| 摘要 |
PURPOSE:To restrict as far as possible the extent of disappearance of information accumulated in an external memory unit even when an abnormal state occurs by changing a spare memory unit that accumulates the same information as that stored in presently used memory unit to a history memory unit at every specified period. CONSTITUTION:The external memory unit 3-1 is used as a presently used memory unit. At a point of time t2, an external memory unit 3-2 is used as a spare memory unit, and external memory units 3-3-3-5 are used as history memory units. When the point of time t2 is arrived, a processing device 1 changes the spare memory unit 3-2 to a history memory unit, and after copying telegrams etc. accumulated in the presently used memory unit 3-1 in the history memory unit 3-5, starts using as a spare memory unit. On arriving at a point of time t3, the processing device 1 changes the spare memory unit 3-5 to a history memory unit, and after copying telegrams etc. accumulated by the presently used memory unit 3-1 in the history memory unit 3-4, starts using as a spare memory unit. |
