Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01 3.00

Question: If a power failure occurs with the trending and other logging files become corrupted?

The trend server has been designed in such a way that a power failure will not cause corruption of the trending files. This can be achieved as the trend server has full control of the files it is accessing. You will however loose the current trend data which is stored in memory buffers. These buffers default to 1024 bytes each, so a 2 second trend would be a bit over 4 minutes of data. The size of this buffer is set by the parameter [trend]TrendBufSize, so you could reduce this buffer size. This is not recommended as it reduces the performance of the trend server. The data which is lost can be re-synchronised from a second trend server so that all data will be recovered. For this to work then the second trend server should be on a separate power supply.

When Citect is logging alarm data to DBF or ODBC databases it has less control of the file access. In these cases it is possible that the file could become corrupted. Corruption of disk files can easily occur during a power failure when you are using the DOS FAT file format. You can also get very bad corruption if you are also using disk compression and write caching with SMARTDRIVE. If want to avoid this problem you should avoid using disk compression and write behind caching. When you disabling write behind caching this will degrade the performance of your system.

You can work around these problems by using a more advanced file format for example those used with file servers like Novell Netware, Windows NT and others. This file servers offer features which track changes to the file system and in the case of a power failure can recover the file system to a known state. For the absolute best protection you should use Novell SFT III with redundant file servers and a backup UPS.

Integration with UPS

The best solution to a power failure is to use a UPS which supports an interface to the Citect PC. These UPS will provide back up power for several minutes to hours depending on the battery size. When the battery is about to fail they can notify the PC via s special interface card or serial port than a power failure is about to occur. You can develop a cicode function which will check for this power fail signal from the UPS and then shutdown Citect so that no files are corrupted.