Upgrade of the Second Level of the Readout Electronics for the CMS Drift Tubes Subdetector

The readout server (ROS), which constitutes the second level of the CMS drift tubes (DT) subdetector readout architecture, is a complex VME 9U board, currently placed in the CMS cavern, in the racks on one side of the detector wheels. The planned upgrade of DT electronics for 2013-2014 includes the relocation of all the second level of readout and trigger electronics in the CMS counting room, allowing among others, the future redesign of a higher performance ROS board based on commercial devices with no radiation tolerance requirements. Simulation studies have been carried out in order to assess the system's performance under the increased luminosity ($10^{35} \textup{cm}^{-2} \textup{s}^{-1}$) planned for the HL-LHC upgrade, ca. 2020. Results show that the ROS board could become a limiting factor due to the event processing time. The capabilities of currently-available FPGAs allow incorporating most of the ROS functions (input deserialization, input buffer, data processing and multiplexing, slow control interface, test-mode operation) into a single device. In particular, Virtex-6 and Spartan-6 series are being considered. In both families, fully-automatic asynchronous deserialization is carried out by the gigabit transceivers, which are not suitable for our application due to the minimum data rate and reduced availability. Nevertheless, asynchronous data reception can be carried out by making use of the dedicated deserializers present in each of the I/O tiles, plus some additional logic and clocking resources. The improved performance of these devices (as compared with current ROS technology) allows reducing the event processing time, effectively increasing maximum system's operation frequency.