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The history PVs have two states: `armed' or `idle'. In `armed'
state the history waits for a triggering event and upon reception,
starts writing data into the buffers. Once in `armed' state, the
history is `busy' and cannot be accessed. Most notably: at the
moment it cannot be disarmed.
After filling up, an `armed' buffer returns to `idle' state and can be
read and rearmed.
A history buffer is armed by setting its RARM field
(The respective PVs are 116-BPM:history.RARM, 132-BPM:history.RARM) to
a non-zero value: