PLC Going to Offline Mode

Check for power on protocol. I suspect that your power glitch is telling the PLC to start fresh at whatever protocol setting is set. In this case, offline.

Agreed.

Some servers have power supply ends that are very sensitive to power changes. In my experience installing backup power for servers I have often had to abandon the use of UPS as the bridge between the different power sources for the server. Xantrex inverters have proved invaluable. The reason is that while the UPS depends on fast switching time to achives uninterruptible power supply, sine wave Xantrex or Outback inverters hold both power sources in real time. Power outage translates to a mere drop off of one of the sources. No fast switching is required for sustainability and no interruption occurs in the power supply end of the server. We have used this design to achieve a permanent solution to the re-boot problem that sometimes results from power interruption/auto-changeover in servers. Rev. Ukaegbu Ogwo (Solartime Electric Ltd., Nigeria)

In order to avoid a momentary loss of power to the PLC the PLC backplane really needs to be plugged directly into the UPS if at all possible.

Also;

Access the PLC processor fault table, review the fault codes, record all faults, and identify the cause of failure. (Example: The PLC may be going off line due to "Loss of comunications.)

Correct the root-cause of all minor and major fault(s) .

Be sure to reset/clear all Processor and I/O faults after you note/record and correct them so that the fault register is purged/cleaned.

REASON: The PLC fault register is normally not a FIFO due to being critical to system operational safety.The register has a finite quantity of memory and will not delete any critical or major faults therefore it gets "stuffed",and once the register is full it will go into overflow anytime a new error is experienced, and will shut the machine down (go offline).

Check the configuration of your "S" bit table assignment and edit accordingly to fit your operating philosophy and operating conditions. (see programming instruction set for explanation)

Consider using the "flash" memory option to store your program.

Keep in mind that once the "flash" memory option is invoked, any future programming editing will require a total erasure (dump) of the existing "flashed" program before loading any changes/edits.

Failure to follow this simple procedure will cause the PLC to revert back to the original "flashed" program and will not save any new changes.

What exactly do you mean by a power change over?

If your SCADA PC is collecting data and the file becomes too large, it may cause communication failures. Make sure you don't have excessively large data files being appended on the PC. If so, you may need to archive that file (by changing its name) and begin again.