The second level of processing in the CTD-SLT consists of
a variety of tasks: 1) input of data from the z-system, 2)
segment parameterisation in LNO coordinates, 3) matching of
r-z hits with r- 
hits on segments, 4) fitting
the r-z data to determine the z-dimension of the
parameters, and 5) coordinate transformation of 3-D segment
parameters in LNO coordinates to 3-D vector hit parameters
in ZEUS coordinates.
The last task is performed in a separate process at level
two to allow a natural division of the concepts of segments
and vector hits in different coordinate systems.
Except for reasons of facilitating debugging, this
division is unnecessary and will be removed in the future.
A crate's worth of z-data arrives at level two from the
z-ROC via a harness channel.
The format of the data is identical to the offline z-data
format.
This data is first unpacked to allow comparison with the
segment hit data.
A list of wire numbers and drifts assigned to segments
arrive, one cell at a time, on a dedicated link between
levels one and two.
As the segment hits arrive the z-data is searched for hits
in the corresponding cell and if found put into internal
cell-structure arrays.
Matching is then performed by comparing DSP drift times with
z-system drift times for hits on the same wire.
The r- segment parameters are determined and then
the z-parameters.
Two types of z-parameters are determined if possible: 1) point information and 2) vector information. The point information consists of the segment mean r, mean z, and errors. If more than one segment with z-information is put on a track these parameters will be used to determine the track z-vertex and dip angle. The vector information consists of the z-vertex, dip angle, and errors. If only one segment with z-information is put on a track these parameters are used for the track.
After z-determination the segment parameters are now available for coordinate transformation. Segment parameters are transformed cell by cell to vector hit parameters and passed on to the track finding processor in its own crate at level three.
The segment and vector hit parameters in each crate are collected together and read out for some GSLT accepts for monitoring. The monitoring data is routed to the global CTD readout transputer via a link with the harness protocol running on it. The global readout transputer concatenates 16 crates worth of segment parameters and vector hit parameters together, adds the ADAMO header, and passes the data on to the EVB. Online histograms of segment and vector hit parameters are obtained from this monitoring data.
All the vector hit finder functions are performed on the first T800 transputer/RAM (TRAM) card on each FADC-ROC board.