Complement to D0 note 706
                        -------------------------

    For each INDIVIDUAL TRIGGER TOWER signal (= one EM signal + one HD
signal), we construct a certain number of energy quantities: e.g. EM
Et(=transverse component) and HD Et, also TOT Px and TOT Py. These
quantities are built by PROM lookup. The lookup will (a) scale the input
signal to the appropriate component AND (b) make corrections using the fast
vertex position from level 0 AND (c) possibly make a low energy cut (=force
to zero all energies below a given Et for physics cut AND below a given
incoming signal level for electronics noise suppression) AND (d) make some
offset and scale adjustments internal to our system (you can probably
forget about this last point). 

    This lookup is unique; I mean only one value is constructed for each
quantity type. We also feature a second lookup ability. That is:
immediately after the first lookup of EM Et and HD Et we can construct an
extra quantity on the EM and HD signals, using the PROM resources spared by
the first lookup. We used to think (i.e. 5 years ago) about constructing
the EM E and HD E signals, but it brings no help to triggering. The other
possible use of this second lookup COULD BE another EM Et and HD Et lookup
with a different low energy cut. 

    From that point on, the information takes TWO DISTINCT PATHS. 
    ~~~~~~~~~~~~~~~~~~
    The simplest path to follow is the arithmetic sums of all this scalar
quantities over the whole detector.   
    global EM Et = sum of Trigger Tower EM Et
    global HD Et = sum of Trigger Tower HD Et
    global TOT Et = global EM Et + global HD Et 
    global EM L2 = sum of Trigger Tower EM second lookup 
    global HD L2 = sum of Trigger Tower HD second lookup 
    global TOT L2 = global EM L2 + global HD L2 
    global Px = signed sum of Trigger Tower Px
    global Py = signed sum of Trigger Tower Py
    final P stage = comparison of Px and Py against a SQRT(Px**2+Py**2)
                    threshold

    Any number (min 0, max probably 16 or 32) of GLOBAL thresholds can be
set for any of these global energy quantities (with the exception of global
Px and Py where it is useless) and the result of each comparison goes to
the andor network of the trigger framework. 

    The other path starts by comparing some of these INDIVIDUAL TRIGGER
TOWER QUANTITIES to some LOCAL THRESHOLDS: for each trigger tower, TWO
decisions are made:

    Is [EM Et greater than threshold] AND [HD Et lower than threshold (i.e
hadronic veto)]?                      ~~~
and Is [TOT Et (=EM Et+HD Et) greater than threshold] ?

    Note that each decision of the first type requires two threshold
values (one EM Et and on HD Et threshold), and each decision of the second
type only requires one threshold value. 

    Since all these thresholds are individually programmed for each trigger
tower (and also to avoid confusion with global thresholds), we chose the
name "reference set" to qualify a list of thresholds of one kind specified
for all the individual trigger towers. 

    The multiplicity is FIXED at four; there are four INDEPENDENT decisions
of each type made for each trigger tower. There are thus FOUR reference
sets of each of these THREE kinds to make decisions of TWO types. 

    After each individual trigger tower decision is made, we count all the
positive decisions over the whole detector:

    global EM Et Trigger Tower count  = count of all Trigger Towers
                                        satisfying EM Et >= threshold 
                                        AND HD Et NOT >= threshold 
    global TOT Et Trigger Tower count = count of all Trigger Towers
                                        satisfying TOT Et >= threshold 

    There are four independent counts of each type (this follows the
reference set multiplicity fixed at four). 

    For each such count, once again, any number (min 0, max probably 8) of
GLOBAL thresholds can be set, and the result of each comparison goes to the
andor network of the trigger framework.