An overview of the first and second level trigger of DELPHI

The DELPHI trigger is organized in four levels. The first two triggers are synchronous with the beam crossover, every 22 mu s, and are implemented in hardware. The first-level trigger decision is taken at 3 mu s and is used primarily as a fast trigger to gate the time protection chamber. Gating this detector off for rejected events reduces space-charge buildup. If the first-level decision is positive, the following beam crossover will be lost to allow the inclusion of data from long-drift-time detectors in the second-level trigger. The second-level decision is taken at 39 mu s. If this decision is positive a front-end readout is initiated and up to 500 mu s is required to free the front-end buffers. To limit the deadtime fraction for each trigger to the order of 2% the first-level trigger rate has to be held to 1 kHz and the second-level rate to 20 Hz. The authors describe the various subtriggers in DELPHI together with the detector local trigger decision boxes contributing to the subtrigger, and they indicate the type of hardware used implement the decision logic. >