Overview of the ECAL Off-Detector Electronics of the CMS Experiment

Abstract

Abstract-Located between the on-detector front-end electronics and the global data acquisition system (DAQ), the off-detector electronics of the CMS electromagnetic calorimeter (ECAL) is involved in both detector readout and trigger system. Working at 40 MHz, the trigger part must, within ten clock cycles, receive and deserialize the data of the front-end electronics, encode the trigger primitives using a nonlinear scale, assure time alignment between channels using a histogramming technique and send the trigger primitives to the regional trigger. In addition, it must classify trigger towers in three classes of interest and send this classification to the readout part. The readout part must select the zero suppression level to be applied depending on the regions of interest determined from the trigger tower classification, deserialize front-end data coming from high-speed (800 Mb/s) serial links, check their integrity, apply zero suppression, build the event and send it to the DAQ, monitor the buffer occupancy and send back pressure to the trigger system when required, provide data spying and monitoring facilities for the local DAQ. The system, and especially the data link speed, the latency constraints and the bit-error rate requirements have been validated on prototypes. Part of the system is about to go to production.

The CMS experiment at the CERN LHC

“…-102 - The ECAL off-detector read-out and trigger architecture [85,86] is illustrated schematically in figure 4.10. The system is composed of different electronic boards sitting in 18 VME-9U crates (the CCS, TCC and DCC modules) and in 1 VME-6U crate (the selective read-out processor, SRP, system).…”

“…The full ECAL data for an event, if all channels are read out, exceeds this target by a factor of nearly 20. Reduction of the data volume, selective read-out, can be performed by the Selective Read-out Processor [86,91] so that the suppression applied to a channel takes account of energy deposits in the vicinity. For the measure of the energy in a region, the trigger tower sums are used.…”