The ZEUS calorimeter contains 12 864 channels and is divided into forward, rear and barrel regions (FCAL, RCAL and BCAL, respectively). A schematic diagram of the readout and trigger transputer network for the entire calorimeter is shown in Fig. 3 [13]. Each rectangle divided by a line in the diagram represents a 2TP-VME module.
Data is read out from the digital cards by readout transputers (over the VMEbus) in 36 VME crates. On each 2TP-VME module in layer one, one transputer is dedicated to readout and data transport to the event-builder. Two local calorimeter event-builders, sub-system crates (SSC), are fed data from three control and switch boxes (CSB). The control and switch boxes are responsible for controlling all transputers in the VME crates. Each control and switch box can handle the signals of 32 transputers (16 2TP-VME modules). Control and switch boxes allow a dynamically configurable network, provide two broadcast mechanisms and a centralized control of the transputer network. The entire transputer network is controlled by transputers in the SSCs. The interfaces to the event-builder are also mounted in the SSC crates.
The control and switch boxes can broadcast messages using a link broadcast board or a link switch board. The link switch board contains a IMS C004 programmable link switch allowing 16 bi-directional link-to-link connections [13]. The link broadcast board allows a message to be broadcast from one transputer link to a maximum of 16 links [14]. The transputer link protocol only supports point-to-point communication. This circuit circumvents the protocol for the case of connecting several receiving transputers to one sending transputer. Each receiving transputer still acknowledges a data packet but the sending transputer is only required to make one acknowledgement, via the circuit.
The calorimeter second level trigger transputers form a three-layer tree network and events are processed in parallel. One of the transputers in the 2TP-VME modules of layer one is used for second level trigger processing (the other performs readout). Trigger layers two and three have transputers (in 2TP-VME modules) which combine data from three or four transputers in the layer below and transmit it to the layer above.
The average latency of the calorimeter data acquisition and second level trigger system is measured to be 7 ms, which is 2 ms above the design value. This is related to the fact that not all processing steps are yet able to run at the designed 1 kHz rate. The control and switch boxes can transfer 3.4 Mbyte/s of data per calorimeter region to the event-builder, which is well above the expected 2.6 Mbyte/s bandwidth.