Since you want to be able to read in alarms based on a certian status; active, acknowledged, all, etc. It would probably be best to creat and utilize a data base (ACCESS, SQL, or other). Since I have little experience with databases, I'll try to explain how to simply use a text file.
The "Write to Spreadsheet File" vi accepts a 2D wave and writes it to a file, which is convenient as the data from your multi-column list box is a 2D wave. Writing to file. "updating" is simply a matter of overwriting the file... check
"Read From Spreadsheet File" will be used to load data from the file... Check.
Since you will be reading in the entire file, you must use code to determine which values to wire to the multi-column list box (say for active alarms only). You will have set up the file so you will know which columns are which. Following your example image, column 2 is Time Acknowledged (column numbers start at zero). So, to get the acknowledged files only, used "Index Array" to extract column 2 (this gives a 1D array of 'Time Acknowledged'). Use a for loop to run through this array. If the entry has a string length greater than zero, it has been acknowledged and we should add the appropriate row to the multi-column list box (only add those with a length of zero if you want NOT Acknowledged).
Do you see the thought process here? Of course, this requires a lot of array manipulation and string evaluations to determine which rows of your file are to be displayed on the multi-column list box. The benefit of databases here is that you can directly query the database for subsets of information and it can be accessed from remote compters easily (but again I've little experience with this).
Just using a file like this, the actual file type doesn't really matter. What matters is how the data is stored within the file (i normally use txt so I can view it manually to check for errors). If you don't want end users to be able to access the file, switch to a data type not easily opened on the PC once the code functions as desired.
I forgot to ask before, how will your alarms be detected? Is this something you must do in the LabVIEW code? If so you may consider a queued state machine http://expressionflow.com/2007/10/01/labview-queued-state-machine-architecture/. The state machine will function as normal with a parallel subvi running which will probe for alarms and, if a new alarm is detected, trigger the state machine to add the new alarm to the file.
