On-line filtering at the CERN-ISR

During the last years it has become quite evident that one should aim at trigger systems with high selectivity in order to improve the purity of data and simultaneously remove load from the off-line processing. At the CERN ISR typical pp interaction rates are in the order of 1-3 MHz. Therefore experiments studying low cross section processes need trigger systems which provide reduction factors in the order of 105 to avoid dead time losses. The use of programmable devices, allowing for flexible and sophisticated decision schemes, enables experiments to gain a decisive factor in rejection after pre-triggering with more conventional devices. An overview of the different methods employed and their performance is given. (orig.)