The DAQ system of OPERA experiment and its specifications for the spectrometers

We present an overview of the data acquisition system (DAQ) and event building of OPERA. OPERA is a long baseline neutrino experiment with a high modularity detector and low event rate. To deal with these features a distributed DAQ system base on Ethernet standards for the data transfer has been chosen. A distributed GPS clock signal is used for synchronizations and time stamp of the data. This architecture allows very modular and flexible event building based on a software trigger strategy. We also present its specific application to the spectrometer sub-detector where RPC trackers are installed. Self-triggerability is a dedicated feature to be also sensitive to out-of-spill events and to possibly allow data taking before the official start of the experiment.