The UA1 calorimeter trigger

Abstract We describe the design, construction and performance of the fast calorimeter trigger used in the UA1 experiment on the CERN p p collider. Calorimeter energy signals are digitized rapidly and used to trigger on possible electrons, hadronic jets, total transverse energy and missing transverse energy, with many possible options. We also describe the minimum-bias pretrigger, based on scintillator hodoscopes.