The HADES DAQ System: Trigger and Readout Board Network

HADES is a di-electron spectrometer operating at GSI, Germany. Currently, the HADES data and trigger system is being upgraded. The main aim is to substantially increase the event rate capabilities by a factor of up to 20 to reach 100 kHz in light systems and 20 kHz in heavy ion reactions. The data rate will be about 400 MByte/s in peak. In this context, the complete readout system has been exchanged to use optical communication. In this contribution we present a general-purpose real-time network protocol that has been developed to meet the new requirements. These include strong timing constraints with latencies less than 5 μs for endpoint-to-endpoint communication through up to 10 intermediate hubs in a star-like network setup. In summary, this network connects over 500 FPGAs distributed over the whole HADES detector. Monitoring and slow control features as well as readout and trigger distribution were joined in a single network protocol. Hence, channel multiplexing with inherent arbitration by priority is implemented. Switching between channels takes less than 100 ns. For control and monitoring, a dedicated channel provides a data bus with a virtual address space spanning the whole network. The configuration is highly flexible and thus adaptable to different experimental setups and hardware.