MARCO CONCEPTUAL

integrity, reach them along the same common and unique path.

Finally, the choice of using standard commodity output links (Gigabit Ethernet) was not only dictated by considerations on cost and simplicity, but resulted in a large flexibility in reconfiguring the downstream part of the TDAQ system, which has been exploited in modifying the design of the L1/L2 computing farm architecture to adapt to emerging challenges, different needs for sharing bandwidth between L0 trigger and main readout information, and the availability of cheaper and more powerful machines, in an easy and cost-effective way without requiring changes in the upstream

part of the system itself. The data are transferred using UDP,5thus with no reliability built-in in the

protocol, because of data performance requirements: this was not an issue in the original TDAQ concept in which each sub-detector used dedicated PCs and only point-to-point links were present. In order to optimize PC usage, a paradigm shift was introduced in which data from the acquisition boards go directly onto a switched network, and possible data losses in the switching fabric have to be monitored.

Broadly, the NA62 experiment comprises about 15 sub-detector systems, most of them sharing similar requirements in terms of precision timing response and readout capabilities; this led to the design of a common, unified and versatile trigger and data acquisition system based on high-

precision TDCs, suited for use in most of NA62 sub-detectors, described in section 13.2. For

the straw spectrometer, with the largest channel count among the TDC-based sub-detectors and an intrinsically poorer time resolution for which less precise TDCs are sufficient, it was later decided

to implement a dedicated solution for an even higher integration and reduced cost (section13.4).

The GigaTracker (GTK) has the largest number of channels in a highly-integrated miniaturized

system, which required a dedicated system (section 13.3). Calorimeters instead use continuous

pulse sampling via FADCs to extract information, with the system described in section13.5.

The L0 hardware trigger is described in section13.6, and the software triggers in section13.7.

Figure 58 illustrates the overall TDAQ scheme of NA62 and the relevant trigger rates and

connections.

13.1 Common signal distribution

Practical considerations for an experiment of the size and time scale of NA62 suggested evaluating the adoption of existing systems and infrastructures. This led to the choice of using the Timing,

Trigger and Control (TTC) [45] system, developed at CERN and adopted by all LHC experiments

for clock and trigger distribution, based on time-multiplexed transmission of synchronous and asynchronous data with 25 ns precision over a low-jitter clock. The TTC is a unidirectional optical fibre based transmission system where two channels are multiplexed and encoded using a 40 MHz clock and transmitted at 160 MHz rate. One channel is used to carry the L0 trigger signal only, while the other carries encoded information concerning resets and trigger types.

Two important differences in the use of the timing system with respect to LHC are related to the fixed-target SPS environment: in NA62 the main clock is centrally generated by a high-quality

clock generator6in the experimental area; its frequency of 40.079 MHz is chosen to fall within the

5User Datagram Protocol (UDP) is a minimal and connectionless network protocol.

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Figure 58. Overview of the NA62 TDAQ Trigger and Data Acquisition system. The average data size in

byte is reported close to the data arrow. For sake of simplicity, only few detectors are displayed, in particular GTK is not represented.

locking range of the QPLL (Quartz Crystal Phase-Locked Loop) jitter-cleaning system [46], used

to guarantee the required high accuracy and stability, but is free-running and otherwise unrelated to any clock from the accelerator system. Moreover, the SPS beam structure is very different from that of the collider, and expected to be roughly uniform in rate for a few-seconds long machine spill, separated from the next one by a variable time which can be as long as a minute. This leads to the machine spill (or burst) being the unit of data-taking in NA62, with all electronics running in a fully synchronized way while it lasts, and performing clean-up tasks in an independent way outside the spill.

All the synchronous elements of the NA62 TDAQ system run on the centrally distributed TTC clock and are synchronously reset by a Start Of Burst (SOB) command delivered through the same links and generated by aligning to 25 ns precision the SPS Warning of Warning of Ejection (WWE), roughly 1 second before the first beam particles are delivered; each system also synchronously stops on a similar common End Of Burst (EOB) signal, after a number of 25 ns clock periods (which can vary from burst to burst but is common to all systems for any given burst). By resetting all coarse time counters on SOB through the same link which delivers the clock, any relative delay between sub-systems due to differences in propagation time from the clock generator is intrinsically corrected for.

The actual clock and L0 trigger signal distribution occurs through the use of a NA62-modified

version of ALICE Local Trigger Unit (LTU) boards [47], of which 12 are deployed (roughly one

per sub-system); these modules take care of time multiplexing the synchronous L0 trigger signal received by the central L0 Trigger Processor (L0TP) and the asynchronous 8-bit Trigger Type word which accompanies each L0 trigger to allow for different processing. Trigger type coding allows for

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