Semantics of RARM

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 negative 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 triggering.