Thanks a lot for the cantcliff's comments. It is very helpful.
The time it takes to fill up SD depends on three figures: SD capacity, bytes used to write each line and write interval. If your interval is 1 seconds, and data of 500 bytes each line, it will not take many years to fill full.
I feel that reading SDW59 is a good solution. For instance, if SDW59 reads 10, so only 5120 bytes remaining, then I may use this to create a bit to show an alert on HMI also stopped from writing data to SD.
But I am just curious to know how the SD write function block was defined when SD is full. I believe the function shall check this every time trying to write to SD. I just could not find more information on manual to indicate what program will behave when SD is full. Must I use controls such as reading SDW59 or other method to monitor SD space on every program scan? I also looked the SD examples and watched webinars. I don't not feel clear about this.
Simply my questions are:
1) What does the block or program do if SD is full during scan?
2) Do I have to monitor the remaining space on SD card? Or is it also safe without?
I am sure Unitronics R&D team would be able to provide more information on this?
I concern about this as I have seen many applications are linked to safety control system. In this case, cause and effect can be crucial for the risk involved.