A fast programmable trigger system for the WASA experiment

A programmable fast trigger system for the WASA experiment at TSL, Uppsala, Sweden is presented. For minimum signal delay times the hardware architecture is divided into two levels. The first level mainly handles fast signals from about 350 plastic scintillating detectors forming hit cluster multiplicity decision, restricting the total number of reaction products. Track alignment checks, co-planarity and deposited energy checks put further restrictions on the acquired events. The first level incorporates 40 generic trigger modules, prescalers, scalers and pattern units. To minimize dead-time losses the first level is working in a feed-through mode with no external strobe signals required, the total delay amounts to 120 ns. The second level utilizes a fast cluster finding algorithm implemented in 40 trigger modules operating on signals from an array of 24×48 CsI crystals. Background is filtered away by restricting the spatial distribution and the number of reaction products. The output from the cluster analyzing second level is provided within 115 ns. The trigger utilizes programmable logic devices and is fully reconfigurable. An I2C-based slow control system is provided for setting discriminator thresholds, pulse duration, channel masking, signal delay and setup of logic conditions