The ROD DSPs can record the number of events per chip per trigger, and add that to a histogram. This is the occupancy per event OPE for that chip.
A two-dimensional histogram consisting of OPE on the x-axis and scan point (threshold) value on the y-axis is shipped off the ROD with the RawScanResult.
The ROD histograms the number of hits per event in bins which have edges 1->4, 5->8, 9->12 etc. The number of empty events (0 hits) is inferred from the total number of events.
An example of an OPE histogram can be seen at http://www-pnp.physics.ox.ac.uk/~daquser/ExamplePlots/OPE_NO.eps (this was made for a noise occupancy, so the y-axis is threshold relative to trim target.
To display this data see DataDisplayerExamples
Note that the OPE data was a new feature on 22nd July 2004, and is not in Release3.0.
OPETool is a Release4.0 tool which analyses the OPE data for NoiseOccupancyTest and outputs a "badness" parameter to the online GUI as well as report a BAD_OPE defect. It can also be used as an offline analysis tool.