The COMPASS data acquisition system

A fully pipelined and massively parallel data acquisition system has been developed for the COMPASS experiment at CERN. The main requirements are to read 250000 detector channels at a trigger rate of up to 100 kHz. Such high rates are only possible when using a hit selection mechanism on the front-end combined with dead-time free readout. For this purpose, a time-to-digital converter (TDC) chip has been developed and is used for all time measurement applications in COMPASS. Distributed, field programmable gate array (FPGA)-based readout-driver modules handle parallel front-end initialization, synchronous trigger and control-signal distribution, and local event building at a processing speed of 160 Mbyte/s. Each of the 160 readout-driver modules connects to 16 front-end boards through independent twisted pair cables (CAT 7, 600 MHz) or optical fibers using an industrial (ESCON), self synchronizing link at 40 Mbyte/s. Automatic configuration through unique module and link identification ensures the flexibility and scalability to very large detector systems. The preprocessed data are transmitted through optical fibers at 160 Mbyte/s to the master event building system. Here global event building is realized on high-performance personal computers (PCs) and Gigabit ethernet network components. The complete events are sent to the central data recording at the CERN main site at an average rate of 40 Mbyte/s and stored in an object oriented database. A reduced system was set up for the commissioning run of COMPASS in 2000. Operation of the full system starts in July 2001.