Gigabit Ethernet Daisy Chain on FPGA for COMET Readout Electronics

The COherent Muon to Electron Transition (COMET) experiment at J-PARC aims to search for the neutrinoless transition of a muon to an electron. We have developed the readout electronics board called readout electronics for straw tube instrument (ROESTI) for the COMET straw tube tracker. We plan to install the ROESTI in the gas manifold of the detector. The number of vacuum feedthroughs needs to be reduced due to space constraints and cost limitations. In order to decrease the number of vacuum feedthroughs drastically, we developed a network processor with a daisy-chain function of Gigabit Ethernet for the field-programmable gate array (FPGA) on the ROESTI. We implemented two SiTCPs, which are hardware-based TCP processors for Gigabit Ethernet, in the network processor. We also added the data path controllers that handle the Ethernet frames and the event data. The network processor enables ROESTI to process the slow control over UDP/IP and to transfer event data over TCP/IP. By using the network processor, we measured the throughput, the stability, and the data loss rate for two-to-six ROESTIs. In any number of boards, the throughput of the event data transfer achieved the theoretical limit of TCP over the Gigabit Ethernet stably and ROESTI stably sent 100% of the data.