Design and Commissioning of the ATLAS Muon Spectrometer RPC Read Out Driver

The RPC subsystem of the ATLAS muon spectrometer 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, tagged with an event number assigned by the trigger logic, and transmit them to the counting room on optical fibre. 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 subsystem 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 board architecture and the event binding algorithm. The boards have been installed in the ATLAS USA15 control room and have been successfully used in the ATLAS commissioning runs.