SLC 5/05's have an option for EEprom program storage, they can then be configured to read the EEProm. Always, if faulted, etc... If you make an online change, and do not save it to the eeprom. Next time it is loaded, the change is gone. You must save to the eeprom, but you cannot do this while the PLC is in run mode. It's one of the downsides of using it.