  The Belle II experiment at the SuperKEKB collider at KEK, Tsukuba, Japan has
successfully started taking data with the full detector in March 2019. Belle II
is a luminosity frontier experiment of the new generation to search for physics
beyond the Standard Model of elementary particles, from precision measurements
of a huge number of B and charm mesons and tau leptons. In order to read out
the events at a high rate from the seven subdetectors of Belle II, we adopt a
highly unified readout system, including a unified trigger timing distribution
system (TTD), a unified high speed data link system (Belle2link), and a common
backend system to receive Belle2link data. Each subdetector frontend readout
system has a field-programmable gate array (FPGA) in which unified firmware
components of the TTD receiver and Belle2link transmitter are embedded. The
system is designed for data taking at a trigger rate up to 30 kHz with a
dead-time fraction of about 1% in the frontend readout system. The trigger rate
is still much lower than our design. However, the background level is already
high due to the initial vacuum condition and other accelerator parameters, and
it is the most limiting factor of the accelerator and detector operation. Hence
the occupancy and radiation effects to the frontend electronics are rather
severe, and they cause various kind of instabilities. We present the
performance of the system, including the achieved trigger rate, dead-time
fraction, stability, and discuss the experience gained during the operation.
