  A trigger system was designed for the external target experiment in the
Cooling Storage Ring (CSR) of the Heavy Ion Research Facility in Lanzhou
(HIRFL). Considering that different detectors are scattered in a large area,
the trigger system is designed based on a master-slave structure and
fiber-based serial data transmission technique. The trigger logic is organized
in hierarchies, and flexible reconfiguration of the trigger function is
achieved based on command register access or overall field-programmable gate
array (FPGA) logic on-line reconfiguration controlled by remote computers. We
also conducted tests to confirm the function of the trigger electronics, and
the results indicate that this trigger system works well.
