BES III TOF Trigger Sub-System

The time-of-flight (TOF) is one of the Beijing Spectrometer III (BES III) subsystems. Its trigger logic is implemented on a 9U VME board equipped with a rear side plug-in transition module. This trigger logic makes prompt decisions to select interesting physics events. These decisions are made by evaluating the relevant hit information from the 272 scintillators that compose the TOF detector. The accessory VME transition modules, which are plugged into the rear of the TOF readout electronics VME crate, transfer the 272 hit signals into the trigger module. A total of 34 fiber optical links are allocated to bring the raw hit information from the TOF readout electronics to the trigger module. In practice, only 28 fiber-optical links have been used. According to the physics requirements, the TOF trigger module generates 7-bit real-time trigger signals that are passed to the global trigger system, where the primary L1 trigger decision is made. In addition, the TOF trigger subsystem also sends 136-bit position-hit data to the Track-Match-Logic modules in the global trigger system for particle track processing in real time. The trigger logic algorithms are implemented in nine Altera EP1K30 and one EP1S40 field-programmable gate array (FPGAs) in the TOF trigger module. Compared to the previous Beijing Spectrometer TOF trigger subsystem, the employment of high-capacity FPGAs in the trigger module has the advantage of reconfiguration if the physics requirements evolve in the future. During the tests carried out after the assembly of the entire BES III system, the TOF trigger subsystem performed reliably and efficiently.