At the LHC bunches of protons are brought to collision at a rate of 40MHz, the resulting interaction rate is in the range of 1GHz. The ATLAS experiment employs several sub-detectors with a total of approximately 100 million data channels for the signal acquisition. Electronics located close to the detector filters from the entire data volume events of about 1MB in size at a rate of 100kHz and sents them over 1,600 optical channels to the DAQ system. The nominal bandwidth per channel amounts to 160MB/s. A two-stage trigger system evaluates each event, first of approximately 10% of the channels. Based on this preliminary decision approximately 3% of the complete events will be analysed subsequently. The accepted events are stored at an average rate of 200Hz. This trigger concept requires an intelligent buffer that bridges the processing time of the first stage and can provide data on request. This intermediate buffer is realised in ATLAS via the Read-Out System (ROS) which consists of 160 PCs with 4 ROBIN cards each.
Every ROBIN receives and buffers three optical channels. The ROS-PCs request or delete the sub-events residing in the ROBINs via the PCI-bus. The functionality of the ROBIN modules is realised via a close coupling of FPGA and microprocessor; simple functions with high rate and data volume are implemented by the FPGA, while the processor handles more complex functions with higher latency and lower rate. The current system with PCI-X connection is able to transfer up to 30% of all events (at 1kB per channel nominal) from the ROBINs to the PC. Options to increase the performance up to 100% are currently being investigated.
ATLAS TDAQ Video (ATLAS Experiment © 2013 CERN):