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The number stored in RARM is the number of ``pre-trigger''
samples to be stored in the buffer. e.g., if RARM=4000, the
buffer ``looks into the past'', e.g. stores 4000 samples
preceding the trigger.
It is possible set RARM to a negativ number thus giving
post-trigger capability. RARM=-8 skips 8 samples after
trigger before putting data into the buffer. in this example,
the history's resulting Spear timestamp would differ by
8 from ``trigger- time''.
The number of pre-trigger samples always must be a multiple
of 8, since the least significant 3 bits of the RARM PV are used
to specify the triggering method. Values of 1..7 indicate
that the history is to be armed on the respective Spear Event,
a value of zero means ``soft triggering'', i.e. writing the RARM field
is the trigger. Since RARM must always be non-zero, it is not
possible to use 0 pre-trigger samples together with software