The Global Trigger in the MTCC (``Magnet Test & Cosmic Challenge'')

During the MTCC, triggers were sent either via an LTC ("Local Trigger Controller") module or via the GT ("Global Trigger"). The system was selected by simply swapping the cables which went from it to the TTCci modules ("Trigger, Timing and Control CMS interface") of the individual subdetectors.

The trigger sources used by the Global Trigger were the muon signals sent by the Drift Tube Track Finder (DTTF) and the Cathode Strip Chamber Track Finder (CSCTF) to the Global Muon Trigger (GMT), which is integrated in the Global Trigger crate (see TDR). The trigger system of the third muon detector of CMS, the Resistive Plate Chambers, was not ready for connection to the GMT during the MTCC (but connection had been tested prior to MTCC), and the calorimeter system was not connected to the Global Trigger either. These systems only sent simple trigger bits when running with the LTC.

The data received by the GMT were processed in the Global Trigger Logic module (GTL) according to a number of simple algorithms, mostly calling for one muon with different energy thresholds, $p_t$ thresholds, and "quality bit" assignments. These algorithm bits were then passed on to the Final Decision Logic module (FDL), where each of these bits could be selected or deselected and downscaled by a factor between 1 and $2^{16}$. Bits which were thus enabled entered a "final OR", which generated a trigger request that was sent on to the Trigger Control System module (TCS). The TCS applied the trigger throttle rules and received state information on the readiness of the various subsystems from Fast Monitoring Modules (FMMs) which make up the synchronous Trigger Throttling System (sTTS). Trigger requests that were compatible with the trigger throttling rules and arrived when all FMMs signaled "ready" were then forwarded via the trigger output module (L1AOUT) to the TTCci's of the subdetectors (the Drift Tube Chambers and the Cathode Strip Chambers).

The readout module of the Global Trigger (Global Trigger Front End module, GTFE) was not yet available during the MTCC. To satisfy the needs of the data acquisition system (DAQ), which expected a record from the GT for each event, the GT also forwarded a trigger signal to the LTC, which in this case sent a dummy readout record to the DAQ.

To verify the correct signal transmission, a "spy readout" system for the GT modules was used. In the case of trigger requests received from the DTTF data recorded by this spy system and by the DTTF's own readout system were compared and found to be identical. Also, it has been tested that only events with muon candidates satisfying the applied trigger cuts have been triggered on.

The correct functioning of the FMMs was tested by varying the trigger rate. By adding large numbers of random triggers in the TCS module to the triggers received from DTTF and CSCTF, the FMMs were forced to go into "warning" or "busy" state, and when lowering the trigger rate again, they went back into "ready" (see Figure).

Thus, apart from the final readout system all important functions of the Global Trigger were tested.