hasin [2], G. Blanchard [4],, I.J. Bloodworth [2], P. Dupieux [4],  L. Efimov [5], D. Evans  [2], G.A. Feofilov [7], M. Gallio [8] ,  G.T. Jones [2], P. Jovanovic [2], A. Jusko [6], J.B. Kinson [2], A. Kirk [2], W. Klempt [3], B. Kocper [6], A. Kolovjari [7], I. Krlik [6], V. Lenti [1], M. Luptk [6], B. Pastircak [6], L. Sandor [6], J. Schukraft [3], W.N. Stokes [2], J. Urban [6], P. Vande Vyvre [3], A. Vascotto[3],  E. Vercellin [8], O. Villalobos Baillie [2].
For the ALICE Collaboration
 [1] Bari, Italy, Dipartimento di Fisica dell'UniversitaU and Sezione INFN
 [2] Birmingham, United Kingdom, School of Physics and Space Research, University of Birmingham
 [3] CERN, Geneva, Switzerland, European Organization for Nuclear Research
 [4]Clermont-Ferrand, France, UniversiteU Blaise Pascal and IN2P3
 [5]JINR, Dubna, Russia, Joint Institute for Nuclear Research
 [6]Kosice, Slovakia, Institute of Experimental Physics, Slovak Academ  of Sciences
 [7]St.Petersburg, Russia, Institute for Physics of St.Petersburg State University
 [8]Turin, Italy, Dipartimento di Fisica Sperimentale dell'UniversitaU and
 Sezione INFN
!!
The ALICE detector is made up of fifteen different sub-detector systems,
having widely different readout requirements. In most cases, the detector is
strobed after about a microsecond, and has a sensitive period of a similar
length. However, the principal ALICE detector, a Time Projection Chamber (TPC),
has a drift time of 100 microseconds, and therefore is sensitive for 200
microseconds.
At present, four detectors contribute trigger signals to ALICE, with the use
of two further detectors, the pixels and the Parallel Plate Chambers (PPC)
under investigation. The four detectors are: (i) the Forward Multiplicity
Detector (FMD), (ii) the Zero Degree Calorimeter (ZDC), (iii) the dimuon
trigger chambers and (iv) the Transition Radiation Detector. The first two
select on centrality, giving relatively modest rate reductions (O (10%)).
In contrast, selections (iii) and (iv) do detailed selections on dimuons and
dielectrons respectively, and can give much lower rates (O (10exp.-5)). In
order to maximize the rates for these rarer events it is essential that they
should sample the full luminosity, i.e. before any scaling is done.
The trigger uses the RD-12 TTC system for distribution of the L1 trigger, for
distribution of events numbers, and for timing applications in the TOF and
FMD detectors. Each sub-detector is treated independently, so that each one
must be able to take data while another is still reading out the previous event
This feature is exploited in the case of rare triggers , where (e.g. for dimuon
triggers) the dimuon arm is allowed to read out with the pixel detector only
The read out time for this system is an order of magnitude lower then for the
TPC, allowing a higher data acquisition rate for this system; although the
physics signals are indeed rare, the counting rates for the associated triggers
are not so low.
In order to meet the instantaneous rates for data transfer, many sub-detectors
have chosen to use front-end buffering. A consequence of this is that the
transfer between the front-end system and the DAQ has been re-evaluated and
is now simpler, taking place only after the final, positive level 2 trigger
decision. This means that an interface between the trigger and the Read Out
Receiver Card (RORC) is no longer required.  
!!
The planned addition of a Transition Radiation Detector (TRD) to the ALICE setup, coupled with a better understanding of the requirements of the front end electronics, have led to a major re-appraisal of the central trigger system for the ALICE detector. The previous system was reviewed at the LEB Workshop in 1996. In the new system, the major rate reduction in Pb-Pb collisions comes after 5.5 microseconds, in order to wait for the decision of the TRD. Data transfer to the data acquisition system is initiated after a positive level 2 decision, 100 microseconds after the event takes place.
!!