TAGnet a twisted-pair protocol for event-coherent DMA transfers in trigger farms

This E.C.D. approach has the following advantages over conventional eventbuilding approaches as far as they applicable at these rates: TAGnet is a link protocol developed for high-rate hardware eventbuilding in trigger farms. Its first implementation is in the level-1 VELO trigger system of LHCb where all data sources (Readout Units) need to receive destination addresses for their DMA engines at a trigger rate of nominally 1 MHz. TAGnet organises event-coherency for the source-destination transfers and provides the proper timing for best utilization of the network bandwidth. The serial TAGnet LVDS link interconnect all Readout Units in a twisted-pair ring, which is controlled by a TAGnet scheduler. 1. A farm CPU requests new events from the scheduler whilst processing a previous one (double buffer, no worst-case buffer requirements like for ”round-robin”) 2. A TAGnet implementation is based on twisted pair, FPGA logic, and one programmable PCI card, hence can be implemented at very low cost and real estate. 3. TAGnet provides added functionality via “messageTAGs” like for online RU buffer-checks, and error reporting from the DMA engines.