The Terabit/s Super-Fragment Builder and Trigger Throttling System for the Compact Muon Solenoid Experiment at CERN

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Abstract
The Data Acquisition System of the Compact Muon Solenoid experiment at the Large Hadron Collider reads out event fragments of an average size of 2 kB from around 650 detector front-ends at a rate of up to 100 kHz. The first stage of event-building is performed by the Super-Fragment Builder employing custom-built electronics and a Myrinet optical network. It reduces the number of fragments by one order of magnitude, thereby greatly decreasing the requirements for the subsequent event-assembly stage. Back-pressure from the down-stream event-processing or variations in the size and rate of events may give rise to buffer overflows in the subdetector's front-end electronics, which would result in data corruption and would require a time-consuming re-sync procedure to recover. The Trigger-Throttling System protects against these buffer overflows. It provides fast feedback from any of the subdetector front-ends to the trigger so that the trigger can be throttled before buffers overflow. This paper reports on new performance measurements and on the recent successful integration of a scaled-down setup of the described system with the trigger and with front-ends of all major subdetectors. The on-going commissioning of the full-scale system is discussed.

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