  The Water Cherenkov Detector Array (WCDA) is one of the major components of
the Large High Altitude Air Shower Observatory (LHAASO). In the WCDA, 3600
Photomultiplier Tubes (PMTs) and the Front End Electronics (FEEs) are scattered
over a 90000 m2 area, while high precision time measurements (0.5 ns RMS) are
required in the readout electronics. To meet this requirement, the clock has to
be distributed to the FEEs with high precision. Due to the "triggerless"
architecture, high speed data transfer is required based on the TCP/IP
protocol. To simplify the readout electronics architecture and be consistent
with the whole LHAASO readout electronics, the White Rabbit (WR) switches are
used to transfer clock, data, and commands via a single fiber of about 400
meters. In this paper, a prototype of data and clock transmission interface for
LHAASO WCDA is developed. The performance tests are conducted and the results
indicate that the clock synchronization precision of the data and clock
transmission is better than 50 ps. The data transmission throughput can reach
400 Mbps for one FEE board and 180 Mbps for 4 FEE boards sharing one up link
port in WR switch, which is better than the requirement of the LHAASO WCDA.
