Due to the non-projective geometry of the ZEUS calorimeter[4], the path taken by a particle from the interaction point often travels through the EMC of one tower and the HAC of an adjacent tower. While ``projective'' combinations of the EMC and HAC sections of different non-projective towers can be made with software in the SLT, TLT,[5] and offline, they must be made directly in the hardware of the FLT.
The creation of a projective geometry from the non-projective mechanical calorimeter geometry is complex. The tower geometry is set by locations of the EMC towers (groups of either four 5 x 20 cm or two 10 x 20 cm sections). The particles that traverse these then ``shadow'' various combinations of HAC towers. The first and 14th BCAL EMC towers shadow the outer ring of the FCAL and RCAL HAC towers, respectively, and do not shadow HAC towers in the BCAL. The top and bottom of the central FCAL and RCAL modules shadow HAC towers above and below them, respectively. The outer 6 or so FCAL and RCAL modules have EMC towers that shadow HAC towers in the adjacent modules. In many of these cases, an individual 20 x 20 cm EMC tower shadows more than 1 (in some cases, as many as 6) HAC towers.
The importance of projectivity is shown by the isolated electron trigger. In order to distinguish an electron from a hadron, the energy in the HAC that would be traversed by a particle that penetrates a certain EMC section must be much smaller than the energy in the EMC section. For many sections of the calorimeter (especially the FCAL and RCAL for theta greater than 20 degrees) a particle that traverses an EMC section of a physical tower does not penetrate the HAC section, but that of an adjacent tower. Therefore, it is inappropriate to use the energy in the HAC section to identify electrons that deposited energy in the EMC section of the same mechanical tower. This check of HAC/EMC energy is done every 96 nsec for each tower in the calorimeter. The two energies to be compared, HAC and EMC, must be fed to the same memory lookup table in order to perform this test. In order to achieve this, the hardware requires that the EMC and HAC pulseheights to be compared be digitized on adjacent electronics channels.
The CAL FLT electronics is constrained to have ``projective'' combinations of EMC and HAC towers cabled to the TEC front end. The pulseheight digitization is done in the CAL FLT Trigger Encoder Cards in the Rucksack. The PMT signals are first brought to the Frond End Cards. In the FCAL four trigger sums are formed from the 12 left or 12 right PMT signals on each FEC. The 4 trigger sums from one right PMT FEC and its corresponding left PMT FEC are combined to form 4 (or less) total trigger sums on each Trigger Sum Card (TSC). On the TSC the analog signal as are further combined to form the CFLT super towers. The total trigger sums from the Trigger Sum Cards are transmitted to the Trigger Encoder Cards in the Rucksack. The Trigger Encoder Cards are able to sum two separate inputs for each digitization channel.