Variable parameters loose after about 6 minutes

Dear all,

Need your help about my problem. Have PLC MELSEC FX2n-16MR with FX-EEPROM-8 and connected to a terminal E300. The problem all variable parameters numbers it save for about 6 minutes and then it suddenly disappeared. All the encoded variable parameter numbers become asterisk character.

It was lately observed the battery of the PLC totally drained that cause an alarm REPLACE BATTERY appeared on the terminal monitor. When the battery replaced the alarm were reset but then saving the parameters is the issue.

Highly appreciated if somebody can help to resolve this issue.

http://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy992d48301/jy992d48301j.pdf

"RAM file registers are 2000 points which can be activated by driving a special auxiliary relay M8074 ON. These registers can then be accessed just like normal program file registers. RAM registers occupy no program steps but do occupy the sampling trace data register area when M8074 is active."

The above may help you out.
Regards,