Hello Malcolmt
Sorry for replying late as I was busy for another project. Both 6&7 switches are ON but i did not understand that why PLC memory failed and it loaded the program from EEPROM as there is no error message received about low battery.
The memory corruption that caused the load from EEPROM would have wiped away any error messages, and there are two main causes aside from low battery:
1) Incoming power problem. Electrical noise from high frequency devices, voltage fluctuations, poor grounding and lightning are common sources.
2) Early sign of power supply failure.
There are probably other possible explanations, but those are two that I have experienced.
The use of EEPROM backup can hamper your ability to perform run time edits...Any time there is a mismatch and you have the settings enabled to Load On Memory Error, you run the risk of this happening. So, either defeat the EEPROM, and rely on a PC with RSLogix5 to recover if the fault returns, or plan scheduled burning of the EEPROM.
I have seen voltage level disturbances cause this to happen, if there is a brownout for just a few cycles. We used an expensive constant voltage transformer to supply a steady 120vac to all the PLC-5s I supported and I only saw one power supply failure among some 2 dozen of them. It was ten years old and it wiped itself twice about 30 days between occurrences, and then another month later, no sign of life from the PS.
As for managing EEPROMs and maintaining online editing power: I have the same issue with a bunch of SLCs, and what I do, is put in the rung comment for LAD2 rung 0:"2012-MM-DD: PC : NOT BURNED TO EEPROM!" followed by a list of descriptions of the un-burned edits and save it to our share drive.
When the weekend rolls around or whenever there's an opportunity, I store to EEPROM and then delete that section of the rung comment.
Paul