  OPERA is an experiment dedicated to the observation of $\nu_\mu$ into
$\nu_\tau$ oscillations in appearance mode using a pure $\nu_\mu$ beam (CNGS)
produced at CERN and detected at Gran Sasso. The experiment exploits a hybrid
technology with emulsions and electronics detectors \cite{opera}. The OPERA
readout is performed through a triggerless, continuously running, distributed
and highly available system. Its global architecture is based on
Ethernet-capable smart sensors with microprocessing and network interface
directly at the front-end stage. An unique interface board is used for the full
detector reading out ADC-, TDC- or Controller-boards. All the readout channels
are synchronized through a GPS-locked common bidirectional clock distribution
system developped on purpose in a PCI format. It offers a second line to
address all channels and the off-line synchronization with the CNGS to select
the events.
