  The Jiangmen Underground Neutrino Observatory (JUNO) is a medium-baseline
neutrino experiment under construction in China, with the goal to determine the
neutrino mass hierarchy. The JUNO electronics readout system consists of an
underwater front-end electronics system and an outside-water back-end
electronics system. These two parts are connected by 100-meter Ethernet cables
and power cables. The back-end card (BEC) is the part of the JUNO electronics
readout system used to link the underwater boxes to the trigger system is
connected to transmit the system clock and triggered signals. Each BEC is
connected to 48 underwater boxes, and in total around 150 BECs are needed. It
is essential to verify the physical layer links before applying real connection
with the underwater system. Therefore, our goal is to build an automatic test
system to check the physical link performance. The test system is based on a
custom designed FPGA board, in order to make the design general, only JTAG is
used as the interface to the PC. The system can generate and check different
data pattern at different speeds for 96 channels simultaneously. The test
results of 1024 continuously clock cycles are automatically uploaded to PC
periodically. We describe the setup of the automatic test system of the BEC and
present the latest test results.
