The CMS ECAL Collaboration Presented by B. Lofstedt CERN EP-CME !! SUMMARY The Upper-Level digital Readout and Trigger system of CMS ECAL consists of around 60 9-U VME crates, each serving 1700 readout channels corresponding to, in the barrel case, 68 trigger towers. This gives for the barrel one crate per supermodule. For the end cap, where the tower mapping is less uniform, the crate mapping will be slightly different. Each crate contains 17 100-channel readout modules (ROSE-100), one Data Concentrator module (DCC) and one standard VME processor card. Each ROSE- 100 has, in the transition board area, two link interface boards. One board is called Opto-Electro-Board (OEB) and contains the opto-electro interfaces for the links from-to the on-detector Very-Front-End (VFE) electronics (120 fibres). The other one, called the Sync-Link-Board (SLB), contains two fast copper (Gb/s) links to the Level-1 local trigger process as well as one bi-directional fast copper (Gb/s) link to the DCC. The system provides the control of the VFE electronics via two 40 Mb/s optical links per ten crystals and it receives one optical link per crystal at an 800 Mb/s sustained rate. It provides information to the regional trigger on the energy, shower profile and time assignment per trigger tower, stores the data during the first level trigger latency. In case of a positive Level-1 decision, the corresponding data is transferred to the DAQ system. In addition, it provides means to assure the synchronisation and integrity of the data, both internally and with respect to the LHC bunch-crossing. The system provides all monitoring and test facilities required to assure proper functionality of the hardware. Also, it performs a local acquisition of monitoring data, such as signals from the light monitoring system, test pulses injected into the VFE electronics, crystal temperatures, monitoring of the APD bias currents, etc. During high-luminosity LHC operation, physics triggers arrive in CMS at a rate of 100kHz. The background physics interactions occur at a much higher rate. An average of 17 minimum bias events occurs per bunch-crossing at a 40 MHz sustained rate. In order to isolate the front-end signals which are in-time with the triggered bunch-crossing, the pulse shapes are digitised over a duration of several consecutive crossings, before and after the trigger, and stored in a time frame for analysis. The length of the time frame expands the data volume of the calorimeter to 2.4 MB/trigger. To reduce the data size down to 120kB/trigger in blocks of 2kB, as required by the central CMS DAQ system, a readout architecture incorporating data reduction algorithms was specially designed for this purpose. The baseline for the data reduction architecture is to use the information extracted for the Level-1 trigger process and to execute the reduction, per crate, in the DCC module. Another possibility, currently under investigation, is to implement a dedicated Centralised Selective Readout Processor (CSRP) in order to facilitate the information exchange across the hardware boundaries. The DCCs will identify the seed regions and communicate them to the CSRP where a map is created of the channels to be read. This information is returned to the DCCs where the selective readout is performed. For each step in the data transfer, formatting and consistency checks are continuously performed, monitoring information are collected and transferred to !! ABSTRACT The readout system for the CMS ECAL detector, comprised of 80’000 PbWO4 crystals, consists of two distinct parts; the on-detector Very-Front-End electronics and the Upper-Level Readout and Trigger system which is purely digital. The two parts are connected by a large number of optical links; one high-speed link per channel (=80’000) for the readout and two slow links per set of ten channels to distribute clock and control information to the on-detector electronics. This paper will concentrate on the Upper-Level part of the system describing the hardware architecture of both the DAQ- and the Trigger-paths. !!