Upgrades to the CMS Cathode Strip Chamber Electronics for HL-LHC

Cathode strip chambers (CSCs) are used to detect muons in theend cap region of the CMS detector. The chambers are arrangedin rings in four planes on each end of the detector. The innerrings of CSCs in each station have the highest flux ofparticles and this presents challenges to efficient readout atthe luminosity, latency, and trigger rate foreseen at theHL-LHC.The existing front end electronics in Stations 2, 3, and 4 arebased on switched capacitor arrays with limited bufferingcapability. Queuing models have shown that significantsaturation of the buffers would occur for the luminosity, Level1 trigger rates, and required Level 1 latency for running atthe HL-LHC. In addition, the expected output rate of data isexpected to exceed the 1 Gbps bandwidth of the optical linksthat carry data to the back end, resulting in loss of eventsynchronization.The upgrade of the inner ring of CSCs addresses these problemsby replacing some types of electronics boards in these ringswith upgraded boards. In particular, the existing cathode frontend boards on the inner rings of Stations 2, 3, and 4 will bereplaced with new digital cathode front end boards. Theseboards follow the design of those installed in the inner ringof Station 1 in the recent long shutdown of the LHC. The boardsuse flash ADCs and digital pipelines in place of the switchcapacitor arrays used previously. The digital pipeline resultsin nearly deadtimeless operation and the capability toaccommodate long latency requirements without loss of data.Also, new DAQ motherboards will be designed with optical outputlinks with higher bandwidth to accept the higher data rate.Finally, the FED system, which is the interface between theCSCs and the central DAQ of the CMS experiment, will bereplaced with a system that can receive the higher input rates.We present the measurements and calculations that predict thebehavior of the CSC electronics under HL-LHC conditions. We describe the design of the electronics systems for the upgradeand show studies of expected performance.