The SRTD first level trigger at ZEUS

A new trigger has been added to the ZEUS Detector during the Winter shutdown for use beginning in the 1995 running period. The Small Angle Rear Tracking Detector (SRTD) has 272 scintillation fingers arranged as four quadrants in two overlapping planes around the beampipe and in front of the Rear Calorimeter (RCAL). The scintillators are viewed by photomultipliers via fiber bundles. As photomultiplier outputs are carried to the data acquisition system they are spied upon by the trigger. High impedance amplifiers, dual level discriminators, and coincidence logic furnish stops to four rapid cycling TDC´s which are cleared by the HERA clock every 96 ns. The trigger data furnished to the Global First Level Trigger by the SRTD on every beam crossing consists of 32 bits. One 8-bit word is generated for each SRTD quadrant. It represents the TDC time (6 bits) with a resolution of 1 ns and the position (2 bits) of the hit or hits closest to the beampipe. The trigger hardware includes programmable thresholds, delays, and circuitry to capture scintillator hit patterns and trigger times of interest. An extensive set of diagnostic and operational software has been developed. The SRTD trigger reduces the ZEUS first level trigger rate by a factor of three without reducing the trigger efficiency