A gigabit/s data acquisition system

A fully pipelined data acquisition system has been developed for the PHOBOS experiment at RHIC in BNL. The system began operation in 1998; after the most recent upgrade in 2004, it reached a trigger rate of 575 Hz, processing speed of 200 MB/s, and maintained a sustained data-sinking rate to HPSS of 50 MB/s. The system receives data from 135,168 silicon pad detectors (12-bit amplitudes) and from 2,000 scintillating detectors (time and amplitudes). The data from the silicon detectors are transmitted through two 120 MB/s fiber links to a 24-node processing system (Mercury Computer System, Inc.) embedded into a VME crate and interconnected using a RACEway switching fabric. Each node (PPC750 300 MHz) compresses a part of the data stream using lossless Huffman algorithm. The four-fold compressed data are sent directly from the nodes to a model TK200 data recorder (Conduant Corporation) through an FPDP link. The event building is done on the fly by adjusting offsets and lengths of a scatter-gather DMA transfer. The TK200 has a disk capacity of 3.2 Tbyte and aggregate bandwidth of 200 MB/s, and is connected to the HPSS over a single gigabit fiber connection. Under laboratory conditions, the TK200 was able to handle 120 MB/s of recording and 70 MB/s of reading simultaneously. The upgraded system was used for data taking during RHIC run 2005 and recorded over 0.5 billion (50 Tbyte of data) of heavy-ion collisions over a period of two months