Conclusions

The ZEUS Calorimeter First Level Trigger system calculates global and regional energy sums and searches for isolated muons and electrons. It completes these tasks in a pipelined fashion in 2 $\mu$sec, accepting new data and shipping out results once every 96 nsec. It uses digital logic with memory lookup tables to provide programmable flexibility for changing calibration, thresholds and even the types of tests made. The results of its calculations are shipped to the Global First Level Trigger (GFLT). The CFLT reduces the present background rate of 10 kHz of tower energy $>$ 500 MeV to about 100 Hz. At full HERA luminosity, the GFLT must reduce the initial beam-gas rate from about 100 kHz to 1 kHz. The GFLT is also pipelined, reaching a decision in 2 $\mu$sec after receipt of the component data. The First Level Trigger system reads in a new event every 96 nsec and returns a trigger to the detector components for that event 5 $\mu$sec after it occurred.

The ZEUS Calorimeter First Level Trigger system has operated reliably and according to design in 3 data-taking runs at HERA, and has accumulated 600 $nb^{-1}$ integrated luminosity of physics data, at a 100 Hz output rate. It analyzes the data from 13K PMT's, collecting signals from a mostly nonprojective calorimeter at a 10 MHz input rate. It consists of a 1792 channel system contained in 16 9U modified VME crates with 14 TECs and 2 double-width Adder Cards, followed by a Trigger Processor in a custom-backplane crate that analyzes 5 GBytes/sec. The modified VME crate cards and backplanes run at 83 MHz with 14 Gbits/sec of data transfer. The system makes extensive use of memory lookup tables so that thresholds, EM/HAC ratios, geometric factors, dead/noisy channel switchoff, calibration and subregion definitions are programmable. This flexibility has proven very useful and has enabled frequent changes in configurations to accomodate changing physics requirements. The isolated electron pattern recognition trigger has proven to be a powerful tool for beam-gas rejection. The analysis of data taken so far indicates sufficient performance to handle luminosity up to the HERA design value.