COMPASS calorimetric trigger in 2012 run: motivation and implementation

For the COMPASS experiment at CERN a dedicated run for studies on chiral perturbation theory is planned for 2012. By impinging pions to a Nickel target, at very low momentum transfer, final states containing an outgoing photon or neutral pions are produced. In order to select these events a new calorimetric trigger, reusing the already existing readout electronics, is implemented. A backplane module as well as a central trigger module is developed on which the hits form the individual cells are collected and further processing is done. The implementation as well as the first test results is presented.