User`s guide

Remarks
To report any subsequent events, the reading count must first drop below
the programmed threshold before reaching the threshold again.
To enable the "Memory Threshold" bit (bit 9 in the Standard Operation
Event register) to be reported to the Status Byte, use the
STATus:OPERation:ENABle command.
Once the "Memory Threshold" bit (bit 9 in the Standard Operation Event
register) is set, it will remain set until cleared by the
STATus:OPERation[:EVENt]? command or *CLS (clear status) command.
(The Standard Operation Condition register will show the current
condition. See STATus Subsystem Introduction for further information.)