Abstract
The PHOBOS data acquisition system based on a RACEway switching network is described. Occupying a single VME crate, the system utilizes 24 PPC-750 CPUs working in parallel to compress data from 135,168 silicon pad detectors, and an UltraSPARC VME host for event building and data storage. Lossless Huffman coding is used for compression; this reduces the event size three times. The two-host disk array is used to stage data before sending them over a Gigabit Ethernet to the RHIC Central Computing Facility. The writing and reading are done simultaneously at 40 MB/sec. All trigger and control logic is formed using universal programmable logic VME modules, which can be programmed in situ, even when the system is running. The data is written in the popular ROOT format; all data processing and run control software is written using the ROOT framework. The slow control and configuration makes use of an Oracle database to store configuration and monitor parameters. The system has been taking data from the Phobos experiment at RHIC since June 2000. The system is capable to take data with the rate of 200 events/sec or 30 MB/sec.
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