The event builder for STAR

STAR is a large multipurpose detector at the Relativistic Heavy Ion Collider (RHIC) at BNL which begins operation this year. The event builder (EVB) has to handle a throughput of 20 MB/s and event rates between 1-100 Hz. The EVB system is built using commercially available components running under VxWorks and Solaris. Implementation language for the system is C++. Connection to multiple data sources (sector crates) is via Scalable Coherent Interface (SCI), a high bandwidth, low latency interconnect. Built events are moved to a UNIX-based buffer systems via SCSI, then distributed to archiving systems and real-time consumers by Gigabit Ethernet and SCSI. Inside EVB, SCSI plays a role both in data transport and as a way to address the "scatter-gather" problem. The EVB allows dynamic virtual partitioning of the DAQ system on an event-by-event basis to facilitate the simultaneous building of different events for different destinations. This multiple stream capability allows minimally invasive monitoring of the data quality and detector components, especially important during the commissioning phase of detectors. Design of the system and results of performance tests with part of the STAR system are presented. Possibilities are discussed for scaling the system to higher rates.