Two level triggering in storage ring experiments

Cylindrical storage ring detectors using more than ten thousand channels of drift-, proportional- and liquid argon chambers have led to an enormous increase in data acquisition rates stored in hundreds of tapes. To reduce the computer time for total analysis of the selected events one has to reject the undesired events as early as possible. Therefore the trigger should be more selective, i.e. not only by counting the number of tracks in an event but to reconstruct the track coordinates to get to know the topology. Because of the short decision time of some microseconds in typical PETRA/DORIS experiments this is done in two steps: at level 1 a parallel or sequential hardwired logic is used for getting the number of tracks, together with some geometrical values; at level 2 a fast or several slower microprocessors are used running on a simple algorithm together with some hardware tricks. For future detectors with lower bunch crossing rates very fast microprocessors may be used also at the first level, at the second level more efficient algorithms are now designed and under test. (11 refs).