The SND calorimeter first level trigger

The first-level trigger (FLT) of the 1632 channel NaI(Tl) scintillation calorimeter of the SND detector at the VEPP-2M collider is described. The FLT utilizes signals from individual calorimeter towers to check the event topology for particular combinations of hit towers with certain distances between towers, number of clusters and their sizes in different parts of the SND calorimeter. The pipeline FLT logic works with a clock rate of 16 MHz which is the collider beam crossing frequency. The latency time of the FLT decision is 500 ns. During the 1995 SND data taking run, the FLT reduced the 100 kHz background rate to 40 Hz. (orig.)