The UA1 trigger processor

Experiment UA1 is a large multipurpose spectrometer at the CERN proton-antiproton collider. The principal trigger is formed on the basis of the energy deposition in calorimeters. A trigger decision taken in under 2.4 microseconds can avoid dead-time losses due to the bunched nature of the beam. To achieve this fast 8-bit charge to digital converters have been built followed by two identical digital processors tailored to the experiment. The outputs of groups of the 2440 photomultipliers in the calorimeters are summed to form a total of 288 input channels to the ADCs. A look-up table in RAM is used to convert the digitised photomultiplier signals to energy in one processor, and to transverse energy in the other. Each processor forms four sums from a chosen combination of input channels, and also counts the number of clusters with electromagnetic or hadronic energy above pre-determined levels. Up to twelve combinations of these conditions, together with external information, may be combined in coincidence or in veto to form the final trigger. Provision has been made for testing using simulated data in an off-line mode, and sampling real data when on-line. (0 refs).