A high speed backplane for the RPC Read Out Driver of the ATLAS Muon Spectrometer

The Read Out Driver (ROD) of the RPC detectors in the ATLAS Muon Spectrometer is a VME64x board designed to perform sub-event building for one of the 32 sectors in which the apparatus is divided. Each ROD receives data fragments from two companion boards placed in the adjacent VME slots. These two boards also send trigger data to the nearest slot, respectively at their right and left hand side. Due to the large amount of data to be transferred with a known latency, an auxiliary bus named RODbus has been designed and deployed in order to link the five boards. In this paper we present the logical and physical architecture of the RODbus, which implements multiple point-to-point LVDS links for high-speed data transfer and parallel TTL busses for commands, flags and flow-control signals. A prototype of the bus embedded into a VME64x 21-slot backplane has been recently developed together with Harting, the firm which designed the connectors specified by the VME64x standard. We also discuss the test results on signal integrity, crosstalk and impedance profile made with Time Domain Reflectometry.