The ZEUS event-builder is a real-time data formatting and transport system [3]. The purpose of the event-builder is to unite the data flows from the 17 components and transfer this data to the third level trigger system. In addition, it is responsible for broadcasting the global second level trigger decision to the components.
The requirements of the event-builder can be calculated from the design second level trigger rate of 100 Hz and estimates of the component maximum data sizes. The maximum required bandwidths are estimated to be 2.9 Mbyte/s at the component interfaces to the event-builder, 13.9 Mbyte/s internally and 2.3 Mbyte/s at the third level trigger interfaces. It is also desirable for the system to have fault tolerance against failure of transmission to the third level trigger and the ability to distribute events evenly over the six third level trigger branches.
These requirements are satisfied by using an asynchronous packet-switching transputer network that uses transputer links for bulk data transfer (Fig. 5). The system consists of 17 component interface modules, six third level trigger interface modules, a cross-bar switch and supervisor unit. In total the event-builder contains more than 50 transputers distributed over 24 VME crates.
The component input interfaces to the event-builder are 2TP-VME modules. The component's global readout processor is responsible for putting the component's event data into a common memory area (TPM) on the module. This could be via one of the IMS T800s or over the VMEbus. Although twelve components use transputer links, this approach keeps the interfaces independent of the external systems. Similarly, 2TP-VME modules are the output interfaces of the event-builder to the third level trigger. The RS422 driver is used for the component transputer links which are 6-20 m from the event-builder, while optical link adapters and 50 m of optical fibre are used for communication with the third level trigger.
A high speed 64 by 64 custom made cross-bar switch allows dynamical linking of any detector component to any branch of third level trigger processor nodes and has a bandwidth of more than 24 Mbyte/s. This custom built cross-bar switch uses a low latency design of IMS C004 communication link switches.
A supervisor unit is responsible for dynamically configuring the cross-bar switch and controlling the system. The supervisor consists of three 2TP-VME modules performing the functions of online monitoring, buffering monitor data, mixing synchronization messages, supervising event building, choosing routing paths and controlling the cross-bar switch. The exchange of synchronization messages between the component interfaces and control unit is performed by a module containing a trinary tree network (three layers deep) of IMS T425 transputers to multiplex and de-multiplex internal synchronization.
The event-builder has been a very reliable component in the ZEUS experiment. Tests have shown that the system can handle a peak trigger rate of 254 Hz. Up to 72 events can be buffered in the output interface modules to the third level trigger and a further 75 events can be buffered at the component interfaces. At the moment link speed would limit the event-builder throughput if the design second level trigger rate and component data volumes were achieved. Procedures to increase the throughput are in progress.