COMPASS is a high-energy particle physics experiment at the Super Proton Synchrotron (SPS) at CERN. The purpose of the fixed target experiment is the study of hadron structure and hadron spectroscopy with high intensity muon and hadron beams.
In 2002, COMPASS started to take data using at first the ALICE data-acquisition software framework DATE. Since 2012, the COMPASS spectrometer has entered its second phase known as COMPASS-II with an increased number of detector channels of about 300,000. In 2014, the DAQ has been upgraded to a compact and more reliable system. The COMPASS experiment nowadays collects data using a novel DAQ architecture utilizing a hardware event builder and modern software tools. The hardware event builder is based on nine custom designed Data Handling Cards (DHC) which replace 30 distributed online computers, around 100 PCI cards, and the ethernet switch of the event building network of the previous system. As a result, the new DAQ is compact and scalable and provides higher bandwidth and better reliability.
The system makes use of the spill structure of the SPS beam by buffering data on different levels of the event builder and averaging the maximum on-spill data rate over the whole SPS cycle. As a result the online DAQ computers work independently from the initial detector rate of 1 GB/sec and deal only with a maximum aggregate rate of 400 MB/sec.
The system utilizes three independent interface networks for synchronization, event building, and data flow control. This functional division simplifies the software architecture, optimally synchronizes processes, and offers an efficient usage of the network bandwidth.
In 2015, it is planned to wire all point-to-point high speed links via a fully programmable crosspoint switch. The crosspoint switch will therefore provide a fully customizable DAQ network topology between front-end electronics, the event building hardware, and the online computers. The adaptability of the system topology allows to compensate for hardware failure in the event builder by activating spare resources replacing broken or malfunctioning modules. Algorithms shall identify hardware failure and synchronously reconfigure the DAQ topology to substitute the broken module by a spare one without human intervention.
In this way, the fully programmable crosspoint switch contributes substantially to an improved system reliability since every broken FPGA module of the event builder as well as the online computers can be exchanged on-the-fly and data loss is reduced.