The NA48 event-building PC farm

The NA48 experiment at the CERN SPS aims to measure the parameter $\Re(\epsilon'/ \epsilon)$ of direct CP violation in the neutral kaon system with an accuracy of $2 \times 10^{-4}$. Based on the requirements of: \\\\ * high event rates (up to 10 kHz) with negligible dead time;\\ * support for a variety of detectors with very wide variation in the number of readout channels;\\ * data rates of up to 150 MByte/s sustained over the beam burst;\\ * level-3 filtering and remote data logging in the CERN computer center; \\\\ the collaboration has designed and built a modular pipelined data flow system with 40 MHz sampling rate. The architecture combines custom-designed components with commercially available hardware for cost effectiveness and flexibility. To increase the available data bandwidth and to add filtering and monitoring capabilities, the original custom-built event builder hardware has been replaced by a farm of 24 Intel PentiumII based PCs running the Linux operating system during the shutdown between the 1997 and 1998 data taking periods. During the data taking period 1998 the system has been successfully operated taking ca. 70 Terabyte of data.