HERA-B: trigger system

The HERA-B experiment at DESY, Germany, analyzes interactions of 920 GeV protons with a fixed target to measure CP violation. The production rates of the decays of interest are more than 10 orders of magnitude smaller than the rate of the background interactions. The HERA-B trigger system must be capable to reduce the high event rate of about 10 MHz to an event rate of about 20 Hz, which can be logged to tape. The implementation of the HERA-B trigger is a challenging task. The hardware and software are described, and an update on the progress at the time of the conference in the summer of 1999 is given.