  The ZEUS Central Tracking Detector utilizes a time difference measurement to
provide a fast determination of the z-coordinate of each hit. The z-by-timing
measurement is achieved by using a Time-to-Amplitude Converter which has an
intrinsic timing resolution of 36 ps, has pipelined readout, and has a multihit
capability of 48 ns. In order to maintain the required sub-nanosecond timing
accuracy, the technique incorporates an automated self-calibration system. The
readout of the z-by-timing data utilizes a fully customized timing control
system which runs synchronously with the HERA beam-crossing clock, and a data
acquisition system implemented on a network of Transputers. Three dimensional
space-points provided by the z-by-timing system are used as input to all three
levels of the ZEUS trigger and for offline track reconstruction. The average
z-resolution is determined to be 4.4 cm for multi-track events from
positron-proton collisions in the ZEUS detector.
