An adaptable fast and compact DAQ for the luminosity measurement at HERA II

A new data acquisition (DAQ) system for the H1 Luminosity measurement at HERA II has been commissioned in 2002 in parallel with the machine; aiming at 1% precision measurement on each of the 186 colliding bunches in less than 1 min, it is based on a commercial mezzanine processing card mounted on a VME station, reading custom-designed analog-to-digital converter (ADC) and time-to-digital converter (TDC) VME cards via a dedicated bus. The protocol on this 20-MHz data bus is implemented on a field-programmable gate array (FPGA), which also performs the triggering for two acquisition modes (triggered and by train), the synchronization of the digitization boards and the formatting of the data in an internal FIFO. The data are then fetched and histogrammed by a dedicated PowerPC processor. The online analysis of the distributions takes place on the VME station. Maximum histogramming rates of 0.5 MHz have been reached in the train mode (three channels readout) and about 110 kHz in the triggered mode (26 channels readout).