  Large liquid argon Time Projection Chambers have been adopted for the DUNE
experiment's far detector, which will be composed of four 17 kton detectors
situated 1.5 km underground at the Sanford Underground Research Facility. This
represents a large increase in scale compared to existing experiments. Both
single- and dual-phase technologies will be validated at CERN, in cryostats
capable of accommodating full-size detector modules, and exposed to low-energy
charged particle beams. This programme, called ProtoDUNE, also allows for
extensive tests of data acquisition strategies. The Front-End LInk eXchange
(FELIX) readout system was initially developed within the ATLAS collaboration
and is based on custom FPGA-based PCIe I/O cards, connected through
point-to-point links to the detector front-end and hosted in commodity servers.
FELIX will be used in the single-phase ProtoDUNE setup to read the data coming
from 2560 anode wires organized in a single Anode Plane Assembly structure.
With a continuous readout at a sampling rate of 2 MHz, the system must deal
with an input rate of 96 Gb/s. An external trigger will preselect time windows
of 5 ms with interesting activity expected inside the detector. Event building
will occur for triggered events, at a target rate of 25 Hz; the readout system
will form fragments from the data samples matching the time window, carry out
lossless compression, and forward the data to event building nodes over 10 Gb/s
Ethernet. This paper discusses the design and implementation of this readout
system as well as first operational experience.
