Experience on a serial-bus based fast readout control system at Belle

We have installed a serial-bus based fast readout control system in the Belle experiment at the KEKB e⁺e^- collider. Every subdetector readout system at Belle comprises common readout boards that initiate the readout sequence upon receiving a trigger signal for every event. In order to control more than hundred of such readout boards with a compact and scalable system, a cascadable tree structure based on one-to-eight switch modules that are interconnected with enhanced category-5 local area network cables was constructed. The cable carries dedicated clock and trigger signals, and a pair of 10-bit 509 Mbps serialbus lines to transfer trigger and control related information. The 10-bit serial-bus is used as an 8-bit parallel lines with 2-bit parity bits for most of the time, and when needed, it is used to embed control sequences in other bit patterns to reset boards, to collect status, and to measure the latency of the serial-bus. A pointto- point path addressing method is implemented as other set of the control sequences, to access every individual node at any location in the tree. This system has replaced the old parallel and LEMO cable based fast control system in a step-by-step manner during summer or winter shutdown periods, for most of the subdetector readout systems at Belle. Our experience on installation and operation after running stably for more than two years is reported.