A 40 MHz trigger-free readout architecture for the LHCb experiment

The LHCb experiment is considering an upgrade towards a trigger-free 40 MHz complete event readout in which the event selection is only performed on a processing farm by a high-level software trigger with access to all detector information. This allows operating LHCb at ten times the current design luminosity and improving the trigger efficiencies in order to collect more than ten times the statistics foreseen in the first phase. In this paper we present the new architecture in consideration. In particular, we investigate new technologies and protocols for the distribution of timing and synchronous control commands, and rate control. This so called Timing and Fast Control (TFC) system will also perform a central destination control for the events and manage the load balancing of the readout network and the event filter farm. The TFC system will be centred on a single FPGA-based multi-master allowing concurrent stand-alone operation of any subset of sub-detectors. The TFC distribution network under investigation consists of a bidirectional optical network based on the high-speed transceivers embedded in the latest generation of FPGAs with special measures to have full control of the phase and latency of the transmitted clock and information. Since data zero-suppression is performed at the detector front-ends, the readout is effectively asynchronous and will require that the synchronous control information carry event identifiers to allow realignment and synchronization checks.