The Read-Out Driver for the RPC of the ATLAS Muon Spectrometer

The spectrometer of the ATLAS experiment has been designed to identify muon tracks with transverse momenta up to 1 TeV/c. Its barrel section is made of monitored drift tubes and resistive plate chambers (RPCs) arranged into 32 sectors, to form two wheels surrounding the interaction point. The RPC subsystem provides the level-1 trigger in the barrel and it is read out by a specific DAQ system. On-detector electronics pack the RPC data in frames with an event number assigned by the trigger logic and transmit them to the counting room optically. Data from each sector are then routed together to a read-out driver (ROD) board. This is a custom processor that parses the frames, checks their coherence and builds a data structure for all the RPCs of one of the 32 sectors of the spectrometer. Each ROD sends the event fragments to a read-out buffer for further event building and analysis. The ROD is a VME64x board, designed around two Xilinx Virtex-II FPGAs and an ARM7 microcontroller. In this paper we describe the system architecture, the event binding algorithms and the monitoring features embedded into the design. The board is in production and will be deployed in the first ATLAS runs.