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1 Introduction

1.3 Definitions, acronyms and abbreviations

1.3.1 - Definitions
1.3.2 - Acronyms
1.3.3 - Abbreviations

1.3.1 Definitions

Event
The combined data output of all detector elements in the experiment relating to a particular bunch crossing. At high luminosity running, this could contain data from several physics processes.

Fragment
The packet of data sent by any ROD to a ROB relating to a particular bunch crossing. All fragments relating to the same bunch crossing make up an event.

Trigger
A decision made by a Trigger System that a particular event is of interest and should be retained in the dataflow. Usually, this decision is based on a subset of data from the event.

Region of Interest
A geographical region of the experiment, identified by the Level-1 Trigger as containing interesting data. This data will be analysed by the Level-2 Trigger to decide if the event is to be processed further. Note that the Level-2 Trigger may identify secondary Regions of Interest which may contain information which is correlated with the primary Region of Interest.

Dataflow
The path along which data flows in the experiment. The dataflow starts in the front-ends and ends in data storage. This term is used to describe the physical hardware (e.g. RODs, ROBs etc.) and the logical concept of a route taken by the experiment's data.

Front-end
The detector sub-systems which generate and send trigger data to the Level-1 Trigger and event data (fragments) to the ROD.

Read-out Driver (ROD)
The last element in the front-end electronics. This module collects multiple data streams from detector elements and merges them into a single stream which is fed to a ROB.

Read-out Link (ROL)
The transmission medium which delivers the fragments from the RODs to the ROBs.

Read-out Buffer (ROB)
A module which receives raw data from the RODs, passes a subset of the data to the Level-2 Trigger and buffers the data until a Level-2 Trigger decision has been reached whereupon, for selected events, it transmits the data to a Level-3 Trigger Processor.

ROB-IN
The input module of the demonstrator ROB.

ROB-OUT
The output module of the demonstrator ROB.

ROB Controller
The module which controls both the ROB-IN and the ROB-out. This may be a part of either.

Level-1 Trigger
A fast processor (implemented in hardware) which receives special trigger data from the front-end. It selects interesting events and issues a L1_accept to the front-end and RoI_message to the Level-2 Trigger.

Level-2 Trigger
A processor system which requests and receives RoI_data from the ROBs and, on analysing it, sends an L2_accept or L2_reject signal for the event to the ROBs.

Level-3 Trigger
A processor system which receives events from the Event Builder. These events have been selected by L2_accept or L3_request signals. The Level-3 Trigger carries out further processing and analysis and may send the event to Data Storage for off-line analysis.

Event Builder
A component which collects fragments, relating to a particular bunch crossing, from all ROBs in the experiment to form an event.

Event_ID
A number which identifies an event.

Bunch_crossing_ID
A number which identifies a bunch crossing. This number will be reset with each LHC orbit (i.e. every 88 ms).

RoI_message
A signal sent from the Level-1 Trigger to the Level-2 Trigger which contains the co-ordinates of the Region of Interest. This used by the Level-2 Trigger to form its RoI_request.

RoI_request
Data sent to the ROBs from the Level-2 Trigger which instructs those ROBs containing Region of Interest data to send that data to the Level-2 Trigger. The RoI_request could contain a ROB address, an event_ID, RoI coordinates and one or more Level-2 destination_address(es).

RoI_data
Region of Interest data sent from a ROB to the Level-2 Trigger in response to an RoI_request.

L1_accept
A signal sent from the Level-1 Trigger to the front-end which causes it to launch an event fragment into the dataflow.

L1_inhibit
A signal sent from a ROB to the Level-1 Trigger which causes the Level-1 Trigger to stop sending L1_accept signals to the front-end. This is to stop the dataflow if the ROB becomes overloaded.

L2_reject
A signal from the Level-2 Trigger to a ROB which causes it to delete a rejected fragment from the buffer.

L2_accept
A signal from the Level-2 Trigger to a ROB which causes it to send an accepted fragment to the Event Builder.

L3_request
A signal from the Level-3 Trigger to a ROB which requests that a particular event fragment be sent to the Event Builder.

L3_done
A signal from the Level-3 Trigger to a ROB which indicates that a fragment sent to Level-3 following an L2_accept or L3_request has been safely received by the Level-3 Trigger. The fragment can therefore be deleted from the buffer.

Destination_address
A number which is appended to an event fragment and which allows it to be routed to a particular Level-2 or Level-3 Trigger processor (could be a processor_ID or a port_ID of a switch or some other ID).

Timing, Trigger and Control
The system which distributes trigger signals (e.g. L1_accept), timing signals (e.g. system clock timestamps) and control signals to the various sub-systems of the experiment.

Run Control
Para to be supplied.

Detector Control System
The system which monitors and controls physical parameters of the sub-systems of the experiment such as gas pressure, flow-rate, high voltage settings, leakage currents etc.

1.3.2 Acronyms

ESA
European Space Agency

CERN
European Laboratory for Particle Physics

ROB
Read-out Buffer

ROD
Read-out Driver

EB
Event Builder

SIF
Standard Internal Format

TTC
Timing, Trigger and Control

1.3.3 Abbreviations

None

Issue: 2.0 - Revision: 0 - Last Modified: 18 June 1996

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