  Gamma Ray Array Detector (GRAD) is one of External Target Facility (ETF)
subsystems at the Heavy Ion Research Facility at Lanzhou. The trigger subsystem
of the GRAD has been developed based on Field Programmable Gate Array (FPGAs)
and PXI interface. The GRAD trigger subsystem makes prompt L1 trigger decisions
to select valid events. These decisions are made by processing the hit signals
from 1024 CsI scintillators of the GRAD. According to the physical
requirements, the GRAD trigger subsystem generates 12-bit trigger signals that
are passed to the ETF global trigger system. In addition, the GRAD trigger
subsystem generates trigger data that are packed and transmitted to the host
computer via PXI bus for off-line analysis. The trigger processing is
implemented in the front-end electronics and one FPGA of the trigger module.
The logic of PXI transmission and reconfiguration is implemented in the other
FPGA of the trigger module. The reliable and efficient performance in the
Gamma-ray experiments demonstrates that the GRAD trigger subsystem is capable
to satisfy the physical requirements.
