  A 256-channel time-of-flight (TOF) electronics system has been developed for
a beam line facility called "Back-n WNS" in China Spallation Neutron Source
(CSNS). This paper shows the structure and performance of electronics system
and the test results in CSNS. A 256-channel photomultiplier tube (PMT) is
chosen as the detector in this system. In order to acquire the time information
from the PMT, an electronics system has been designed. The electronics system
mainly includes one front-end board (FEB), four time-to-digital converter (TDC)
boards and one clock distribution module (CDM). There are 256 channels on FEB
and 64 channels on each TDC board. The FEB is connected to the PMT with
high-density connectors and the TDC boards are connected to the FEB through 2m
cables. The TDC boards are 6U size so that they can be PCI extensions for
Instrumentation (PXI) cards. Data from TDC boards can be transferred to the PXI
control card through the backboard. In order to make four TDC boards work
synchronously, a CDM outputs four clock signals to TDC boards which are
distributed from one clock source. The TDC boards achieve a timing resolution
of 3.5ns by test with a signal generator. The TOF measurement system has been
used in CSNS.
