Highly scalable asynchronous trigger system for PET using UDP/IP

Asynchronous coincidence triggering in PET can be realized by using UDP over 1 Gbps Ethernet. This will bypass problems related to point-to-point connections and tree-structures in real-time coincidence trigger units. By using UDP over Ethernet, the trigger system is easy to scale, and off-the-shelf components can be used. However, due to unknown and network load dependent delay times of UDP communication packages, and due to possible package losses, the system must react tolerantly towards these occurrences. In our FPGA based readout system, event packages (consisting of an event time, energy, and channel number) are stored in a FIFO. The event times are sent to a Central Trigger Unit (CTU) through a UDP/IP link where trigger decisions are made and broadcasted back to the readout cards. The broadcasted trigger times are compared to existing event times to decide if the event package is discarded or sent to a computer farm. Since the UDP packages with event times and trigger times are self-contained there is no need for sorting algorithms usually associated with UDP networking. The UDP/IP stack is implemented solely in hardware without using large amount of resources.