  In the Large High Altitude Air Shower Observatory (LHAASO), the Water
Cherenkov Detector Array (WCDA) is one of the key parts. The WCDA consists of
3600 Photomultiplier Tubes (PMTs) scattered in a 90000 m2 area, and both high
precision time and charge measurements are required over a large dynamic range
from 1 to 4000 Photo Electrons (P.E.). To achieve time measurement precision
better than 500 ps RMS, high quality clock distribution and automatic phase
compensation are needed among the 400 Front End Electronics (FEE) modules. To
simplify the readout electronics architecture, clock, data, and commands are
transferred simultaneously over 400-meter fibers, while high speed data
transfer interface is implemented based on TCP/IP protocol. Design and testing
of the readout electronics prototype for WCDA is presented in this paper. Test
results indicate that a charge resolution better than 10% RMS @ 1 P.E. and 1%
RMS @ 4000 P.E., and a time resolution better than 300 ps RMS are successfully
achieved over the whole dynamic range, beyond the application requirement.
