Ethernet-Based Data, Acquisition System for the Do Experiment at Fermilab

The DO detector at Fermilab has been collecting data with its new, Ethernet-based Data Acquisition (DAQ) System since spring 2002. The system was built almost exclusively with commercially available components, and designed to read in event fragments at a rate of 1 kHz from about 70 VME crates, assemble the events with a typical size of 250 kB and pass them to the Level 3 trigger farm, composed of about 160 CPUs, for final event selection. Accepted events are sent to the online system to he stored on tape at an average rate of 50 Hz. The event fragments are read out over VME using Single Board Computers (SBCs) and sent to the trigger farm through 100 Mb/s and 1 Gb/s Ethernet connections via a series of Ethernet switches. This paper gives an overview of the main components of the DAQ and trigger system and reports on its performance during the first year of operation.