Design and performance of the DZero data acquisition system

The D0 Run 2 data acquisition system features a novel design permitting complex event routing in a high speed, data driven architecture. Data blocks associated with multiple events flow freely from ~70 frontend digitization crates. Eight separate readout concentrators funnel these blocks onto optical fiber paths to the Level 3 trigger processor farm. The processor farm is divided into segments, each of which is fed by a segment bridge. Data blocks flow to each bridge in turn, where they are either routed to a Level 3 node (or nodes) in the segment, or passed onto the next segment bridge. Blocks that do not flow to a processor node are returned to the readout concentrator. Thus the system has a built-in mechanism for rate limitation. The Event Tag Generator (ETG) and segment bridges facilitate the event routing decisions in real time. The routing is based on the trigger bits from the hardware trigger. This and the availability of nodes in each segment determine to which node(s) the event is routed. We present operational and performance details of this system, designed for comfortably more than the nominal 1 kHz input; being modular and parallel, it can be easily upgraded