Level three of the CTD-SLT is primary responsible for matching vector hits in two-crate masks to form tracks and determining the tracks parameters. These functions are performed on the second T800 TRAM card on each FADC-ROC board. Cables connected between the FADC crates link the level three nodes together to form the SLT readout chain. Level three SLT processors form the bottom of the SLT harness network.
Before track finding commences a two-crate mask of vector hits is formed. Each processor receives a crate's worth of vector hit data on a dedicated input link from the level two processor in its own crate. This input data arrives cell by cell and is concatenated together for the entire crate and fanned out on two channels; one channel is internal to the processor itself and the second is an external link to the next lower numbered crate in the SLT chain. In each processor the vector hits from two crates are concatenated together to form a two-crate mask for vector hit matching and track parameter determination.
Each track finding processor sends and receives track data from its nearest neighbours. Track data is received from the level three processor in the lower numbered crate, concatenated with the track data in the crate (using the same process that merges vector hit data from two-crates), and passed on to the higher numbered crate. Track data thus flows along the chain on the same links as the vector hit data but in the opposite direction. Hence track data from the entire CTD emerges from the level three processor in crate 15, for input to the global CTD-SLT processor (level four).
The track parameters determined in each processor are read out for some GSLT accepts for monitoring. The monitoring data is routed to the CTD global readout transputer using the remaining link on the track finding transputer, which runs the harness protocol. The global readout transputer concatenates 16 crates worth of track parameters together, adds the ADAMO header, and passes the data on to the EVB. Online histograms of track parameters are derived from this monitoring data.