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2 General Description

2.1 Product perspective

The ROBs form a layer in the Read-out architecture where events selected by the Level-1 Trigger are stored until the Level-2 Trigger has decided whether to accept or reject them and (if accepted) until they are transferred to the Level-3 Trigger. There may be around 2,000 ROBs in the final installation.

The detailed operation of the ROB in the dataflow has not been fixed and the dataflow protocol has still to be defined. The official dataflow architecture can be viewed at:

ftp://www.cern.ch/pub/Atlas/TP/NEW/HTML/tp9new/node12.html#1611

However, in order to aid the understanding of this document, an example of a simplified read-out architecture surrounding the ROB is shown in Figure 1. A description of how this example read-out functions is as follows;

  1. Special trigger data is sent for every bunch crossing from selected sub-detectors to the Level-1 Trigger. When the Level-1 Trigger selects an event, it issues an L1_accept signal to all the sub-detectors and a RoI_message to the Level-2 Trigger. It then sends the trigger data for the selected event to one or more ROBs (i.e the trigger data is launched into the dataflow and thus is appended to the event).

  2. On receiving an L1_accept, the sub-detectors transfer data, via the RODs and Read-out Links to the ROBs. The data waits in the ROBs.

  3. The RoI_message is processed by the Level-2 Trigger which issues RoI_requests to the ROBs which correspond to the Regions of Interest. Note that only those ROBs which contain RoI_data are requested to send data to the Level-2 Trigger.

  4. Selected RoI_data is transferred from the ROB to the Level-2 Trigger.

  5. The Level-2 Trigger analyses the RoI_data and issues an L2_accept or L2_reject.

  6. On an L2_reject, the ROBs delete the fragment; on an L2_accept or L3_request they may reformat the data and pass the fragment to the Event Builder.

  7. The Event Builder merges fragments from participating ROBs to form events and passes these to the appropriate Level-3 Trigger processor (as defined by the destination_address).

  8. The Level-3 Trigger filters events in software and may write selected events to storage.

Figure 1 A simplified functional diagram of the ATLAS Read-out Architecture

Issue: 1.2 - Revision: 0 - Last Modified: 18 June 1996

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