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192 trigger (physics algorithm or technical trigger) bits; veto masks can be used fortechnical triggers.4.3.1 Decision LogicThe CMS DAQ subsystem can be split into 8 independent partitions interested indifferent parts of the event stream. The actual FINOR contains the decision for eachof these partitions, forming an 8-bit FINOR word.The trigger decision (per partition) in the global trigger is highly programmable.The basic building blocks of event selection decisions are predefined conditions whichare applied to the input objects. Table 4.1 shows the defined condition types. Theseconditions, along with 64 external conditions (like presence of beam from the LHCBeam Pick-up Timing Experiment (BPTX) detectors or signals from CASTOR, ZDCand Beam Scintillator Counters (BSC)) can be combined using boolean operators into128 algorithms.Up to 64 binary signals from external systems (BPTX, HCAL, CSC, DT, RPC,BSC. . . ) are received directly by GT as technical triggers.The following factors influence the FINOR word:• Prescaling: each trigger bit (algorithm or technical trigger bit) is assigned aprescale factor, selecting only a given ratio of the triggers of that type. Prescalingis needed to select samples of very common processes without exceeding thetarget trigger rate. Actually, at the beginning of the run, sets of prescale factorsare defined which are applied sequentially in time to compensate for the decreaseof the luminosity during the run (see Sec. 3.1.1).• Masks: for each partition in the FINOR word the trigger can apply a triggermask on each trigger bit. This mask causes the result of the correspondingtrigger bit to be ignored in the final OR.• Trigger Veto Mask for the technical trigger bits: if a technical trigger fires andthe associated veto bit is set, the event is not selected (forcing the FINOR to 0).4.3.2 Global Trigger HardwareThe GT system consists of a number of VME electronics boards housed in a VME9U crate. This crate also contains the GMT and the Trigger Control System (TCS)boards. All boards in the crate are connected through the backplane.Common FeaturesAll the boards that make up the global trigger contain a VME interface, allowing theonline software to configure and monitor the operation of the boards through exposedregisters and memories. In addition, the custom chips in the global trigger are allbased on FPGA technology and can be reprogrammed with new firmware over theVME interface.44