or suggest a work around.
Munir.
Attachments:
Untitled-1.jpg ‏66 KB

Hi Munir,
the datatype you want to use is not supported in Signal Express. So if you choose this datatype you always get this error massage. As a work around I can recomment you to take the Double Waveform datatype this is supported, create a shared variable for each array element (waveform) and read this variable in Signal Express. This should help you out.
If you got any questions feel free to ask me.
Regards
TomBaum

Shared Variable and classes

Hi friends,
1.can you tell me the Disadvantages of Shared Variables?
2.What is classes ,how to make it?
Raj

Hi! Raj
Shared Variables
shared variables share data between loops on a single diagram or between VIs across the network. For doing that they break the LabVIEW dataflow paradigm.
Disadvantages:
1. Using variables allows for race conditions and incurs more overhead than passing the data by wire(break the LabVIEW dataflow paradigm).
Race conditions happen when you, for example, write data to a shared resource from multiple operations faster than you read data from the same shared resource.
2.You must have a project open to create a shared variable
You can also read more about shared variables at http://zone.ni.com/devzone/cda/tut/p/id/4679
Attachments:
Race condition.PNG ‏57 KB

Problema di polimorfismo con Read/Write delle Shared variable

Salve,
Cercando di modificare l'esempio di progetto "Shared Variable Client/Server" in un progetto dove sono i diversi client a mandare dati al Server ( ovvero l'esatto contrario di quello che fa )
mi sono inbattuto nel problema illustrato nelle due figure che seguono.
Ovvero, sostituendo "Writable PSP Variable" ad "Readable" nella scheda "Connect"(FIGURA 1) mi da un errore nella scheda "Read/Write" (FIGURA 2), ovviamente anche qui ho sostituito a "Read Variable" un "Write Variable".
Mentre utilizzando il Data Socket funziona, ma non mi sono chiari i vantaggi e gli svantaggi nell'utilizzare quest'approccio.
Qual'è la strada migliore tra Shared Variable e Data Socket per creare un sistema Client/Server così costituito, ovvero con un cospicuo numero di client da servire ( destinato a crescere )che mandano simultaneamente un discreto quantitativo di dati ( circa quelli di una seriale a 115200bps ) ad un server
FIGURA 1
IMMAGINE 2

Translation
Hi,
Trying
to modify the sample project "Shared Variable Client / Server" in a
project where different clients to send data to the server (ie the
exact opposite of what it does)
I untapped in the problem illustrated in the two figures that follow.
That
is, replacing "Writable PSP Variable" to "readable" in "Connect"
(Figure 1) gives me an error in "Read / Write" (FIGURE 2), and of
course here I replaced "Read Variable" a "Write Variable.
While using the Data Socket works, but I have clear advantages and disadvantages of using this approach.
What
is the best route between Shared Variable and Data Socket to create a
client / server system so constituted, or with a large number of
customers to serve (to grow) that simultaneously sends a fair amount of
data (approximately those of a serial to 115200bps) to a server
The images points to a gmail location. It is not properly attached.

Shared Variable Polling Rate

Hi all,
I use the NI-DSM on the windows machine running an LV application that shares variables on the network.
In the NI-DSM you can setup the update rate and I can see that the rate changes as expected.
BUT, on my IPad I still have only a rate of 200ms thus 5 Values per second.
Bastian

NI_MikeB wrote:
Hi
You cannot change the rate of update for the shared variable on Data Dashboard. You can alter it if use Poll Web Service but it will still only go to 300ms. Please bear in mind, as a remote device, that you typically want to limit update rate. And, as of version 2 you can send arrays of values and supply multiple values at once.
Mike
As a remote device you typically want to limit update rate? Why?
Here is my idea. The company I am working for is producing position sensor integrated circuits. I build LV executables to access the ICs RAM, and reading the sensor data via an serial interface. This should be done at least at a data rate of 25 values per second. So if you want to show someone how our sensor works it should be faster than the eye can recognize value changes (smooth updates).
Don't get me wrong. The Dash Board App is great. But I want more.
I like the idea to have just one PC running, measuring the data via an USB device and sending the data via WLAN to several IPADs. Thumb up!
Bastian

Problem Sharing Variables between 2 machines

I am having problems trying to create a project that would share variables between multiple computers. I am using Labview 8.2.1 (I am new to 8.2.1 and Shared Variable).
here is what i did:
1. I created the project with Client and Server VI's and a library containing shared variables, which both of them call
2. I placed this project onto 2 computers (XP on a local workgroup, no firewall, see each other).
3. On one of the computers I bound all shared variables to their counterparts on the other computer
The system is "almost working"
The problems are:
- I initially used wrong type variables for few items. I then modified the variables in both VI's and Shared Variable libraries and repeated the Steps 1-3 above.
But the sysrtem has some sort of a buffer somwhere, because it keeps bringing the old types of shared variables back (when I monitor them using Shared Variable Monitor).
- When I do binding to source, the system shows wrong variable types (for example DBL instead of U8).
- It takes almost a second to write and read shared variable (my network is quite fast)
I saw that the Labview Example with Server and multiple Clients uses 2 sets of shared variable and they bind - separate sets (different names) fro client and server within the same project. Do I have to do this or is this done just to simplify bindinga nd future distribution?
Is there a buffer/setup/file that I need to clear to get rid of "old" shared variable assignements? (I tried undeploy/deploy - does not help...)
Thank you very much,
Gleb.

Hi Gleb,
It sounds like the shared variable is not getting completely undeployed from the shared variable engine. I would recommend opening the shared variable manager and undeploying the library from there just to see that it is no longer in the engine. Then, redeploy the library and rebind your variables to the host library that you have deployed.
It is not necessary to have different sets of variables for client and host. The important part of the variable is what it is bound to, not necessarily what it is named. So just undeploy your variable, double check the settings of the host variable, then redeploy using the variable manager and rebind your client variables.
I hope this helps,
Brian Coalson
Software Engineer
National Instruments

Binding a shared variable to a NI-PSP data object does not work

Hi,
I want to share data between a RT-target and one or more hosts (LV 8.6.1). The network shared variables are deployed to the RT-Target. According to NI accessing shared variables from another project or host has to be done by defining a shared variable on the host and aliasing it to the NI-PSP data object on the target.
I did that and the host shared variable generated an error (0x8BBB0011) during runtime.
Next I aliased to a shared variable deployed on the host from another project. This did work.
Another thing I tried was to bind the variable from the RT-target to a display element:
This is working !!! And as you can see the path of the NI-PSP data object is exactly the same ! So what is the difference between binding a data object to a shared variable and to a display element?
Is there a bug in the SVE or am I missing something here?
The host project:
The publisher VI
Hope, someone has an answer.
Regards
Matthias Quade
Solved!
Go to Solution.
Attachments:
AliasTestWrite-RT.vi ‏8 KB
AliasTestConsumer.vi ‏8 KB

Dear Mr. Quade,
thank you for posting at the National Instruments Forum. There is a known issue with the path of the bound variable with LabVIEW 8.6.1
Please download the patch for LabVIEW 8.6.1, it should solve your problem:
http://joule.ni.com/nidu/cds/view/p/id/1255/lang/de
Best regards from Munich
MarianO

Shared variable vs. data binding

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