LabVIEW
How to make a value non-volatile?
@David99999 wrote:
What I'm noticing is that when I close out the remote client app, the value is no longer what the user had set it to when they last had it open.
How exactly do you know what the "values" is and what else has access to that value? (for example do you notice the value changed when the client disconnects or only once the client reconnects? Could it be the client has some initialization code that reset is on reconnect?)
How do the two side communicate values? There are so many possibilities!
To make it persistent on the cRIO, you need to write it to e.g. a local ini file that gets read on startup. (with proper default handling if the file is missing)
Your comments are a bit unclear but if I understand correctly what might happen is that your TCP Server tries to receive values from the client and if the client is connected and sends those values all is peachy. But if that client dies or has some other communication error you do error out but still overwrite the setting with an empty string. You have of course to make sure that you only overwrite that setting (global variable, FGV or some tag engine value) when you have successfully received a value from the client.
Otherwise you need to do some error indication. That could be to set a different variable to indicate that the communication has been lost and your cRIO device now operates in disconnected mode, with probably the last valid received value. But don't update the value itself unless your system has some security implications that could cause loss of resources or even worse human health if it continues to operate while disconneced.
As to make your value persistent when your application or controller restarts: Whenever you receive a valid value from the client, write the value also into your ini file. On starting you can read that value back and store a default value if it doesn't yet exist in the ini file.
If you create a FGV you can put this all nicely into this VI. Have an "Init" case that reads the value from the ini file, a "Write" case that updates the value when you receive a valid one from the client and also stores it in the ini file and a "Read" case that you can use to read the current value for use anywhere in your application. You can easily make the write to the ini file inside the FGV conditional by detecting if the new incoming value is different to the already stored value and only write to the ini file if there is a change between the two.
One caveat about FGVs, try to avoid a Read-Modify-Write sequence at all costs. This is a very easy way to introduce race conditions. You should never do a modify of the stored value by first reading it, do something with it and then writing it back, but always integrate the modification into the FGV itself as a separate case method.
