  A new fanning topology is proposed to precisely fan out fast control signals
in the Beijing Spectrometer (BES III) end-cap time-of-flight (ETOF)
electronics. However, uncertainty in transfer latency is introduced by the new
fanning channel, which will degrade the precision of fast control. In this
paper, latency uncertainty elimination for the BESIII ETOF upgrade is
introduced. The latency uncertainty is determined by a Time-Digital-Converter
(TDC) embedded in a Field-Programmable Gate Array (FPGA) and is eliminated by
re-capturing at synchronous and determinate time. Compared with the existing
method of Barrel-cap TOF (BTOF), it has advantages of flexible structure, easy
calibration and good adaptability. Field tests on the BES III ETOF system show
that this method effectively eliminates transfer latency uncertainty.
