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D. Froidevaux
cases, separated from their performance. The numerous and challenging subsystem components were thus developed along two independent paths. The first
development path concentrated on the identification and implementation of the full
functionality needed for operation in the final DAQ. The second path concentrated
on the issues that arise when the functions identified in the first path are executed at
the performance levels required by the final DAQ system.
Following these principles, ATLAS has pursued an R&D programme, which
has resulted in a system that could be implemented for the early luminosities of
the LHC, and could be scaled to the expected needs at the full design luminosity,
since the system architecture is such that in a number of incremental steps, the
performance of the system can be increased proportionally.
16.6.3.1 Data Acquisition
The main elements of the ATLAS DAQ system are described in more detail
below:
• Detector readout system: this consists of modules which read the data corresponding to a single bunch crossing out of the front-end electronics upon the
reception of a L1 trigger accept signal. There are approximately 1600 such
modules in the ATLAS readout.
• Event builder: this is the collection of networks, which provide the interconnections between the detector readout and the HLT. It provides (and monitors) the
data flow and employs a large switching fabric. ATLAS has two such networks,
one for the L2 trigger and one for the event filter.
• HLT systems: these are the processors, which deal with the events provided by
the detector readout. They execute the HLT algorithms to select the events to be
kept for storage and offline processing.
• Controls and monitors: these consist of all the elements needed to control the
flow of data (events) through the DAQ system, as well as the elements needed to
configure and operate the DAQ. This includes all the provisions for special runs,
e.g. for calibrations, that involve special setups for both the detectors, the trigger
and the readout. The other major functionality is the monitoring of the various
detector elements, of the operation of the L1 and HLT and of the state of the DAQ
system and its elements.
The factorisation of the DAQ function into tasks, which can be made almost
independent of each other, facilitates the design of a modular system which can be
developed, tested and installed in parallel. To ensure this factorisation, the different
operational environments of the four functional stages must be decoupled. This is
achieved via the introduction of buffering of adequate depth in between each of these
stages. The primary purpose of these buffers is to match the very different operating
rates of the elements at each stage. As an example, at a rate of 100,000 events per
second, the readout system delivers an event every 10 μs. On the other hand, the
event building process requires, even assuming a 100% efficiency of 2 Gb/s links,
D. Froidevaux
cases, separated from their performance. The numerous and challenging subsystem components were thus developed along two independent paths. The first
development path concentrated on the identification and implementation of the full
functionality needed for operation in the final DAQ. The second path concentrated
on the issues that arise when the functions identified in the first path are executed at
the performance levels required by the final DAQ system.
Following these principles, ATLAS has pursued an R&D programme, which
has resulted in a system that could be implemented for the early luminosities of
the LHC, and could be scaled to the expected needs at the full design luminosity,
since the system architecture is such that in a number of incremental steps, the
performance of the system can be increased proportionally.
16.6.3.1 Data Acquisition
The main elements of the ATLAS DAQ system are described in more detail
below:
• Detector readout system: this consists of modules which read the data corresponding to a single bunch crossing out of the front-end electronics upon the
reception of a L1 trigger accept signal. There are approximately 1600 such
modules in the ATLAS readout.
• Event builder: this is the collection of networks, which provide the interconnections between the detector readout and the HLT. It provides (and monitors) the
data flow and employs a large switching fabric. ATLAS has two such networks,
one for the L2 trigger and one for the event filter.
• HLT systems: these are the processors, which deal with the events provided by
the detector readout. They execute the HLT algorithms to select the events to be
kept for storage and offline processing.
• Controls and monitors: these consist of all the elements needed to control the
flow of data (events) through the DAQ system, as well as the elements needed to
configure and operate the DAQ. This includes all the provisions for special runs,
e.g. for calibrations, that involve special setups for both the detectors, the trigger
and the readout. The other major functionality is the monitoring of the various
detector elements, of the operation of the L1 and HLT and of the state of the DAQ
system and its elements.
The factorisation of the DAQ function into tasks, which can be made almost
independent of each other, facilitates the design of a modular system which can be
developed, tested and installed in parallel. To ensure this factorisation, the different
operational environments of the four functional stages must be decoupled. This is
achieved via the introduction of buffering of adequate depth in between each of these
stages. The primary purpose of these buffers is to match the very different operating
rates of the elements at each stage. As an example, at a rate of 100,000 events per
second, the readout system delivers an event every 10 μs. On the other hand, the
event building process requires, even assuming a 100% efficiency of 2 Gb/s links,