The Linux-based single board computer for front end crates in the DZERO Data Acquisition System

The Level 3 Trigger/Data Acquisition system (L3DAQ) for the DZERO detector was redesigned from scratch for the Fermilab Tevatron's RunII. The L3DAQ moves data at a rate of 250 MB/sec from 63 readout crates to one of 82 Level 3 Trigger farm nodes over Ethernet. The readout crates are VME based, and each consists of a number of detector specific digitizer cards and a controller. The readout crates also contain a single L3DAQ Single Board Computer (SBC). Crate data size per event ranges from several kilobytes up to 24 kilobytes for the largest; and the data must he collected from each of the VME cards at an event readout rate of 1 kHz. The SBC, a VMIC 7750, is an off-the-shelf component and runs Linux with several common and custom patches. This paper will describe the custom driver we've written to control the VME readout, modifications to the Linux kernel, and the user level readout code.