Omron Programmable Terminals NT631C - Manual (Page 156)

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Memory Initialization Section 6-4
The maximum number of records is 256 for sort by occurrence (order of error
occurrence) and 255 for sort by frequency (order of the number of times each
error has occurred).
Use the alarm history record initialization function to reset the alarm history
record data to 0, for example when the screen data has been changed, etc.
Also, if Screen (Use Ring Buffer) under History Setting in the PT Environment
Settings – System Settings has not been checked, no more records will be
stored after the maximum number of records has been kept, and therefore the
records must be periodically initialized. If Screen (Use Ring Buffer) is
checked, when the maximum number of records is reached the oldest record
is deleted and the newest data is recorded.
Reference: The NT631/NT631C also allows the alarm history record data to be initialized
by controlling the PT status control area from the host. For details, refer to 2-
2-1 PT Status Control Area in the Reference Manual.