ATLAS Level-1 Endcap Muon Trigger for Run 3

The Large Hadron Collider is expected to operate with a centre-of-mass energy of 14~TeV and an instantaneous luminosity of $2.0 \times 10^{34}\ \rm{cm^{-2} s^{-1}}$ for Run~3 scheduled from 2021 to 2024. In order to cope with the high event rate, an upgrade of the ATLAS trigger system is required. The level-1 endcap muon trigger system identifies muons with high transverse momentum by combining data from a fast muon trigger detector, called Thin Gap Chambers. In the ongoing Phase-1 upgrade the present Small Wheel is replaced with the New Small Wheel and additional Resistive Plate Chambers are installed in the inner region of the ATLAS muon spectrometer for the endcap muon trigger. Precision track information from the new detectors can be used as part of the muon trigger logic to enhance the performance significantly. The trigger processor board, Sector Logic, has been upgraded to handle the additional data from the new detectors. The new Sector Logic board has a modern FPGA to make use of Multi-Gigabit transceiver technology, which is used to receive data from the new detectors. The readout system for trigger data has also been re-designed to minimize the use of custom electronics and instead use commercial computers and network switches, by using a commercial TCP/IP for the data transfer. The new readout system uses a software-based data-handling, which allows for real-time data monitoring. This paper describes the development of the \mbox{level-1} endcap muon trigger and its readout system for Run~3. The use-case of the software-based readout system is also introduced