  The 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 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 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
which 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.
