SCAN vs MONITOR

Both SCAN and MONITOR use CA monitors, but they differ in how samples are buffered.

MONITOR estimates the number of samples based on the PV’s sampling period, allocates a buffer to hold that many samples, and fills it up. The buffer capacity is computed as:

int buffer_capacity = ((int) Math.round(Math.max((write_period/pvSamplingPeriod)*sampleBufferCapacityAdjustment, 1.0))) + 1;

  • pvSamplingPeriod — the sampling_period from the PV’s PVTypeInfo

  • write_period — the engine’s write period from archappl.properties (secondsToBuffer); defaults to 10 seconds

  • sampleBufferCapacityAdjustment — a system-wide buffer-size adjustment set in archappl.properties

For example, if write_period is 10 seconds and pvSamplingPeriod is 1 second, the buffer holds 11 samples. For a PV changing faster than 1 Hz, you will see 11 samples, a gap where the buffer overflows, then 11 more samples on the next buffer switch.

SCAN updates one slot at whatever rate is sent from the IOC, then a separate thread reads that slot every pvSamplingPeriod seconds and writes it to the buffer. This gives exactly one sample per pvSamplingPeriod regardless of the IOC update rate, at the cost of potentially missing intermediate values.