  The prototype of time digitizing system for the upgrade of BESIII endcap TOF
(ETOF) is introduced in this paper. The ETOF readout electronics has a
formation of distributed architecture that hit signal from multi-gap resistive
plate chamber (MRPC) is signaled as LVDS by front-end electronics (FEE) and
sent to the back-end time digitizing system via long shield differential
twisted pair cables. The ETOF digitizing system consists of 2 VME crates each
of which contains modules of time digitizing, clock, trigger and fast control
etc. The time digitizing module (TDIG) of this prototype can support up to 72
electrical channels of hit information measurement. The fast control (FCTL)
module can operate at barrel or endcap mode. The barrel FCTL fans fast control
signals from the trigger system out to endcap FCTLs, merges data from endcaps
and transfers to the trigger system. Without modifying the barrel TOF
structure, this time digitizing architecture benefits for improving ETOF
performance without degrading barrel TOF measuring. Lab experiments show that
the time resolution of this digitizing system can be less than 20ps, and the
data throughput to DAQ can be about 92Mbps. Beam experiments show that the
complete time resolution can be less than 45ps.
