A Data Merger for the NA48 experiment at CERN

The NA48 experiment at CERN is designed to measure the magnitude of direct CP-violation in the neutral kaon system with an accuracy which is four times greater than previous work. To achieve the required statistical accuracy, it is envisaged that the data acquisition over the course of the experiment will have to run at 100 Mbytes/sec. The NA48 Data Merger receives data from several sources in the detector in parallel on optical fibres. Incoming data are stored in several input buffers. The input buffers are read out sequentially on a backplane known as RPATH which implements a simplified protocol. Data arrive at the Data Merger Output Formatter which concatenates the input buffer data packets into an `event´ which is then sent via a HIPPI link to a crossbar switch and finally to a workstation farm for processing. In order to achieve the performance goals of 100 Mbytes/sec with event building at 10 kHz, all key components of the Data Merger (the input buffers, RPATH and Data Merger Output Formatter) have been custom-designed for this experiment. This paper describes how each component functions and highlights the novel techniques used to achieve this high data bandwidth